Approximately 400 elementary particles are now known. Some of them “live” for a very short time, quickly turning into other particles, managing during their existence to fly distances equal to the radius of the atomic nucleus (10 -12 - 10 -13 cm). The minimum time available for experimental measurement is characterized by a value of approximately 10 -26 s. Some elementary particles turned out to be unexpectedly heavy - even heavier than individual atoms.
Modern physicists pay a lot of attention to the systematization of elementary particles, revealing the internal unity both between them and between the fundamental types of interaction corresponding to them - strong, weak, electromagnetic and gravitational.
The intensity of weak interaction is 10-11 orders of magnitude (10 10 -10 11 times) less than the intensity of nuclear forces. That is why it was called weak, its radius of action is less than 10 -15 cm. Electromagnetic interaction at distances commensurate with the radius of action of nuclear forces is only 10 2 -10 3 times weaker. The weakest at these distances is the gravitational interaction, the intensity of which is many orders of magnitude lower than the weak interaction.
Even the weak interaction is many orders of magnitude greater than the gravitational interaction. And the force of the Coulomb, electric repulsion of two electrons is 10 42 times greater than the magnitude of their gravitational attraction. If we imagine that the electromagnetic forces that “attract” electrons to the atomic nucleus weaken to the level of gravitational forces, then the hydrogen atom would become larger than the part of the Universe visible to us. Gravitational forces increase very slowly as distances decrease. They become predominant only in fantastically small intervals of less than 10 -32 cm, which remain inaccessible for experimental research. With the help of the experiment, it is now possible to “look through” distances close to 10 -16 cm.
These four types of fundamental (lying in the very foundation of matter) interactions are carried out through the exchange of corresponding particles, which serve as a kind of carriers of these interactions. The radius of action of forces depends on the mass of particles. Electromagnetic interaction is carried by photons (the rest mass is zero), gravitational interaction is carried by gravitons (yet hypothetical, experimentally not established particles, the mass of which should also be zero). These two interactions, carried by massless particles, have a large, possibly infinite range of action. Moreover, only gravitational interaction generates attraction between identical particles, the other three types of interactions determine the repulsion of particles of the same name. Gluons are carriers of the strong interaction that binds protons and neutrons in atomic nuclei. This interaction is characteristic of heavy particles called hadrons. The weak interaction is carried by vector bosons. This interaction is characteristic of light particles - leptons (electrons, positrons, etc.).
The diversity of the microworld presupposes its unity through the interconvertibility of particles and fields. Particularly important is the transformation of a “pair” - a particle and an antiparticle - into particles of a different “type”. The first to discover was the transformation of electrons and positrons into electromagnetic field quanta - photons and the reverse process of “generation” of pairs from photons with sufficiently high energy.
Currently, the development of the problem of systematization of elementary particles is associated with the idea of the existence quarks - particles with a fractional electric charge. Now they are considered “the most elementary” in the sense that all strongly interacting particles - hadrons - can be “built” from them. From the perspective of quark theory, the level of elementary particles is the region of objects consisting of quarks and antiquarks. Moreover, although the latter are considered at this level of knowledge to be the simplest, most elementary of known particles, they themselves have complex properties - charge, “charm” (“charm”), “color” and other unusual quantum physical properties. Just as in chemistry one cannot do without the concepts of “atom” and “molecule,” so elementary particle physics cannot do without the concept of “quark.”
So the list hadrons - heavy particles characterized by strong interaction - consists of three particles: quark, antique and connecting them gluon. Along with them, there are about ten light particles - leptons (electrons, positrons, neutrinos, etc.) - which correspond to the weak interaction. Also known photon - carrier of electromagnetic interaction. And still hypothetical, only theoretically predicted, remains graviton, which is associated with gravitational interaction. Nothing is yet known about the internal structure of leptons, photons and gravitons. Now there is already a more or less specific idea of synthesis, the relationship of weak, strong and electromagnetic types of interaction. It is discovered that it is possible to explain their relationship with gravitational interaction. All this testifies to the gradual realization in reality of the fundamentally unlimited possibility of theoretical thinking in understanding the unity of the world, which remains infinitely diverse in its manifestations within the framework of unity.
References for Chapter 10
Barashenkov V. S. Are there boundaries of science: the quantitative and qualitative inexhaustibility of the material world. - M., 1982.
Heisenberg V. Physics and philosophy: Part and whole. - M., 1989.
Zeldovich Ya.B., Khlopov M.Yu. The drama of ideas in the knowledge of nature: Particles, fields, charges. - M., 1988.
Markov M.A. About the nature of matter. - M., 1976.
Pakhomov B.Ya. The formation of a modern physical picture of the world. -M., 1985.
Sachkov Yu.V. Introduction to the world of probability. - M., 1971.
CHAPTER 11
Ministry of the Russian Federation
Saratov Law Institute
Samara branch
Department of PI and PCTRP
Essay
On the topic of: ” Elementary particles ”
Completed by: cadet 421 training group
police private
Sizonenko A.A.
Checked by: department teacher
Kuznetsov S.I.
Samara 2002
Plan
1) Introduction.
2)
3) Basic properties of elementary particles. Interaction classes .
4)
5)
a) Unitary symmetry.
b) Quark model of hadrons
6)
7) Conclusion. Some general problems of the theory of elementary particles.
Introduction .
E . h. in the exact meaning of this term - primary, further indecomposable particles, of which, by assumption, all matter consists. In the concept of "E.h." in modern physics, the idea of primordial entities that determine all known properties of the material world is expressed, an idea that originated in the early stages of the development of natural science and has always played an important role in its development.
The concept of "E.h." formed in close connection with the establishment of the discrete nature of the structure of matter at the microscopic level. Discovery at the turn of the 19th-20th centuries. the smallest carriers of the properties of matter - molecules and atoms - and the establishment of the fact that molecules are built from atoms, for the first time made it possible to describe all known substances as combinations of a finite, albeit large, number of structural components - atoms. Further identification of the presence of constituent atoms - electrons and nuclei, establishment of the complex nature of nuclei, which turned out to be built from only two types of particles (protons and neutrons) , significantly reduced the number of discrete elements that form the properties of matter, and gave reason to assume that the chain of constituent parts of matter ends with discrete structureless formations - E. ch. Such an assumption, generally speaking, is an extrapolation of known facts and cannot be rigorously substantiated. It is impossible to say with certainty that particles that are elementary in the sense of the above definition exist. Protons and neutrons, for example, which for a long time were considered to be electrons, as it turned out, have a complex structure. The possibility cannot be ruled out that the sequence of structural components of matter is fundamentally infinite. It may also turn out that the statement “consists of...” at some stage of the study of matter will turn out to be devoid of content. In this case, the definition of “elementary” given above will have to be abandoned. The existence of an electron element is a kind of postulate, and testing its validity is one of the most important tasks in physics.
The term "E.h." often used in modern physics not in its exact meaning, but less strictly - to name a large group of smallest particles of matter, subject to the condition that they are not atoms or atomic nuclei (the exception is the simplest nucleus of the hydrogen atom - the proton). Research has shown that this group of particles is unusually broad. In addition to the mentioned proton (p), neutron (n) and electron (e -), it includes: photon (g), pi-mesons (p), muons (m), neutrinos of three types (electron v e, muon v m and related to the so-called. heavy lepton v t), so-called strange particles (K-mesons and hyperons) , various resonances discovered in 1974-77 y-particles, “charmed” particles, upsilon particles (¡) and heavy leptons (t + , t -) - in total more than 350 particles, mostly unstable. The number of particles included in this group continues to grow and is most likely unlimited; Moreover, most of the listed particles do not satisfy the strict definition of elementaryity, since, according to modern concepts, they are composite systems (see below). Use of the name "E.h." to all these particles has historical reasons and is associated with that period of research (early 30s of the 20th century), when the only known representatives of this group were the proton, neutron, electron and a particle of the electromagnetic field - the photon. It was then natural to consider these four particles to be elementary, since they served as the basis for the construction of the matter surrounding us and the electromagnetic field interacting with it, and the complex structure of the proton and neutron was not known.
The discovery of new microscopic particles of matter gradually destroyed this simple picture. The newly discovered particles, however, were close in many respects to the first four known particles. Their unifying property is that they are all specific forms of existence of matter, not associated into nuclei and atoms (sometimes for this reason they are called “subnuclear particles”). While the number of such particles was not very large, the belief remained that they play a fundamental role in the structure of matter, and they were classified as E. particles. The increase in the number of subnuclear particles, the identification of a complex structure in many of them showed that they, as a rule, do not have elementary properties, but the traditional name "E. ch." preserved for them.
In accordance with established practice, the term "E. h." will be used below as a general name. subnuclear particles. In cases where we are talking about particles that claim to be the primary elements of matter, the term “true E. particle” will be used, if necessary.
Brief historical information.
The discovery of electron particles was a natural result of the general successes in the study of the structure of matter achieved by physics at the end of the 19th century. It was prepared by comprehensive studies of the optical spectra of atoms, the study of electrical phenomena in liquids and gases, the discovery of photoelectricity, X-rays, and natural radioactivity, which indicated the existence of a complex structure of matter.
Historically, the first electron element discovered was the electron, the carrier of the negative elementary electric charge in atoms. In 1897, J. J. Thomson established that the so-called. cathode rays are formed by a stream of tiny particles called electrons. In 1911, E. Rutherford, passing alpha particles from a natural radioactive source through thin foils of various substances, found that the positive charge in atoms is concentrated in compact formations - nuclei, and in 1919 he discovered protons - particles with a unit positive charge and a mass 1840 times greater than the mass of an electron. Another particle that is part of the nucleus, the neutron, was discovered in 1932 by J. Chadwick while studying the interaction of alpha particles with beryllium. A neutron has a mass close to that of a proton, but has no electrical charge. The discovery of the neutron completed the identification of particles - the structural elements of atoms and their nuclei.
The conclusion about the existence of a particle of an electromagnetic field - a photon - originates from the work of M. Planck (1900). Assuming that the energy of electromagnetic radiation from an absolutely black body is quantized, Planck obtained the correct formula for the radiation spectrum. Developing Planck's idea, A. Einstein (1905) postulated that electromagnetic radiation (light) is actually a flow of individual quanta (photons), and on this basis explained the laws of the photoelectric effect. Direct experimental evidence of the existence of the photon was given by R. Millikan (1912-1915) and A. Compton (1922; see Compton effect).
The discovery of the neutrino, a particle that hardly interacts with matter, originates from the theoretical guess of W. Pauli (1930), which, due to the assumption of the birth of such a particle, made it possible to eliminate difficulties with the law of conservation of energy in the processes of beta decay of radioactive nuclei. The existence of neutrinos was experimentally confirmed only in 1953 (F. Reines and K. Cowan, USA).
From the 30s to the early 50s. The study of electron particles was closely related to the study of cosmic rays. In 1932, K. Anderson discovered a positron (e +) in cosmic rays - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle discovered (see below). The existence of e+ directly followed from the relativistic theory of the electron, developed by P. Dirac (1928-31) shortly before the discovery of the positron. In 1936, American physicists K. Anderson and S. Neddermeyer discovered, while studying osmic rays, muons (both signs of electric charge) - particles with a mass of approximately 200 electron masses, but otherwise surprisingly similar in properties to e -, e +.
In 1947, also in cosmic rays, S. Powell's group discovered p + and p - mesons with a mass of 274 electron masses, which play an important role in the interaction of protons with neutrons in nuclei. The existence of such particles was suggested by H. Yukawa in 1935.
Late 40's - early 50's. were marked by the discovery of a large group of particles with unusual properties, called “strange”. The first particles of this group, K + - and K - -mesons, L-, S + -, S - -, X - - hyperons, were discovered in cosmic rays, subsequent discoveries of strange particles were made at accelerators - installations that create intense flows of fast protons and electrons. When accelerated protons and electrons collide with matter, they give birth to new electron particles, which become the subject of study.
Since the early 50s. accelerators became the main tool for studying electron particles. In the 70s. The energies of particles accelerated in accelerators amounted to tens and hundreds of billions of electronvolts ( Gav). The desire to increase particle energies is due to the fact that high energies open up the possibility of studying the structure of matter at shorter distances, the higher the energy of colliding particles. Accelerators have significantly increased the rate of obtaining new data and in a short time expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study their properties in more detail, in particular the features of their decay, and soon led to an important discovery: elucidating the possibility of changing the characteristics of some microprocesses during the operation of mirror reflection (see Spatial inversion) - so-called violation of spaces. parity (1956). Commissioning of proton accelerators with energies in the billions ev allowed the discovery of heavy antiparticles: antiproton (1955), antineutron (1956), anti-sigma hyperons (1960). In 1964, the heaviest hyperon, W - (with a mass of about two proton masses) was discovered. In the 1960s A large number of extremely unstable (compared to other unstable electron particles) particles, called “resonances,” were discovered at accelerators. The masses of most resonances exceed the mass of a proton. The first of them, D 1 (1232), has been known since 1953. It turned out that resonances make up the main part of the electron frequency.
In 1962, it was discovered that there are two different neutrinos: electron and muon. In 1964 in the decays of neutral K-mesons. non-preservation of the so-called combined parity (introduced by Li Tsung-dao and Yang Zhen-ning and independently by L. D. Landau in 1956; see Combined inversion) , meaning the need to revise the usual views on the behavior of physical processes during the operation of time reflection (see CPT Theorem) .
In 1974, massive (3-4 proton masses) and at the same time relatively stable y-particles were discovered, with a lifetime unusually long for resonances. They turned out to be closely related to the new family of electron particles - “charmed” ones, the first representatives of which (D 0, D +, L c) were discovered in 1976. In 1975, the first information was obtained about the existence of a heavy analogue of the electron and muon (heavy lepton t). In 1977, β-particles with a mass of about ten proton masses were discovered.
Thus, over the years since the discovery of the electron, a huge number of different microparticles of matter have been identified. The world of E. h. turned out to be quite complex. The properties of the discovered electron particles were unexpected in many respects. To describe them, in addition to the characteristics borrowed from classical physics, such as electric charge, mass, and angular momentum, it was necessary to introduce many new special characteristics, in particular to describe strange electron particles . - strangeness (K. Nishijima, M. Gell-Man, 1953), “fascinated” by E . h. - “charm” (American physicists J. Bjorken, S. Glashow, 1964); The names of the given characteristics already reflect the unusual nature of the properties of the elements they describe.
From its first steps, the study of the internal structure of matter and the properties of electrons was accompanied by a radical revision of many established concepts and ideas. The laws governing the behavior of matter in the small turned out to be so different from the laws of classical mechanics and electrodynamics that they required completely new theoretical constructions for their description. Such new fundamental constructions in theory were the particular (special) and general theories of relativity (A. Einstein, 1905 and 1916; see Relativity theory, Gravity) and quantum mechanics (1924-27; N. Bohr, L. de Broglie, V. Heisenberg, E. Schrödinger, M. Born) . The theory of relativity and quantum mechanics marked a true revolution in the science of nature and laid the foundations for describing the phenomena of the microworld. However, quantum mechanics turned out to be insufficient to describe the processes occurring in electron particles. The next step was needed - the quantization of classical fields (the so-called secondary quantization) and the development of quantum field theory. The most important stages along the path of its development were: the formulation of quantum electrodynamics (P. Dirac, 1929), the quantum theory of b-decay (E. Fermi, 1934), which laid the foundation for the modern theory of weak interactions, quantum mesodynamics (Yukawa, 1935). The immediate predecessor of the latter was the so-called. b-theory of nuclear forces (I.E. Tamm, D.D. Ivanenko, 1934; see Strong interactions). This period ended with the creation of a consistent computing apparatus for quantum electrodynamics (S. Tomonaga, R. Feynman, J. Schwinger; 1944-49), based on the use of the renormalization technique (see Quantum field theory). This technique was subsequently generalized to other variants of quantum field theory.
Quantum field theory continues to develop and improve and is the basis for describing the interactions of electron particles. This theory has a number of significant successes, and yet it is still very far from complete and cannot claim to be a comprehensive theory of electron particles. The origin of many properties of electrons h. and the nature of their inherent interactions remain largely unclear. It is possible that more than one restructuring of all ideas and a much deeper understanding of the relationship between the properties of microparticles and the geometric properties of space-time will be required before the theory of electron particles will be constructed.
Basic properties of elementary particles. Interaction classes.
All electron particles are objects of extremely small masses and sizes. Most of them have masses on the order of the proton mass, equal to 1.6×10 -24 g (only the electron mass is noticeably smaller: 9×10 -28 g). The experimentally determined sizes of the proton, neutron, and p-meson are equal in order of magnitude to 10 -13 cm. The sizes of the electron and muon could not be determined; it is only known that they are less than 10 -15 cm. The microscopic masses and sizes of electron particles form the basis quantum specificity of their behavior. The characteristic wavelengths that should be attributed to electron particles in quantum theory (where is Planck’s constant, m is the mass of the particle, c is the speed of light) are close in order of magnitude to the typical dimensions at which their interaction occurs (for example, for p- meson 1.4×10 -13 cm). This leads to the fact that quantum laws are decisive for electron particles.
The most important quantum property of all electron particles is their ability to be created and destroyed (emitted and absorbed) when interacting with other particles. In this respect they are completely analogous to photons. E. particles are specific quanta of matter, more precisely, quanta of the corresponding physical fields (see below). All processes involving electron particles proceed through a sequence of acts of absorption and emission. Only on this basis can one understand, for example, the process of the birth of a p + meson in the collision of two protons (p + p ® p + n+ p +) or the process of annihilation of an electron and a positron, when instead of the disappeared particles, for example, two g-quanta appear ( e + +e - ® g + g). But the processes of elastic scattering of particles, for example e - +p ® e - + p, are also associated with the absorption of initial particles and the birth of final particles. The decay of unstable electron particles into lighter particles, accompanied by the release of energy, follows the same pattern and is a process in which decay products are born at the moment of the decay itself and do not exist until that moment. In this respect, the decay of an electron particle is similar to the decay of an excited atom into an atom in the ground state and a photon. Examples of electrochemical decays include: ; p + ® m + + v m ; К + ® p + + p 0 (the “tilde” sign above the particle symbol hereinafter marks the corresponding antiparticles).
Various processes with E. h. differ markedly in the intensity of their occurrence. In accordance with this, the interactions of electromagnetic particles can be phenomenologically divided into several classes: strong, electromagnetic, and weak interactions. All electron particles also have gravitational interaction.
Strong interactions are identified as interactions that give rise to processes that occur with the greatest intensity among all other processes. They also lead to the strongest bond of electrons. It is the strong interactions that determine the bond of protons and neutrons in the nuclei of atoms and provide the exceptional strength of these formations, which underlies the stability of matter under terrestrial conditions.
Electromagnetic interactions are characterized as interactions that are based on a connection with an electromagnetic field. The processes caused by them are less intense than the processes of strong interactions, and the connection between the electron forces generated by them is noticeably weaker. Electromagnetic interactions, in particular, are responsible for the connection of atomic electrons with nuclei and the connection of atoms in molecules.
Weak interactions, as the name itself shows, cause very slowly occurring processes with electron particles. Their low intensity can be illustrated by the fact that neutrinos, which have only weak interactions, unhinderedly penetrate, for example, the thickness of the Earth and the Sun. Weak interactions also cause slow decays of the so-called. quasi-stable electron particles. The lifetimes of these particles lie in the range of 10 -8 -10 -10 sec, while typical times for strong interactions of electron particles are 10 -23 -10 -24 sec.
Gravitational interactions, well known for their macroscopic manifestations, in the case of electron particles at characteristic distances of ~10 -13 cm produce extremely small effects due to the small masses of electron particles.
The strength of various classes of interactions can be approximately characterized by dimensionless parameters associated with the squares of the constants of the corresponding interactions. For strong, electromagnetic, weak and gravitational interactions of protons with an average process energy of ~1 GeV, these parameters correlate as 1:10 -2: l0 -10:10 -38. The need to indicate the average energy of the process is due to the fact that for weak interactions the dimensionless parameter depends on the energy. In addition, the intensities of various processes themselves depend differently on energy. This leads to the fact that the relative role of various interactions, generally speaking, changes with increasing energy of the interacting particles, so that the division of interactions into classes, based on a comparison of the intensities of processes, is reliably carried out at not too high energies. Different classes of interactions, however, also have other specific features associated with different properties of their symmetry (see Symmetry in physics), which contributes to their separation at higher energies. Whether this division of interactions into classes will be preserved in the limit of the highest energies remains unclear.
Depending on their participation in certain types of interactions, all studied electron particles, with the exception of the photon, are divided into two main groups: hadrons (from the Greek hadros - large, strong) and leptons (from the Greek leptos - small, thin, light). . Hadrons are characterized primarily by the fact that they have strong interactions, along with electromagnetic and weak interactions, while leptons participate only in electromagnetic and weak interactions. (The presence of gravitational interactions common to both groups is implied.) The hadron masses are close in order of magnitude to the proton mass (m p); The p-meson has the minimum mass among hadrons: t p "m 1/7×t p. The masses of leptons known before 1975-76 were small (0.1 m p), however, the latest data apparently indicate the possibility of the existence of heavy leptons with the same masses as hadrons. The first studied representatives of hadrons were the proton and neutron, and leptons - the electron. The photon, which has only electromagnetic interactions, cannot be classified as either hadrons or leptons and should be separated into a separate group. According According to the ideas developed in the 70s, the photon (a particle with zero rest mass) is included in the same group with very massive particles - the so-called intermediate vector bosons, which are responsible for weak interactions and have not yet been observed experimentally (see section Elementary particles and quantum field theory).
Characteristics of elementary particles.
Each element, along with the specific interactions inherent in it, is described by a set of discrete values of certain physical quantities, or its characteristics. In some cases, these discrete values are expressed through integer or fractional numbers and some common factor - a unit of measurement; These numbers are spoken of as quantum numbers of E. numbers and only they are specified, omitting the units of measurement.
The common characteristics of all electron particles are mass (m), lifetime (t), spin (J), and electric charge (Q). There is still no sufficient understanding of the law by which the masses of electron particles are distributed and whether there is any unit of measurement for them.
Depending on the lifetime, electron particles are divided into stable, quasi-stable, and unstable (resonances). Stable, within the accuracy of modern measurements, are the electron (t > 5×10 21 years), proton (t > 2×10 30 years), photon and neutrino. Quasi-stable particles include particles that decay due to electromagnetic and weak interactions. Their lifetimes are > 10 -20 sec (for a free neutron even ~ 1000 sec). Elementary particles that decay due to strong interactions are called resonances. Their characteristic lifetimes are 10 -23 -10 -24 sec. In some cases, the decay of heavy resonances (with a mass of ³ 3 GeV) due to strong interactions is suppressed and the lifetime increases to values of ~10 -20 sec.
The spin of an E. h. is an integer or half-integer multiple of the value. In these units, the spin of p- and K-mesons is 0, for the proton, neutron and electron J = 1/2, for the photon J = 1. There are particles with a higher spin. The magnitude of the spin of an electron particle determines the behavior of an ensemble of identical (identical) particles, or their statistics (W. Pauli, 1940). Particles of half-integer spin are subject to Fermi-Dirac statistics (hence the name fermions), which requires antisymmetry of the wave function of the system with respect to the permutation of a pair of particles (or an odd number of pairs) and, therefore, “prohibits” two particles of half-integer spin from being in the same state (Pauli principle). Particles of integer spin are subject to Bose-Einstein statistics (hence the name bosons), which requires the symmetry of the wave function with respect to permutations of particles and allows any number of particles to be in the same state. The statistical properties of electron particles turn out to be significant in cases where several identical particles are formed during the birth or decay. Fermi-Dirac statistics also plays an extremely important role in the structure of nuclei and determines the patterns of filling atomic shells with electrons, which underlie D. I. Mendeleev’s periodic system of elements.
The electric charges of the studied E. particles are integer multiples of the value e "1.6×10 -19 k, and are called the elementary electric charge. For the known E. particles Q = 0, ±1, ±2.
In addition to the indicated quantities, energy particles are additionally characterized by a number of quantum numbers and are called internal. Leptons carry a specific lepton charge L of two types: electronic (L e) and muonic (L m); L e = +1 for electron and electron neutrino, L m = +1 for negative muon and muon neutrino. Heavy lepton t; and the neutrino associated with it, apparently, are carriers of a new type of lepton charge L t.
For hadrons L = 0, and this is another manifestation of their difference from leptons. In turn, significant parts of hadrons should be attributed to a special baryon charge B (|E| = 1). Hadrons with B = +1 form a subgroup of baryons (this includes the proton, neutron, hyperons, baryon resonances), and hadrons with B = 0 form a subgroup of mesons (p- and K-mesons, bosonic resonances). The name of the subgroups of hadrons comes from the Greek words barýs - heavy and mésos - medium, which at the initial stage of research into electron particles reflected the comparative values of the masses of baryons and mesons known at that time. Later data showed that the masses of baryons and mesons are comparable. For leptons B = 0. For photons B = 0 and L = 0.
Baryons and mesons are divided into the already mentioned aggregates: ordinary (non-strange) particles (proton, neutron, p-mesons), strange particles (hyperons, K-mesons) and charmed particles. This division corresponds to the presence of special quantum numbers in hadrons: strangeness S and charm (English charm) Ch with permissible values: 151 = 0, 1, 2, 3 and |Ch| = 0, 1, 2, 3. For ordinary particles S = 0 and Ch = 0, for strange particles |S| ¹ 0, Ch = 0, for charmed particles |Ch| ¹ 0, and |S| = 0, 1, 2. Instead of strangeness, the quantum number hypercharge Y = S + B is often used, which apparently has a more fundamental meaning.
Already the first studies with ordinary hadrons revealed the presence among them of families of particles that are similar in mass, with very similar properties with respect to strong interactions, but with different electric charge values. The proton and neutron (nucleons) were the first example of such a family. Later, similar families were discovered among strange and (in 1976) among charmed hadrons. The commonality of properties of particles included in such families is a reflection of the existence in them of the same value of a special quantum number - isotopic spin I, which, like ordinary spin, takes integer and half-integer values. The families themselves are usually called isotopic multiplets. The number of particles in a multiplet (n) is related to I by the relation: n = 2I + 1. Particles of one isotopic multiplet differ from each other in the value of the “projection” of the isotopic spin I 3, and
An important characteristic of hadrons is also the internal parity P, associated with the operation of spaces, inversion: P takes values of ±1.
For all electron particles with nonzero values of at least one of the charges O, L, B, Y (S) and the charm Ch, there are antiparticles with the same values of mass m, lifetime t, spin J and for hadrons of isotopic spin 1, but with opposite signs of all charges and for baryons with the opposite sign of internal parity P. Particles that do not have antiparticles are called absolutely (truly) neutral. Absolutely neutral hadrons have a special quantum number - charge parity (i.e. parity with respect to the charge conjugation operation) C with values of ±1; examples of such particles are the photon and p 0 .
Quantum numbers of electrons are divided into exact (that is, those that are associated with physical quantities that are conserved in all processes) and imprecise (for which the corresponding physical quantities are not conserved in some processes). Spin J is associated with the strict law of conservation of angular momentum and is therefore an exact quantum number. Other exact quantum numbers: Q, L, B; According to modern data, they are preserved during all transformations of the electron element. The stability of the proton is a direct expression of the conservation of B (for example, there is no decay p ® e + + g). However, most hadron quantum numbers are imprecise. Isotopic spin, while conserved in strong interactions, is not conserved in electromagnetic and weak interactions. Strangeness and charm are preserved in the strong and electromagnetic interactions, but not in the weak interactions. Weak interactions also change the internal and charge parity. The combined parity of the CP is preserved with a much greater degree of accuracy, but it is also violated in some processes caused by weak interactions. The reasons causing the non-conservation of many quantum numbers of hadrons are unclear and, apparently, are associated both with the nature of these quantum numbers and with the deep structure of electromagnetic and weak interactions. The conservation or non-conservation of certain quantum numbers is one of the significant manifestations of differences in the classes of interactions of electron particles.
Classification of elementary particles.
Unitary symmetry. The classification of leptons does not yet present any problems; the large number of hadrons, known already in the early 50s, provided the basis for the search for patterns in the distribution of masses and quantum numbers of baryons and mesons, which could form the basis for their classification. The identification of isotopic multiplets of hadrons was the first step on this path. From a mathematical point of view, the grouping of hadrons into isotopic multiplets reflects the presence of symmetry associated with the rotation group (see Group) , more formally, with a group S.U.(2) - a group of unitary transformations in a complex two-dimensional space. It is assumed that these transformations operate in some specific internal space - “isotopic space”, different from the usual one. The existence of isotopic space is manifested only in the observable properties of symmetry. In mathematical language, isotopic multiplets are irreducible representations of the symmetry group S.U. (2).
The concept of symmetry as a factor determining the existence of various groups and families of electron particles in modern theory is dominant in the classification of hadrons and other electron particles. It is assumed that the internal quantum numbers of electron particles, which make it possible to distinguish certain groups of particles, are related with special types of symmetries arising due to the freedom of transformations in special “internal” spaces. This is where the name “internal quantum numbers” comes from.
A careful examination shows that strange and ordinary hadrons together form broader associations of particles with similar properties than isotopic multiplets. They are called supermultiplets. The number of particles included in the observed supermultiplets is 8 and 10. From the point of view of symmetries, the emergence of supermultiplets is interpreted as a manifestation of the existence of a symmetry group in hadrons wider than the group S.U.(2), namely: S.U.(3) - groups of unitary transformations in three-dimensional complex space (M. Gell-Man and independently Y. Neeman, 1961). The corresponding symmetry is called unitary symmetry. Group S.U.(3) has, in particular, irreducible representations with the number of components 8 and 10, corresponding to the observed supermultiplets: octet and decuplet. Examples include the following groups of particles with the same values JP:
Common to all particles in a supermultiplet are the values of two quantities, which in their mathematical nature are close to isotopic spin and therefore are often called unitary spin. For an octet, the values of the quantum numbers associated with these quantities are equal to (1, 1), for a decuplet - (3, 0).
Unitary symmetry is less precise than isotopic symmetry. In accordance with this, the difference in the masses of particles included in octets and decuplets is quite significant. For the same reason, the division of hadrons into supermultiplets is relatively simple for electron particles of not very high masses. At large masses, when there are many different particles with similar masses, this partitioning is less reliable. However, in the properties of elementary particles there are many different manifestations of unitary symmetry.
The inclusion of charmed hadrons in the systematics of elementary particles allows us to talk about supersupermultiplets and the existence of an even broader symmetry associated with the unitary group S.U.(4). There are no examples of completely filled supersupermultiplets yet. S.U.(4)-symmetry is broken even more strongly than S.U.(3)-symmetry, and its manifestations are less pronounced.
The discovery of symmetry properties in hadrons associated with unitary groups and patterns of division into multiplets that correspond to strictly defined representations of these groups was the basis for the conclusion about the existence of special structural elements in hadrons - quarks.
Quark model of hadrons. From its very first steps, the development of work on the classification of hadrons was accompanied by attempts to identify among them particles that were more fundamental than the rest, which could become the basis for the construction of all hadrons. This line of research was started by E. Fermi and Yang Chen-ning (1949), who suggested that such fundamental particles are the nucleon (N) and antinucleon (), and p-mesons are their bound states (). With the further development of this idea, strange baryons were also included among the fundamental particles (M. A. Markov, 1955; Japanese physicist S. Sakata, 1956; L. B. Okun, 1957). Models built on this basis described meson multiplets well, but did not provide a correct description of baryon multiplets. The most important element of these models - the use of a small number of fermions to “construct” hadrons - was organically included in the model that most successfully solves the problem of describing all hadrons - the quark model (Austrian physicist G. Zweig and independently M. Gell-Man, 1964).
In the original version, the model was based on the assumption that all known hadrons are built from three types of particles of spin 1/2, called p-, n-, l-quarks, which do not belong to the number of observed hadrons and have very unusual properties. The name "quarks" is borrowed from the novel by J. Joyce (see Quarks) . The modern version of the model assumes the existence of at least four types of quarks. The fourth quark is necessary to describe charmed hadrons.
The idea of quarks is suggested by unitary symmetry. The mathematical structure of unitary groups opens up the possibility of describing all representations of the group S.U. (n) (and hence all hadron multiplets) based on the simplest group representation containing n component. In case of a group S.U.(3) there are three such components. It is only necessary to assume the existence of particles associated with this simplest representation. These particles are quarks. The quark composition of mesons and baryons was deduced from the fact that meson supermultiplets contain, as a rule, 8 particles, and baryons - 8 and 10 particles. This pattern is easily reproduced if we assume that mesons are composed of quarks q and an antiquark - symbolically: , and baryons of three quarks - symbolically: IN = (qqq). Due to the properties of the group S.U.(3) 9 mesons are divided into supermultiplets of 1 and 8 particles, and 27 baryons are divided into supermultiplets containing 1, 10 and twice 8 particles, which explains the observed separation of octets and decuplets.
The addition of a fourth quark (and, if necessary, new additional quarks) to the scheme is carried out while maintaining the basic assumption of the quark model about the structure of hadrons:
B = (qqq).
All experimental data are in good agreement with the given quark composition of hadrons. There are apparently only small deviations from this structure, which do not significantly affect the properties of hadrons.
The indicated structure of hadrons and mathematical properties of quarks, as objects associated with a certain (simplest) representation of the group S.U.(4), lead to the following. quantum numbers of quarks (Table 2). The unusual - fractional - values of the electric charge are noteworthy. Q, and B, S And Y, not found in any of the observed electron particles. With index a for each type of quark q i (i = 1, 2, 3, 4) a special characteristic of quarks is associated - “color”, which is not present in the studied hadrons. Index a takes values 1, 2, 3, i.e., each type of quark q i presented in three varieties q i a (N.N. Bogolyubov and co-workers, 1965; American physicists I. Nambu and M. Khan, 1965; Japanese physicist I. Miyamoto, 1965). The quantum numbers of each type of quark do not change when the “color” changes and therefore table. 2 applies to quarks of any “color”.
The whole variety of hadrons arises due to various combinations R -, P-, g- and With-quarks forming bound states. Ordinary hadrons correspond to bound states constructed only from R- And n-quarks [for mesons with the possible participation of combinations and ]. Presence in a bound state along with R- And n-quarks of one g- or With-quark means that the corresponding hadron is strange ( S= -1) or charmed ( Ch =+ 1). A baryon can contain two and three g-quarks (respectively With-quark), i.e., double and triple strange (charm) baryons are possible. Combinations of different numbers of g- and With- quarks (especially in baryons), which correspond to “hybrid” forms of hadrons (“strange charm”). Obviously, the larger the g- or With-quarks contains a hadron, the heavier it is. If we compare the ground (non-excited) states of hadrons, this is exactly the picture that is observed (see Table 1, as well as Tables 3 and 5).
Since the spin of quarks is equal to 1/2, the above quark structure of hadrons results in an integer spin for mesons and a half-integer spin for baryons, in full accordance with experiment. Moreover, in states corresponding to the orbital momentum l= 0, in particular in the ground states, the spin of mesons should be equal to 0 or 1 (for antiparallel ґ¯ and parallel ґґ orientation of quark spins), and the spin of baryons should be 1/2 or 3/2 (for spin configurations ¯ґґ and ґґґ) . Taking into account that the internal parity of the quark-antiquark system is negative, the values JP for mesons at l= 0 are equal to 0 - and 1 - , for baryons - 1 / 2 + and 3 / 2 + . These are the values JP observed in hadrons having the smallest mass at given values I And Y(see Table 1).
Since indexes i, k, l in the structural formulas the values run through 1, 2, 3, 4, the number of mesons Mik with a given spin should be equal to 16. For baryons Bikl the maximum possible number of states for a given spin (64) is not realized, since by virtue of the Pauli principle, for a given total spin, only three-quark states are allowed that have a well-defined symmetry with respect to permutations of indices i, k, 1, namely: fully symmetric for spin 3/2 and mixed symmetry for spin 1/2. This condition is l = 0 selects 20 baryon states for spin 3/2 and 20 for spin 1/2.
A more detailed examination shows that the value of the quark composition and symmetry properties of the quark system makes it possible to determine all the basic quantum numbers of the hadron ( J, P, B, Q, I, Y, Ch), excluding mass; determining the mass requires knowledge of the dynamics of the interaction of quarks and the mass of quarks, which is not yet available.
Correctly conveying the specifics of hadrons with the lowest masses and spins at given values Y And Ch, The quark model also naturally explains the overall large number of hadrons and the predominance of resonances among them. The large number of hadrons is a reflection of their complex structure and the possibility of the existence of various excited states of quark systems. It is possible that the number of such excited states is unlimited. All excited states of quark systems are unstable with respect to rapid transitions due to strong interactions into underlying states. They form the bulk of the resonances. A small fraction of resonances also consists of quark systems with parallel spin orientations (with the exception of W -). Quark configurations with antiparallel spin orientation, related to the basic. states, form quasi-stable hadrons and a stable proton.
Excitations of quark systems occur both due to changes in the rotational motion of quarks (orbital excitations) and due to changes in their spaces. location (radial excitations). In the first case, an increase in the mass of the system is accompanied by a change in the total spin J and parity R system, in the second case the increase in mass occurs without change J P . For example, mesons with JP= 2 + are the first orbital excitation ( l = 1) mesons with J P = 1 - . The correspondence of 2 + mesons and 1 - mesons of identical quark structures is clearly seen in the example of many pairs of particles:
Mesons r" and y" are examples of radial excitations of r- and y-mesons, respectively (see.
Orbital and radial excitations generate sequences of resonances corresponding to the same initial quark structure. The lack of reliable information about the interaction of quarks does not yet allow us to make quantitative calculations of excitation spectra and draw any conclusions about the possible number of such excited states. When formulating the quark model, quarks were considered as hypothetical structural elements that open up the possibility of a very convenient description of hadrons. Subsequently, experiments were carried out that allow us to talk about quarks as real material formations inside hadrons. The first were experiments on the scattering of electrons by nucleons at very large angles. These experiments (1968), reminiscent of Rutherford's classical experiments on the scattering of alpha particles on atoms, revealed the presence of point charged formations inside the nucleon. Comparison of the data from these experiments with similar data on neutrino scattering on nucleons (1973-75) made it possible to draw a conclusion about the average squared value of the electric charge of these point formations. The result turned out to be surprisingly close to the value 1 / 2 [(2 / 3 e) 2 +(1 / 3 e) 2 ]. The study of the process of hadron production during the annihilation of an electron and a positron, which supposedly goes through the sequence of processes: ® hadrons, indicated the presence of two groups of hadrons genetically associated with each of the resulting quarks, and made it possible to determine the spin of the quarks. It turned out to be equal to 1/2. The total number of hadrons born in this process also indicates that quarks of three varieties appear in the intermediate state, i.e., quarks are three-colored.
Thus, the quantum numbers of quarks, introduced on the basis of theoretical considerations, have been confirmed in a number of experiments. Quarks are gradually acquiring the status of new electron particles. If further research confirms this conclusion, then quarks are serious contenders for the role of true electron particles for the hadronic form of matter. Up to lengths ~ 10 -15 cm quarks act as structureless point formations. The number of known types of quarks is small. In the future, it may, of course, change: one cannot guarantee that at higher energies hadrons with new quantum numbers, owing their existence to new types of quarks, will not be discovered. Detection Y-mesons confirms this point of view. But it is quite possible that the increase in the number of quarks will be small, that general principles impose limits on the total number of quarks, although these limits are not yet known. The structurelessness of quarks also perhaps reflects only the achieved level of research into these material formations. However, a number of specific features of quarks give some reason to assume that quarks are particles that complete the chain of structural components of matter.
Quarks differ from all other electron particles in that they have not yet been observed in a free state, although there is evidence of their existence in a bound state. One of the reasons for the non-observation of quarks may be their very large mass, which prevents their production at the energies of modern accelerators. It is possible, however, that quarks fundamentally, due to the specific nature of their interaction, cannot be in a free state. There are theoretical and experimental arguments in favor of the fact that the forces acting between quarks do not weaken with distance. This means that infinitely more energy is required to separate quarks from each other, or, otherwise, the emergence of quarks in a free state is impossible. The inability to isolate quarks in a free state makes them a completely new type of structural units of matter. It is unclear, for example, whether it is possible to raise the question of the constituent parts of quarks if the quarks themselves cannot be observed in a free state. It is possible that under these conditions, parts of the quarks do not physically manifest themselves at all, and therefore the quarks act as the last stage in the fragmentation of hadronic matter.
Elementary particles and quantum field theory.
To describe the properties and interactions of electron particles in modern theory, the concept of physics is essential. field, which is assigned to each particle. A field is a specific form of matter; it is described by a function specified at all points ( X)space-time and possessing certain transformation properties in relation to transformations of the Lorentz group (scalar, spinor, vector, etc.) and groups of “internal” symmetries (isotopic scalar, isotopic spinor, etc.). An electromagnetic field with the properties of a four-dimensional vector And m (x) (m = 1, 2, 3, 4) is historically the first example of a physical field. The fields that are compared with E. particles are of a quantum nature, that is, their energy and momentum are composed of many parts. portions - quanta, and the energy E k and the momentum p k of the quantum are related by the relation of the special theory of relativity: E k 2 = p k 2 c 2 + m 2 c 2 . Each such quantum is an electron particle with a given energy E k , momentum p k and mass m. The quanta of the electromagnetic field are photons, the quanta of other fields correspond to all other known electron particles. The field, therefore, is a physical reflection of the existence of an infinite collections of particles - quanta. The special mathematical apparatus of quantum field theory makes it possible to describe the birth and destruction of a particle at each point x.
The transformation properties of the field determine all quantum numbers of E. particles. The transformation properties in relation to space-time transformations (the Lorentz group) determine the spin of particles. Thus, a scalar corresponds to spin 0, a spinor - spin 1/2, a vector - spin 1, etc. The existence of such quantum numbers as L, B, 1, Y, Ch and for quarks and gluons "color" follows from the transformation properties of fields in relation to transformations of “internal spaces” (“charge space”, “isotopic space”, “unitary space”, etc.). The existence of “color” in quarks, in particular, is associated with a special “colored” unitary space. The introduction of “internal spaces” in the theoretical apparatus is still a purely formal device, which, however, can serve as an indication that the dimension of physical space-time, reflected in the properties of the E. Ch., is actually greater than four - the dimension of space-time characteristic of all macroscopic physical processes. The mass of an electron is not directly related to the transformation properties of fields; this is their additional characteristic.
To describe the processes occurring with electron particles, it is necessary to know how various physical fields are related to each other, that is, to know the dynamics of the fields. In the modern apparatus of quantum field theory, information about the dynamics of fields is contained in a special quantity expressed through fields - the Lagrangian (more precisely, the Lagrangian density) L. Knowledge of L allows, in principle, to calculate the probabilities of transitions from one set of particles to another under the influence of various interactions. These probabilities are given by the so-called. scattering matrix (W. Heisenberg, 1943), expressed through L. The Lagrangian L consists of the Lagrangian L, which describes the behavior of free fields, and the interaction Lagrangian, L, constructed from the fields of different particles and reflecting the possibility of their mutual transformations. Knowledge of Lz is decisive for describing processes with E. h.
The form of L3 is uniquely determined by the transformation properties of the fields of the relative Lorentz group and the requirement of invariance with respect to this group (relativistic invariance). For a long time, however, the criteria for finding L3 were not known (with the exception of electromagnetic interactions), and information about the interactions of electromagnetic particles obtained from experiment, in most cases did not allow a reliable choice between different possibilities. Under these conditions, a phenomenological approach to describing interactions has become widespread, based either on the selection of the simplest forms of L ins, leading to observable processes, or on the direct study of the characteristic properties of the elements of the scattering matrix. Along this path, significant success has been achieved in describing processes with electron particles for various selected energy regions. However, many parameters of the theory were borrowed from experiment, and the approach itself could not claim universality.
In the period 50-70s. Significant progress has been made in understanding the structure of L3, which has made it possible to significantly refine its form for strong and weak interactions. A decisive role in this progress was played by the clarification of the close connection between the symmetry properties of the interactions of the electron particles and the shape of the Lv.
The symmetry of the interactions of electron particles is reflected in the existence of laws of conservation of certain physical quantities and, consequently, in the conservation of the quantum numbers of electron particles associated with them (see Conservation laws). Exact symmetry, which occurs for all classes of interactions, corresponds to the presence of exact quantum numbers in electrons; approximate symmetry, characteristic only for certain classes of interactions (strong, electromagnetic), leads to inaccurate quantum numbers. The difference between classes of interactions noted above in relation to the conservation of quantum numbers of electrons reflects differences in the properties of their symmetry.
Known form L vz el. m. for electromagnetic interactions is a consequence of the existence of an obvious symmetry of the Lagrangian L with respect to the multiplication of the complex fields j of charged particles included in it in combinations of type j*j (here * means complex conjugation) by the factor e ia, where a is an arbitrary real number. This symmetry, on the one hand, gives rise to the law of conservation of electric charge, on the other hand, if we require the fulfillment of symmetry under the condition that a arbitrarily depends on the point x of space-time, it unambiguously leads to the Lagrangian of interaction:
L up el. m. = j m el. m. (x) A m (x) (1)
where j m el. m. - four-dimensional electromagnetic current (see Electromagnetic interactions). As it turns out, this result has general significance. In all cases when the interactions exhibit “internal” symmetry, i.e. the Lagrangian is invariant under transformations of the “internal space”, and the corresponding quantum numbers arise in E. numbers, it should be required that invariance take place for any dependence of the transformation parameters on the point x (so-called local gauge invariance; Yang Zhen-ning, American physicist R. Mills, 1954). Physically, this requirement is due to the fact that interaction cannot be instantly transmitted from point to point. This condition is satisfied when among the fields included in the Lagrangian there are vector fields (analogs of A m (x)), which change during transformations of “internal” symmetry and interact with the fields of particles in a very specific way, namely:
L in = å r=1 n j m r (x) V m r (x), (2)
where j m r (x) are currents composed of particle fields, V m r (x) are vector fields, often called gauge fields. Thus, the requirement of locality of “internal” symmetry fixes the form of L and identifies vector fields as universal carriers of interactions. The properties of vector fields and their number "n" are determined by the properties of the "internal" symmetry group. If the symmetry is exact, then the mass of the field quantum V m r is equal to 0. For approximate symmetry, the mass of the vector field quantum is nonzero. The type of current j m r is determined by the fields of particles with non-zero quantum numbers associated with the “internal” symmetry group.
Based on the principles outlined above, it turned out to be possible to approach the question of the interaction of quarks in a nucleon. Experiments on the scattering of neutrinos and antineutrinos by nucleons have shown that the momentum of the nucleon is only partially (about 50%) transferred by quarks, and the rest of it is transferred by another type of matter that does not interact with neutrinos. Presumably this part of matter consists of particles that are exchanged between quarks and due to which they are held in the nucleon. These particles are called "gluons" (from the English glue - glue). From the above point of view on interactions, it is natural to consider these particles to be vector particles. In modern theory, their existence is associated with symmetry, which determines the appearance of “color” in quarks. If this symmetry is exact (color SU (3) symmetry), then gluons are massless particles and their number is eight (American physicist I. Nambu, 1966). The interaction of quarks with gluons is given by L vz with structure (2), where the current j m r is composed of quark fields. There is also reason to assume that the interaction of quarks, caused by the exchange of massless gluons, leads to forces between quarks that do not decrease with distance, but this has not been rigorously proven.
In principle, knowledge of the interaction between quarks could be the basis for describing the interaction of all hadrons with each other, i.e., all strong interactions. This direction in hadron physics is developing rapidly.
The use of the principle of the determining role of symmetry (including approximate) in the formation of the interaction structure also made it possible to advance in understanding the nature of the Lagrangian of weak interactions. At the same time, a deep internal connection between weak and electromagnetic interactions was revealed. In this approach, the presence of pairs of leptons with the same lepton charge: e - , v e and m - , v m , but with different masses and electric charges is regarded not as random, but as reflecting the existence of broken symmetry of the isotonic type (group SU (2)). Application of the principle of locality to this “internal” symmetry leads to the characteristic Lagrangian (2), in which terms responsible for electromagnetic and weak interactions simultaneously arise (American physicist S. Weinberg, 1967; A. Salam, 1968):
L air = j m el. m. + A m + j m sl. h. W m + + j m sl. h. W m - + j m sl. n. Z m 0 (3)
Here j m sl. h. , j m sl. n. - charged and neutral currents of weak interactions, built from the fields of leptons, W m +, W m -, Z m 0 - fields of massive (due to symmetry breaking) vector particles, which in this scheme are carriers of weak interactions (the so-called intermediate bosons), A m - photon field. The idea of the existence of a charged intermediate boson was put forward a long time ago (H. Yukawa, 1935). It is important, however, that in this model of a unified theory of electron magnetic and weak interactions, a charged intermediate boson appears on an equal basis with a photon and a neutral intermediate boson. Processes of weak interactions caused by neutral currents were discovered in 1973, which confirms the correctness of the approach just outlined to the formulation of the dynamics of weak interactions. Other options for writing the Lagrangian L with a large number of neutral and charged intermediate bosons are also possible; Experimental data are not yet sufficient for the final choice of the Lagrangian.
Intermediate bosons have not yet been discovered experimentally. From the available data, the masses W ± and Z 0 for the Weinberg-Salam model are estimated to be approximately 60 and 80 GeV.
The electromagnetic and weak interactions of quarks can be described within a model similar to the Weinberg-Salam model. Consideration of electromagnetic and weak hadron interactions on this basis gives good agreement with the observed data. A common problem in constructing such models is the still unknown total number of quarks and leptons, which does not allow determining the type of initial symmetry and the nature of its violation. Therefore, further experimental studies are very important.
The single origin of electromagnetic and weak interactions means that in theory the weak interaction constant disappears as an independent parameter. The only constant remains the electric charge e. The suppression of weak processes at low energies is explained by the large mass of intermediate bosons. At energies in the center of mass system comparable to the masses of intermediate bosons, the effects of electromagnetic and weak interactions should be of the same order. The latter, however, will differ in the non-conservation of a number of quantum numbers (P, Y, Ch, etc.).
There are attempts to consider on a unified basis not only electromagnetic and weak interactions, but also strong interactions. The starting point for such attempts is the assumption of the same nature of all types of interactions of electron particles (without gravitational interaction). The observed strong differences between interactions are considered to be due to significant symmetry breaking. These attempts have not yet been sufficiently developed and face serious difficulties, in particular in explaining the differences in the properties of quarks and leptons.
The development of a method for obtaining the Lagrangian of interaction, based on the use of symmetry properties, was an important step on the path leading to the dynamic theory of elementary particles. There is every reason to think that gauge field theories will be an essential component of further theoretical constructions.
Conclusion
Some general problems of the theory of elementary particles. The latest development of the physics of electron particles clearly distinguishes from all electron particles a group of particles that significantly determine the specifics of the processes of the microworld. These particles are possible candidates for the role of true electron particles. These include: particles with spin 1/2 - leptons and quarks, as well as particles with spin 1 - gluons, photons, massive intermediate bosons, which carry out different types of interactions of particles with spin 12 . This group most likely should also include a particle with spin 2 - the graviton; a quantum of the gravitational field that connects all electron particles. In this scheme, many questions, however, require further research. It is not known what the total number of leptons, quarks and various vector (with J = 1) particles is and whether there are physical principles that determine this number. The reasons for the division of particles with spin 1/2 into 2 different groups: leptons and quarks are unclear. The origin of the internal quantum numbers of leptons and quarks (L, B, 1, Y, Ch) and such characteristics of quarks and gluons as “color” is unclear. What degrees of freedom are associated with internal quantum numbers? Only such characteristics of an electron particle as J and P are associated with ordinary four-dimensional space-time. What mechanism determines the masses of a true electron particle? What is the reason for the presence of different classes of interactions in electrons with different symmetry properties? These and other questions will have to be resolved by the future theory of E. ch.
The description of the interactions of electron particles, as noted, is associated with gauge field theories. These theories have a developed mathematical apparatus that allows calculations of processes with electron particles (at least in principle) at the same level of rigor as in quantum electrodynamics. But in their present form, gauge field theories have one serious drawback, common with quantum electrodynamics - in them, in the process of calculations, meaningless infinitely large expressions appear. Using a special technique for redefining observable quantities (mass and charge) - renormalization - it is possible to eliminate infinities from the final results of calculations. In the most well-studied electrodynamics, this does not yet affect the agreement of theoretical predictions with experiment. However, the renormalization procedure is a purely formal bypass of the difficulty existing in the theoretical apparatus, which at some level of accuracy should affect the degree of agreement between calculations and measurements.
The appearance of infinities in calculations is due to the fact that in the Lagrangians of interactions the fields of different particles are referred to one point x, i.e. it is assumed that the particles are pointlike, and four-dimensional space-time remains flat down to the smallest distances. In reality, these assumptions are apparently incorrect for several reasons: a) true E. elements, most likely, are material objects of finite extent; b) the properties of space-time in the small (on the scale determined by the so-called fundamental length) are most likely radically different from its macroscopic properties; c) at the smallest distances (~ 10 -33 cm), a change in the geometric properties of space-time due to gravity affects. Perhaps these reasons are closely related. Thus, it is taking into account gravity that most naturally leads to the size of a true E. particle of the order of 10 -33 cm, and the foundation, length l 0 can be associated with the gravitational constant f: "10 -33 cm. Any of these reasons should lead to a modification of the theory and the elimination of infinities, although the practical implementation of this modification can be quite complex.
It seems very interesting to take into account the influence of gravity at short distances. Gravitational interaction can not only eliminate divergences in quantum field theory, but also determine the very existence of primary matter (M. A. Markov, 1966). If the density of a true E.H. substance is sufficiently large, gravitational attraction can be the factor that determines the stable existence of these material formations. The dimensions of such formations should be ~10 -33 cm. In most experiments they will behave like point objects, their gravitational interaction will be negligible and will appear only at the smallest distances, in the region where the geometry of space changes significantly.
Thus, the emerging trend towards the simultaneous consideration of various classes of interactions of electron particles should most likely be logically completed by including gravitational interaction in the general scheme. It is on the basis of simultaneous consideration of all types of interactions that it is most likely to expect the creation of a future theory of electron particles.
Bibliography
1) Markov M.A. About the nature of matter. M., 1976
2) Gaziorovich S. Physics of elementary particles, trans. from English, M. 1969
3) Kokkede Ya., Theory of quarks, trans. from English, M., 1971
4) I., Ioffe B.L., Okun L.B., New elementary particles, "Advances in Physical Sciences", 1975, v. 117, v. 2, p. 227
5) Bogolyubov N.N., Shirkov D.V., Introduction to the theory of quantized fields, 3rd ed., M., 1976;
6) News of fundamental physics, trans. from English, M., 1977, pp. 120-240 .
Federal State Educational Institution
higher professional education
"SOUTH FEDERAL UNIVERSITY"
Faculty of Economics
Elementary particles.
Their classification and main properties.
Performed
1st year student, 11th group
Bublikova Ekaterina
Rostov-on-Don – 2009
Introduction. The world of elementary particles.
Gravity.
Electromagnetic interaction.
Weak interaction.
Strong interaction.
Fundamental physical interactions.
Characteristics of subatomic particles.
History of the discovery of elementary particles.
Classification of elementary particles.
2.5. Quark theory.
2.6. Particles are carriers of interactions.
3. Theories of elementary particles.
3.1. Quantum electrodynamics.
3.2. Theory of electroweak interaction.
3.3. Quantum chromodynamics.
3.4. On the way to... The Great Unification.
List of used literature.
The world of elementary particles.
In the middle and second half of the twentieth century, truly amazing results were obtained in those branches of physics that study the fundamental structure of matter. First of all, this manifested itself in the discovery of a whole host of new subatomic particles. They are usually called elementary particles, but not all of them are truly elementary. Elementary particles in the precise meaning of this term are primary, further indecomposable particles, of which all matter is supposed to consist, but many of them, in turn, consist of even more elementary particles.
The world of subatomic particles is truly diverse. Currently, more than 350 elementary particles are known. These include protons and neutrons that make up atomic nuclei, as well as electrons orbiting the nuclei. But there are also particles that are practically never found in the matter around us. If the average lifetime of a neutron located outside the atomic nucleus is 15 minutes, then the lifetime of such short-lived particles is extremely short, it amounts to the smallest fractions of a second. After this extremely short time, they disintegrate into ordinary particles. There are an amazing number of such unstable short-lived particles: several hundred of them are already known. However, it cannot be considered that unstable elementary particles “consist” of stable ones, if only because the same particle can decay in several ways into different elementary particles.
Each elementary particle (with the exception of absolutely neutral particles) has its own antiparticle.
Physicists discovered the existence of elementary particles when studying nuclear processes, so until the middle of the 20th century, elementary particle physics was a branch of nuclear physics. Currently, elementary particle physics and nuclear physics are close but independent branches of physics, united by the commonality of many problems considered and the research methods used. The main task of elementary particle physics is the study of the nature, properties and mutual transformations of elementary particles.
In the 1960s and 1970s, physicists were completely baffled by the number, variety, and strangeness of the newly discovered subatomic particles. There seemed to be no end to them. It is completely unclear why there are so many particles. Are these elementary particles chaotic and random fragments of matter? Or perhaps they hold the key to understanding the structure of the Universe? The development of physics in subsequent decades showed that there is no doubt about the existence of such a structure. At the end of the twentieth century, physics begins to understand the significance of each of the elementary particles.
The world of subatomic particles is characterized by a deep and rational order. This order is based on fundamental physical interactions.
1. Fundamental physical interactions.
In his daily life, a person is faced with many forces acting on his body. Here is the force of the wind or the oncoming flow of water, air pressure, a powerful release of explosive chemicals, human muscular strength, the weight of heavy objects, the pressure of light quanta, the attraction and repulsion of electrical charges, seismic waves that sometimes cause catastrophic destruction, and volcanic eruptions that led to the death of civilization, etc. Some forces act directly upon contact with the body, others, for example, gravity, act at a distance, through space. But, as it turned out as a result of the development of theoretical natural science, despite such great diversity, all forces acting in nature can be reduced to just four fundamental interactions: gravitational, electromagnetic, weak and strong. It is these interactions that are ultimately responsible for all changes in the world; they are the source of all transformations of bodies and processes. Elementary particles are divided into groups according to their abilities for various types of fundamental interactions. The study of the properties of fundamental interactions is the main task of modern physics.
1.1. Gravity.
In the history of physics, gravity (gravity) became the first of the four fundamental interactions to be the subject of scientific research. After its appearance in the 17th century. Newton's theory of gravity - the law of universal gravitation - managed for the first time to realize the true role of gravity as a force of nature. Gravity has a number of features that distinguish it from other fundamental interactions.
The most surprising feature of gravity is its low intensity. The magnitude of the gravitational interaction between the components of a hydrogen atom is 10n, where n = -39, based on the force of interaction of electric charges. It may seem surprising that we feel gravity at all, since it is so weak. How can she become the dominant force in the Universe?
It's all about the second amazing feature of gravity - its universality. Nothing in the Universe is free from gravity. Each particle experiences the action of gravity and is itself a source of gravity. Since every particle of matter exerts a gravitational pull, gravity increases as larger clumps of matter form. We feel gravity in everyday life because all the atoms of the Earth work together to attract us. And although the effect of the gravitational attraction of one atom is negligible, the resulting force of attraction from all atoms can be significant.
Gravity - long-range force of nature. This means that, although the intensity of gravitational interaction decreases with distance, it spreads in space and can affect bodies very distant from the source. On an astronomical scale, gravitational interactions tend to play a major role. Thanks to long-range action, gravity prevents the Universe from falling apart: it holds planets in orbits, stars in galaxies, galaxies in clusters, clusters in the Metagalaxy.
The gravitational force acting between particles is always an attractive force: it tends to bring the particles closer together. Gravitational repulsion has never been observed before (although in the traditions of quasi-scientific mythology there is a whole field called levitation - the search for the "facts" of antigravity). Since the energy stored in any particle is always positive and gives it positive mass, particles under the influence of gravity always tend to get closer.
What is gravity, a certain field or a manifestation of the curvature of space-time - there is still no clear answer to this question. There are different opinions and concepts of physicists on this matter.
1.2. Electromagnetic interaction.
Electrical forces are much larger than gravitational forces. Unlike the weak gravitational interaction, the electrical forces acting between bodies of normal size can be easily observed. Electromagnetism has been known to people since time immemorial (auroras, lightning flashes, etc.).
For a long time, electrical and magnetic processes were studied independently of each other. A decisive step in the knowledge of electromagnetism was made in the middle of the 19th century by J. C. Maxwell, who united electricity and magnetism in a unified theory of electromagnetism - the first unified field theory.
The existence of the electron was firmly established in the 90s of the last century. It is now known that the electric charge of any particle of matter is always a multiple of the fundamental unit of charge - a kind of “atom” of charge. Why this is so is an extremely interesting question. However, not all material particles are carriers of electric charge. For example, the photon and neutrino are electrically neutral. In this respect, electricity differs from gravity. All material particles create a gravitational field, while only charged particles are associated with an electromagnetic field. The carrier of electromagnetic interaction between charged particles is the electromagnetic field, or field quanta - photons.
Like electric charges, like magnetic poles repel, and opposite ones attract. However, unlike electric charges, magnetic poles do not occur individually, but only in pairs - a north pole and a south pole. Since ancient times, attempts have been known to obtain, by dividing a magnet, only one isolated magnetic pole - a monopole. But they all ended in failure. Perhaps the existence of isolated magnetic poles in nature is excluded? There is no definite answer to this question yet. Some theoretical concepts allow for the possibility of a monopole.
Like electrical and gravitational interactions, the interaction of magnetic poles obeys the inverse square law. Consequently, electric and magnetic forces are “long-range”, and their effect is felt at large distances from the source. Thus, the Earth's magnetic field extends far into outer space. The Sun's powerful magnetic field fills the entire Solar System. There are also galactic magnetic fields.
Electromagnetic interaction determines the structure of atoms and is responsible for the vast majority of physical and chemical phenomena and processes. Electromagnetic interaction also leads to the emission of electromagnetic waves.
1.3. Weak interaction.
Physics has moved slowly towards identifying the existence of the weak interaction. The weak force is responsible for particle decays, and its manifestation was therefore confronted with the discovery of radioactivity and the study of beta decay.
Beta decay has revealed an extremely strange feature. Research led to the conclusion that this decay violates one of the fundamental laws of physics - the law of conservation of energy. It seemed that in this decay part of the energy disappeared somewhere. In order to “save” the law of conservation of energy, W. Pauli suggested that, together with the electron, during beta decay, another particle flies out. It is neutral and has an unusually high penetrating ability, as a result of which it could not be observed. E. Fermi called the invisible particle "neutrino".
Neutrino (Italian neutrino, diminutive of neutrone - neutron), a stable uncharged elementary particle with spin 1/2 and possibly zero mass. Neutrinos are classified as leptons. They participate only in weak and gravitational interactions and therefore interact extremely weakly with matter. There are electron neutrinos, always paired with an electron or positron, muon neutrinos, paired with a muon, and tau neutrinos, associated with a heavy lepton. Each type of neutrino has its own antiparticle, which differs from neutrinos in the sign of the corresponding lepton charge and helicity: neutrinos have left-handed helicity (spin is directed against the motion of the particle), and antineutrinos have right-handed helicity (spin is in the direction of motion).
But the prediction and detection of neutrinos is only the beginning of the problem, its formulation. It was necessary to explain the nature of neutrinos, but there remained a lot of mystery here. The fact is that both electrons and neutrinos were emitted by unstable nuclei. But it was irrefutably proven that there are no such particles inside nuclei. How did they arise? It was suggested that electrons and neutrinos do not exist in the nucleus in a “ready form”, but are somehow formed from the energy of the radioactive nucleus. Further research showed that the neutrons included in the nucleus, left to their own devices, after a few minutes decay into a proton, electron and neutrino, i.e. instead of one particle, three new ones appear. The analysis led to the conclusion that known forces could not cause such a disintegration. It was apparently generated by some other, unknown force. Research has shown that this force corresponds to some weak interaction.
It is much weaker than electromagnetic, although stronger than gravitational. It spreads over very short distances. The radius of weak interaction is very small and is about 2*10^(-16) cm. The weak interaction stops at a minimum distance from the source and therefore cannot affect macroscopic objects, but is limited to individual subatomic particles. All elementary particles except the photon participate in weak interaction. It determines most of the decays of elementary particles, the interaction of neutrinos with matter, etc. The weak interaction is characterized by a violation of parity, strangeness, and “charm.” A unified theory of weak and electromagnetic interaction was created in the late 60s by S. Weinberg, S. Glashow and A. Salam. It describes the interactions of quarks and leptons, carried out through the exchange of four particles: massless photons (electromagnetic interaction) and heavy intermediate vector bosons - particles W+, W- and Z°, which are carriers of the weak interaction (experimentally discovered in 1983). This single interaction came to be called electroweak. Since Maxwell's theory of the electromagnetic field, the creation of this theory was the largest step towards the unity of physics.
1.4. Strong interaction.
The last in the series of fundamental interactions is the strong interaction, which is a source of enormous energy. The most typical example of energy released by the strong force is our Sun. In the depths of the Sun and stars, starting from a certain time, thermonuclear reactions caused by strong interaction continuously occur. But man has also learned to release strong interactions: a hydrogen bomb has been created, technologies for controlled thermonuclear reactions have been designed and improved.
Physics came to the idea of the existence of strong interaction during the study of the structure of the atomic nucleus. Some force must hold the protons in the nucleus, preventing them from scattering under the influence of electrostatic repulsion. Gravity is too weak for this; Obviously, some new interaction is needed, moreover, stronger than electromagnetic. It was subsequently discovered. It turned out that although the strong interaction significantly exceeds all other fundamental interactions in its magnitude, it is not felt outside the nucleus. The radius of action of the new force turned out to be very small. The strong force drops off sharply at a distance from a proton or neutron greater than about 10^(-15) m.
In addition, it turned out that not all particles experience strong interactions. It is experienced by protons and neutrons, but electrons, neutrinos and photons are not subject to it. This means that only hadrons participate in the strong interaction.
The strong interaction exceeds the electromagnetic interaction by about 100 times. The theoretical explanation of the nature of the strong interaction has been difficult to develop. A breakthrough occurred in the early 60s, when the quark model was proposed. In this theory, neutrons and protons are considered not as elementary particles, but as composite systems built from quarks. The modern theory of the strong interaction is quantum chromodynamics.
Thus, in fundamental physical interactions the difference between long-range and short-range forces is clearly visible. On the one hand, there are interactions of unlimited range (gravity, electromagnetism), and on the other, interactions of short range (strong and weak). The world of physical elements as a whole unfolds in the unity of these two polarities and is the embodiment of the unity of the extremely small and the extremely large - short-range action in the microworld and long-range action throughout the Universe.
1.5. The problem of the unity of physics.
Knowledge is a generalization of reality, and therefore the goal of science is the search for unity in nature, linking disparate fragments of knowledge into a single picture. In order to create a unified system, it is necessary to open a connecting link between various branches of knowledge, some fundamental relationship. The search for such connections and relationships is one of the main tasks of scientific research. Whenever it is possible to establish such new connections, the understanding of the surrounding world deepens significantly, new ways of knowing are formed that point the way to previously unknown phenomena.
Establishing deep connections between different areas of nature is both a synthesis of knowledge and a method that guides scientific research along new, untrodden roads. Newton's discovery of the connection between the attraction of bodies under terrestrial conditions and the movement of planets marked the birth of classical mechanics, on the basis of which the technological basis of modern civilization is built. The establishment of a connection between the thermodynamic properties of gas and the chaotic movement of molecules put the atomic-molecular theory of matter on a solid basis. In the middle of the last century, Maxwell created a unified electromagnetic theory that covered both electrical and magnetic phenomena. Then, in the 20s of the twentieth century, Einstein made attempts to combine electromagnetism and gravity in a single theory.
But by the middle of the twentieth century, the situation in physics had changed radically: two new fundamental interactions were discovered - strong and weak, i.e. when creating a unified physics, one has to take into account not two, but four fundamental interactions. This somewhat cooled the ardor of those who hoped for a quick solution to this problem. But the idea itself was not seriously questioned, and the enthusiasm for the idea of a single description did not go away.
There is a point of view that all four (or at least three) interactions represent phenomena of the same nature and their unified theoretical description must be found. The prospect of creating a unified theory of the world of physical elements based on a single fundamental interaction remains very attractive. This is the main dream of physicists of the twentieth century. But for a long time it remained only a dream, and a very vague one.
However, in the second half of the twentieth century, the prerequisites for the realization of this dream and the confidence that this is not a matter of the distant future appeared. It looks like it could soon become a reality. A decisive step towards a unified theory was made in the 60-70s with the creation first of the theory of quarks, and then of the theory of electroweak interaction. There is reason to believe that we are on the threshold of a more powerful and deeper unification than ever before. There is a growing belief among physicists that the contours of a unified theory of all fundamental interactions - the Grand Unification - are beginning to emerge.
2. Classification of elementary particles.
2.1. Characteristics of subatomic particles.
The discovery at the turn of the nineteenth and twentieth centuries of the smallest carriers of the properties of matter - molecules and atoms - and the establishment of the fact that molecules are built from atoms, for the first time made it possible to describe all known substances as combinations of a finite, albeit large, number of structural components - atoms. Further identification of the presence of constituent atoms - electrons and nuclei, establishment of the complex nature of nuclei, which turned out to be built from only two types of particles (protons and neutrons) , significantly reduced the number of discrete elements that form the properties of matter. It is impossible to say with certainty that particles that are elementary in the sense of the above definition exist. Protons and neutrons, for example, which for a long time were considered elementary, as it turned out, have a complex structure. The possibility cannot be ruled out that the sequence of structural components of matter is fundamentally infinite. It may also turn out that the statement “consists of...” at some stage of the study of matter will turn out to be devoid of content. In this case, the definition of “elementary” given above will have to be abandoned. The existence of elementary (subatomic) particles is a kind of postulate, and testing its validity is one of the most important tasks of physics.
The characteristics of subatomic particles are mass, electric charge, spin (intrinsic angular momentum), particle lifetime, magnetic moment, spatial parity, charge parity, lepton charge, baryon charge, strangeness, “charm”, etc.
When they talk about the mass of a particle, they mean its rest mass, since this mass does not depend on the state of motion. A particle with zero rest mass moves at the speed of light (photon). No two particles have the same mass. The electron is the lightest particle with a non-zero rest mass. The proton and neutron are almost 2000 times heavier than the electron. And the heaviest known elementary particle (Z - particle) has a mass 200,000 times the mass of an electron.
The electric charge varies over a fairly narrow range and is always a multiple of the fundamental unit of charge - the charge of the electron (-1). Some particles, such as the photon and neutrino, have no charge at all.
An important characteristic of a particle is spin. It has no classical analogue and, of course, indicates the “internal complexity” of a microobject. True, sometimes they try to compare with the concept of spin a model of an object rotating around its axis (the word “spin” itself is translated as “spindle”). This model is visual, but incorrect. In any case, it cannot be taken literally. The term “rotating microobject” found in the literature does not mean the rotation of the microobject, but only the presence of a specific internal angular momentum. In order for this moment to “turn” into the classical angular momentum (and thus the object would actually begin to rotate), it is necessary to require the fulfillment of the condition s >> 1 (much more than one). However, this condition is never met. The spin is also always a multiple of some fundamental unit, which is chosen to be ½. All particles of the same type have the same spin. Typically, particle spins are measured in units of Planck's constant ћ. It can be an integer (0, 1, 2,...) or a half-integer (1/2, 3/2,...). Thus, a proton, neutron and electron have a spin of S, and the spin of a photon is equal to 1. Particles with a spin of 0, 3/2, 2 are known. A particle with a spin of 0 looks the same at any angle of rotation. Particles with spin 1 take the same form after a full rotation of 360°. A particle with spin 1/2 takes on its previous appearance after a rotation of 720°, etc. A particle with spin 2 returns to its previous position after half a turn (180°). Particles with a spin greater than 2 have not been discovered, and perhaps they do not exist at all. Knowing the spin of a microobject allows us to judge the nature of its behavior in a group of its own kind (in other words, it allows us to judge the statistical properties of the microobject). It turns out that, according to their statistical properties, all microobjects in nature are divided into two groups: a group of microobjects with an integer spin and a group of microobjects with a half-integer spin.
Microobjects of the first group are capable of “populating” the same state in an unlimited number, and the more strongly this state is “populated,” the higher the number. Such microobjects are said to obey Bose-Einstein statistics. For short, they are simply called bosons. Microobjects of the second group can “populate” states only one by one. And if the state in question is occupied, then no microobject of this type can get into it. Such micro-objects are said to obey Fermi-Dirac statistics, and for brevity they are called fermions. Of the elementary particles, bosons include photons and mesons, and fermions include leptons (in particular electrons), nucleons, and hyperons.
Particles are also characterized by their lifetime. Based on this criterion, particles are divided into stable and unstable. Stable particles are the electron, proton, photon and neutrino. A neutron is stable when in the nucleus of an atom, but a free neutron decays in about 15 minutes. All other known particles are unstable, their lifetime ranges from a few microseconds to 10n seconds (where n = -23). This means that when this time expires, they spontaneously, without any external influences, disintegrate, turning into other particles. For example, a neutron spontaneously decays into a proton, an electron, and an electron antineutrino. It is impossible to predict exactly when the indicated decay of a particular neutron will occur, because each specific decay event is random. Each unstable elementary particle is characterized by its own lifetime. The shorter the lifetime, the greater the probability of particle decay. Instability is inherent not only in elementary particles, but also in other micro-objects. The phenomenon of radioactivity (spontaneous transformation of isotopes of one chemical element into isotopes of another, accompanied by the emission of particles) shows that atomic nuclei can be unstable. Atoms and molecules in excited states also turn out to be unstable: they spontaneously go into the ground or less excited state.
Instability, determined by probabilistic laws, is, along with the presence of spin, the second highly specific property inherent in microobjects. It can also be considered as an indication of a certain “internal complexity” of a micro-object.
However, instability is a specific, but by no means obligatory, property of a microobject. Along with unstable ones, there are many stable micro-objects: photon, electron, proton, neutrino, stable atomic nuclei, as well as atoms and molecules in the ground state.
Lepton charge (lepton number) is an internal characteristic of leptons. It is designated by the letter L. For leptons it is +1, and for antileptons -1. There are: electronic lepton charge, which is possessed only by electrons, positrons, electron neutrinos and antineutrinos; muonic lepton charge, which is possessed only by muons and muon neutrinos and antineutrinos; lepton charge of heavy leptons and their neutrinos. The algebraic sum of the lepton charge of each type is conserved with very high accuracy across all interactions.
Baryon charge (baryon number) is one of the internal characteristics of baryons. Denoted by the letter B. All baryons have B = +1, and their antiparticles have B = -1 (for other elementary particles B = 0). The algebraic sum of baryon charges included in a system of particles is conserved under all interactions.
Strangeness is an integer (zero, positive or negative) quantum number that characterizes hadrons. The strangeness of particles and antiparticles is opposite in sign. Hadrons with S equal to 0 are called strange. Strangeness is preserved in the strong and electromagnetic interactions, but is violated in the weak interaction.
“Charm” (charm) is a quantum number characterizing hadrons (or quarks). It is preserved in the strong and electromagnetic interactions, but is violated by the weak interaction. Particles with a non-zero charm value are called "charmed" particles.
Magneton is a unit of measurement of magnetic moment in the physics of the atom, atomic nucleus and elementary particles. The magnetic moment, caused by the orbital motion of electrons in an atom and their spin, is measured in Bohr magnetons. The magnetic moment of nucleons and nuclei is measured in nuclear magnetons.
Parity is another characteristic of subatomic particles. Parity is a quantum number that characterizes the symmetry of the wave function of a physical system or an elementary particle under some discrete transformations: if during such a transformation the function does not change sign, then the parity is positive; if it does, then the parity is negative. For absolutely neutral particles (or systems) that are identical to their antiparticles, in addition to spatial parity, one can introduce the concepts of charge parity and combined parity (for other particles, replacing them with antiparticles changes the wave function itself).
Spatial parity is a quantum mechanical characteristic that reflects the symmetry properties of elementary particles or their systems during mirror reflection (spatial inversion). This parity is denoted by the letter P and is conserved in all interactions except weak ones.
Charge parity - the parity of an absolute neutral elementary particle or system, corresponding to the operation of charge conjugation. Charge parity is also conserved in all interactions except weak ones.
Combined parity is the parity of an absolutely neutral particle (or system) relative to the combined inversion. Combined parity is conserved in all interactions, with the exception of decays of the long-lived neutral K meson caused by the weak interaction (the reason for this violation of combined parity has not yet been clarified).
2.2. History of the discovery of elementary particles.
The idea that the world is made of fundamental particles has a long history. For the first time, the idea of the existence of the smallest invisible particles that make up all surrounding objects was expressed 400 years BC by the Greek philosopher Democritus. He called these particles atoms, that is, indivisible particles. Science began to use the idea of atoms only at the beginning of the 19th century, when on this basis it was possible to explain a number of chemical phenomena. In the 30s of the 19th century, in the theory of electrolysis developed by M. Faraday, the concept of an ion appeared and the elementary charge was measured. But from about the middle of the 19th century, experimental facts began to appear that cast doubt on the idea of the indivisibility of atoms. The results of these experiments suggested that atoms have a complex structure and that they contain electrically charged particles. This was confirmed by the French physicist Henri Becquerel, who discovered the phenomenon of radioactivity in 1896.
This was followed by the discovery of the first elementary particle by the English physicist Thomson in 1897. It was the electron that finally acquired the status of a real physical object and became the first known elementary particle in human history. Its mass is approximately 2000 times less than the mass of a hydrogen atom and is equal to:
m = 9.11*10^(-31) kg.
The negative electric charge of an electron is called elementary and is equal to:
e = 0.60*10^(-19) Cl.
Analysis of atomic spectra shows that the electron spin is equal to 1/2, and its magnetic moment is equal to one Bohr magneton. Electrons obey Fermi statistics because they have half-integer spin. This is consistent with experimental data on the structure of atoms and the behavior of electrons in metals. Electrons participate in electromagnetic, weak and gravitational interactions.
The second discovered elementary particle was the proton (from the Greek protos - first). This elementary particle was discovered in 1919 by Rutherford, while studying the products of fission of atomic nuclei of various chemical elements. Literally, a proton is the nucleus of an atom of the lightest isotope of hydrogen - protium. The proton spin is 1/2. A proton has a positive elementary charge +e. Its mass is:
m = 1.67*10^(-27) kg.
or approximately 1836 electron masses. Protons are part of the nuclei of all atoms of chemical elements. After this, in 1911, Rutherford proposed a planetary model of the atom, which helped scientists in further research into the composition of atoms.
In 1932, J. Chadwick discovered the third elementary particle, the neutron (from the Latin neuter - neither one nor the other), which has no electrical charge and has a mass of approximately 1839 times the mass of an electron. The neutron spin is also 1/2.
The conclusion about the existence of a particle of an electromagnetic field - a photon - originates from the work of M. Planck (1900). Assuming that the energy of electromagnetic radiation from an absolutely black body is quantized (i.e., consists of quanta), Planck obtained the correct formula for the radiation spectrum. Developing Planck's idea, A. Einstein (1905) postulated that electromagnetic radiation (light) is actually a flow of individual quanta (photons), and on this basis explained the laws of the photoelectric effect. Direct experimental evidence of the existence of the photon was given by R. Millikan in 1912 - 1915 and A. Compton in 1922.
The discovery of the neutrino, a particle that hardly interacts with matter, dates back to W. Pauli’s theoretical guess in 1930, which, due to the assumption of the birth of such a particle, made it possible to eliminate difficulties with the law of conservation of energy in the beta decay processes of radioactive nuclei. The existence of neutrinos was experimentally confirmed only in 1953 by F. Reines and K. Cowan.
But matter consists of more than just particles. There are also antiparticles - elementary particles that have the same mass, spin, lifetime and some other internal characteristics as their “twins” - particles, but differ from particles in the signs of electric charge and magnetic moment, baryon charge, lepton charge, strangeness and etc. All elementary particles, except absolutely neutral ones, have their own antiparticles.
The first discovered antiparticle was the positron (from the Latin positivus - positive) - a particle with the mass of an electron, but a positive electric charge. This antiparticle was discovered in cosmic rays by American physicist Carl David Anderson in 1932. Interestingly, the existence of the positron was theoretically predicted by the English physicist Paul Dirac almost a year before the experimental discovery. Moreover, Dirac predicted the so-called processes of annihilation (disappearance) and the birth of an electron-positron pair. The annihilation of a pair itself is one of the types of transformations of elementary particles that occurs when a particle collides with an antiparticle. During annihilation, a particle and an antiparticle disappear, turning into other particles, the number and type of which are limited by conservation laws. The reverse process of annihilation is the birth of a couple. The positron itself is stable, but in matter it exists for a very short time due to annihilation with electrons. The annihilation of an electron and a positron is that when they meet, they disappear, turning into γ- quanta (photons). And in a collision γ- When a quantum occurs with any massive nucleus, an electron-positron pair is born.
In 1955, another antiparticle was discovered - the antiproton, and a little later - the antineutron. An antineutron, like a neutron, has no electrical charge, but it undoubtedly belongs to antiparticles, since it participates in the process of annihilation and the birth of a neutron-antineutron pair.
The possibility of obtaining antiparticles led scientists to the idea of creating antimatter. Antimatter atoms should be built in this way: in the center of the atom there is a negatively charged nucleus, consisting of antiprotons and antineutrons, and positrons with a positive charge revolve around the nucleus. In general, the atom also turns out to be neutral. This idea received brilliant experimental confirmation. In 1969, at a proton accelerator in the city of Serpukhov, Soviet physicists obtained nuclei of antihelium atoms. Also in 2002, 50,000 antihydrogen atoms were produced at the CERN accelerator in Geneva. But, despite this, accumulations of antimatter in the Universe have not yet been discovered. It also becomes clear that at the slightest interaction of antimatter with any substance, their annihilation will occur, which will be accompanied by a huge release of energy, several times greater than the energy of atomic nuclei, which is extremely unsafe for people and the environment.
At present, antiparticles of almost all known elementary particles have been experimentally discovered.
A major role in the physics of elementary particles is played by conservation laws that establish equality between certain combinations of quantities characterizing the initial and final state of the system. The arsenal of conservation laws in quantum physics is larger than in classical physics. It was replenished with laws of conservation of various parities (spatial, charge), charges (leptonic, baryon, etc.), internal symmetries characteristic of one or another type of interaction.
Isolating the characteristics of individual subatomic particles is an important, but only the initial stage of understanding their world. At the next stage, we still need to understand what the role of each individual particle is, what its functions are in the structure of matter.
Physicists have found that, first of all, the properties of a particle are determined by its ability (or inability) to participate in strong interactions. Particles participating in strong interactions form a special class and are called hadrons. Particles that participate in the weak interaction and do not participate in the strong interaction are called leptons. In addition, there are particles that carry interactions.
2.3. Leptons.
Leptons are considered truly elementary particles. Although leptons may or may not have an electrical charge, they all have a spin of 1/2. Among leptons, the most famous is the electron. The electron is the first of the discovered elementary particles. Like all other leptons, the electron appears to be an elementary (in the proper sense of the word) object. As far as is known, the electron does not consist of any other particles.
Another well-known lepton is the neutrino. Neutrinos are the most common particles throughout the Universe. The Universe can be imagined as a boundless neutrino sea, in which islands in the form of atoms are occasionally found. But despite such prevalence of neutrinos, it is very difficult to study them. As we have already noted, neutrinos are almost elusive. Without participating in either strong or electromagnetic interactions, they penetrate through matter as if it were not there at all. Neutrinos are some kind of “ghosts of the physical world.”
Muons are quite widespread in nature, accounting for a significant portion of cosmic radiation. In many respects, the muon resembles an electron: it has the same charge and spin, participates in those interactions, but has a large mass (about 207 electron masses) and is unstable. In about two millionths of a second, the muon decays into an electron and two neutrinos. In the late 1970s, a third charged lepton was discovered, called the tau lepton. This is a very heavy particle. Its mass is about 3500 electron masses. But in all other respects it behaves like an electron and a muon.
In the 60s, the list of leptons expanded significantly. It was found that there are several types of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Thus, the total number of neutrino varieties is three, and the total number of leptons is six. Of course, each lepton has its own antiparticle; thus the total number of different leptons is twelve. Neutral leptons participate only in weak interactions; charged - in the weak and electromagnetic. All leptons participate in gravitational interactions, but are not capable of strong ones.
2.4. Hadrons.
If there are just over a dozen leptons, then there are hundreds of hadrons. Such a multitude of hadrons suggests that hadrons are not elementary particles, but are built from smaller particles. All hadrons are found in two varieties - electrically charged and neutral. Among hadrons, the most famous and widespread are the neutron and proton, which in turn belong to the class of nucleons. The remaining hadrons are short-lived and decay quickly. Hadrons participate in all fundamental interactions. They are divided into baryons and mesons. Baryons include nucleons and hyperons.
To explain the existence of nuclear forces of interaction between nucleons, quantum theory required the existence of special elementary particles with a mass greater than the mass of the electron, but less than the mass of the proton. These particles, predicted by quantum theory, were later called mesons. Mesons were discovered experimentally. There turned out to be a whole family of them. All of them turned out to be short-lived unstable particles, living in a free state for billionths of a second. For example, a charged pi-meson or pion has a rest mass of 273 electron masses and a lifetime:
t = 2.6*10^(-8) s.
Further, during studies at charged particle accelerators, particles with masses exceeding the mass of a proton were discovered. These particles were called hyperons. Even more of them were discovered than mesons. The hyperon family includes: lambda-, sigma-, xi- and omega-minus hyperons.
The existence and properties of most known hadrons were established in accelerator experiments. The discovery of many different hadrons in the 50-60s greatly puzzled physicists. But over time, hadrons were classified by mass, charge and spin. Gradually a more or less clear picture began to emerge. Specific ideas emerged on how to systematize the chaos of empirical data and reveal the mystery of hadrons in scientific theory. The decisive step here was taken in 1963, when the theory of quarks was proposed.
2.5. Quark theory.
The theory of quarks is a theory of the structure of hadrons. The main idea of this theory is very simple. All hadrons are made of smaller particles called quarks. This means that quarks are more elementary particles than hadrons. Quarks are hypothetical particles because were not observed in the free state. The baryon charge of quarks is 1/3. They carry a fractional electrical charge: they have a charge whose value is either -1/3 or +2/3 of the fundamental unit - the charge of the electron. A combination of two and three quarks can have a total charge of zero or one. All quarks have spin S, so they are classified as fermions. The founders of the theory of quarks, Gell-Mann and Zweig, in order to take into account all the hadrons known in the 60s, introduced three types (colors) of quarks: u (from up - upper), d (from down - lower) and s (from strange - strange) .
Quarks can combine with each other in one of two possible ways: either in triplets or in quark-antiquark pairs. Relatively heavy particles - baryons - are made up of three quarks. The best known baryons are the neutron and the proton. Lighter quark-antiquark pairs form particles called mesons - “intermediate particles”. For example, a proton consists of two u-quarks and one d-quark (uud), and a neutron consists of two d-quarks and one u-quark (udd). In order for this “trio” of quarks not to decay, a holding force, a certain “glue”, is needed.
It turns out that the resulting interaction between neutrons and protons in the nucleus is simply a residual effect of the more powerful interaction between the quarks themselves. This explained why strong interactions seem so complex. When a proton “sticks” to a neutron or another proton, the interaction involves six quarks, each of which interacts with all the others. A significant part of the force is spent on firmly gluing a trio of quarks, and a small part is spent on fastening two trios of quarks to each other. But later it turned out that quarks also participate in weak interactions. The weak interaction can change the color of a quark. This is how neutron decay occurs. One of the d-quarks in the neutron turns into a u-quark, and the excess charge carries away the electron that is born at the same time. Similarly, by changing the flavor, the weak interaction leads to the decay of other hadrons.
The fact that all known hadrons could be obtained from various combinations of the three fundamental particles was a triumph for the theory of quarks. But in the 70s, new hadrons were discovered (psi particles, upsilon meson, etc.). This dealt a blow to the first version of the quark theory, since there was no longer room for a single new particle in it. All possible combinations of quarks and their antiquarks have already been exhausted.
The problem was solved by introducing three new colors. They were named c - quark (charm), b - quark (from bottom - bottom, and more often beauty - beauty, or charm), and subsequently another color was introduced - t (from top - top).
Until now, quarks and antiquarks have not been observed in free form. However, there is practically no doubt about the reality of their existence. Moreover, a search is underway for “real” elementary particles following quarks - gluons, which are carriers of interactions between quarks, because Quarks are held together by the strong interaction, and gluons (color charges) are carriers of the strong interaction. The field of particle physics that studies the interaction of quarks and gluons is called quantum chromodynamics. Just as quantum electrodynamics is the theory of electromagnetic interaction, quantum chromodynamics is the theory of strong interaction. Quantum chromodynamics is a quantum field theory of the strong interaction of quarks and gluons, which is carried out through the exchange between them - gluons (analogues of photons in quantum electrodynamics). Unlike photons, gluons interact with each other, which leads, in particular, to an increase in the strength of interaction between quarks and gluons as they move away from each other. It is assumed that it is this property that determines the short-range action of nuclear forces and the absence of free quarks and gluons in nature.
According to modern concepts, hadrons have a complex internal structure: baryons consist of 3 quarks, mesons - of a quark and an antiquark.
Although there is some dissatisfaction with the quark scheme, most physicists consider quarks to be truly elementary particles - point-like, indivisible and without internal structure. In this respect they resemble leptons, and it has long been assumed that there must be a deep relationship between these two distinct but structurally similar families.
Thus, the most probable number of truly elementary particles (not counting carriers of fundamental interactions) at the end of the twentieth century is 48. Of these: leptons (6x2) = 12 and quarks (6x3)x2 = 36.
2.6. Particles are carriers of interactions.
The list of known particles is not limited to the listed particles - leptons and hadrons, which form the building material of matter. This list does not include, for example, a photon. There is also another type of particles that are not directly the building material of matter, but provide all four fundamental interactions, i.e. form a kind of “glue” that prevents the world from falling apart. Such particles are called carriers of interactions, and a particular type of particle transfers its interactions.
The carrier of electromagnetic interaction between charged particles is the photon. Photon is a quantum of electromagnetic radiation, a neutral particle with zero mass. The photon spin is 1.
The theory of electromagnetic interaction was introduced by quantum electrodynamics.
The carriers of the strong interaction are gluons. These are hypothetical electrically neutral particles with zero mass and spin 1. Like quarks, gluons have the quantum characteristic of “color.” Gluons are carriers of interaction between quarks, because tie them in pairs or threes.
The carriers of the weak interaction are three particles - W+, W- and Z° bosons. They were discovered only in 1983. The radius of the weak interaction is extremely small, so its carriers must be particles with large rest masses. According to the uncertainty principle, the lifetime of particles with such a large rest mass should be extremely short - only about 10n sec (where n = -26). The radius of interaction carried by these particles is very small because such short-lived particles do not have time to move very far.
It is suggested that the existence of a carrier of the gravitational field - the graviton - is also possible (in those theories of gravity that consider it not (only) as a consequence of the curvature of space-time, but as a field). Theoretically, a graviton is a quantum of the gravitational field, having zero rest mass, zero electric charge and spin 2. In principle, gravitons can be detected in experiment. But since the gravitational interaction is very weak and practically does not manifest itself in quantum processes, it is very difficult to directly detect gravitons, and so far no scientist has succeeded.
The classification of particles into leptons, hadrons and carriers of interactions exhausts the world of subatomic particles known to us. Each type of particle plays its role in shaping the structure of matter and the Universe.
3. Theories of elementary particles.
3.1. Quantum electrodynamics (QED).
Quantum theory combines quantum mechanics, quantum statistics and quantum field theory.
Quantum mechanics (wave mechanics) is a theory that establishes the method of description and laws of motion of microparticles in given external fields. It allows us to describe the movement of elementary particles, but not their generation or destruction, i.e., it is used only to describe systems with a constant number of particles. Quantum mechanics is one of the main branches of quantum theory. Quantum mechanics for the first time made it possible to describe the structure of atoms and understand their spectra, establish the nature of chemical bonds, explain the periodic system of elements, etc. Since the properties of macroscopic bodies are determined by the movement and interaction of the particles that form them, the laws of quantum mechanics underlie the understanding of most macroscopic phenomena. Thus, quantum mechanics made it possible to understand many properties of solids, explain the phenomena of superconductivity, ferromagnetism, superfluidity, and much more. Quantum mechanical laws underlie nuclear energy, quantum electronics, etc. Unlike classical theory, all particles act in quantum mechanics as carriers of both corpuscular and wave properties, which do not exclude, but complement each other. The wave nature of electrons, protons and other particles is confirmed by particle diffraction experiments. The state of a quantum system is described by a wave function, the square of the modulus of which determines the probability of a given state and, consequently, the probabilities for the values of physical quantities that characterize it. It follows from quantum mechanics that not all physical quantities can simultaneously have exact values. The wave function obeys the principle of superposition, which explains, in particular, the diffraction of particles. A distinctive feature of quantum theory is the discreteness of possible values for a number of physical quantities: the energy of electrons in atoms, angular momentum and its projection onto an arbitrary direction, etc.; in classical theory, all these quantities can only change continuously. A fundamental role in quantum mechanics is played by Planck's constant - one of the main scales of nature, separating the areas of phenomena that can be described by classical physics from areas for the correct interpretation of which quantum theory is necessary. Planck's constant is named after M. Planck. It is equal to:
Ћ = h/2π ≈ 1.0546. 10 ^(-34) J. s
A generalization of quantum mechanics is quantum field theory - this is a quantum theory of systems with an infinite number of degrees of freedom (physical fields). Quantum field theory is the main apparatus of the physics of elementary particles, their interactions and interconversions. The need for such a theory is generated by quantum-wave dualism, the existence of wave properties in all particles. In quantum field theory, interaction is represented as a result of the exchange of field quanta. This theory includes the theory of electromagnetic (quantum electrodynamics) and weak interactions, which appear in modern theory as a single whole (electroweak interaction), and the theory of strong (nuclear) interaction (quantum chromodynamics).
Quantum statistics is the statistical physics of quantum systems consisting of a large number of particles. For particles with an integer spin, this is the Bose Einstein statistics, and for particles with a half-integer spin, this is the Fermi-Dirac statistics.
In the middle of the twentieth century, a theory of electromagnetic interaction was created - quantum electrodynamics QED - this is a theory of interaction between photons and electrons, thought out to the smallest detail and equipped with a perfect mathematical apparatus. QED is based on a description of electromagnetic interaction using the concept of virtual photons - its carriers. This theory satisfies the basic principles of both quantum theory and relativity.
At the center of the theory is the analysis of the acts of emission or absorption of one photon by one charged particle, as well as the annihilation of an electron-positron pair into a photon or the generation of such a pair by photons.
If in the classical description electrons are represented as a solid point ball, then in QED the electromagnetic field surrounding the electron is considered as a cloud of virtual photons that relentlessly follows the electron, surrounding it with energy quanta. After an electron emits a photon, it produces a (virtual) electron-positron pair, which can annihilate to form a new photon. The latter can be absorbed by the original photon, but can generate a new pair, etc. Thus, the electron is covered with a cloud of virtual photons, electrons and positrons, which are in a state of dynamic equilibrium. Photons appear and disappear very quickly, and electrons do not move in space along well-defined trajectories. It is still possible in one way or another to determine the starting and ending points of the path - before and after scattering, but the path itself in the interval between the beginning and end of the movement remains uncertain.
The description of interaction using a carrier particle led to an expansion of the concept of a photon. The concepts of a real (quantum of light visible to us) and a virtual (fleeting, ghostly) photon, which is “seen” only by charged particles undergoing scattering, are introduced.
To test whether the theory agreed with reality, physicists focused on two effects that were of particular interest. The first concerned the energy levels of the hydrogen atom, the simplest atom. According to QED, the levels should be slightly shifted relative to the position they would occupy in the absence of virtual photons. The second decisive test of QED concerned the extremely small correction to the electron's own magnetic moment. The theoretical and experimental results of testing QED coincide with the highest accuracy - more than nine decimal places. Such a striking correspondence gives the right to consider QED the most advanced of the existing natural scientific theories.
Following this triumph, QED was adopted as a model for the quantum description of the other three fundamental interactions. Of course, fields associated with other interactions must correspond to other carrier particles.
3.2. Theory of electroweak interaction.
In the 70s of the twentieth century, an outstanding event occurred in natural science: two fundamental interactions out of four physics were combined into one. The picture of the fundamental principles of nature has become somewhat simpler. Electromagnetic and weak interactions, seemingly very different in nature, actually turned out to be two varieties of a single electroweak interaction. The theory of electroweak interaction had a decisive influence on the further development of elementary particle physics at the end of the twentieth century.
The main idea in constructing this theory was to describe the weak interaction in the language of the concept of a gauge field, according to which the key to understanding the nature of interactions is symmetry. One of the fundamental ideas in physics of the second half of the twentieth century is the belief that all interactions exist only to maintain a certain set of abstract symmetries in nature. What does symmetry have to do with fundamental interactions? At first glance, the very assumption of the existence of such a connection seems paradoxical and incomprehensible.
First of all, about what is meant by symmetry. It is generally accepted that an object has symmetry if the object remains unchanged as a result of one or another operation to transform it. Thus, a sphere is symmetrical because it looks the same when rotated at any angle relative to its center. The laws of electricity are symmetrical regarding the replacement of positive charges with negative ones and vice versa. Thus, by symmetry we mean invariance under a certain operation.
There are different types of symmetries: geometric, mirror, non-geometric. Among the non-geometric ones there are so-called gauge symmetries. Gauge symmetries are abstract in nature and are not directly fixed. They are associated with a change in the reference level, scale or value of some physical quantity. A system has gauge symmetry if its nature remains unchanged under this kind of transformation. So, for example, in physics, work depends on the difference in heights, and not on the absolute height; voltage - from the potential difference, and not from their absolute values, etc. The symmetries on which the revision of the understanding of the four fundamental interactions is based are precisely of this kind. Gauge transformations can be global or local. Gauge transformations that vary from point to point are known as "local" gauge transformations. There are a number of local gauge symmetries in nature, and an appropriate number of fields are needed to compensate for these gauge transformations. Force fields can be considered as a means by which nature creates its inherent local gauge symmetry. The significance of the concept of gauge symmetry is that it theoretically models all four fundamental interactions found in nature. All of them can be considered as gauge fields.
Representing the weak interaction as a gauge field, physicists proceed from the fact that all particles participating in the weak interaction serve as sources of a new type of field - a field of weak forces. Weakly interacting particles, such as electrons and neutrinos, carry a “weak charge,” which is analogous to an electric charge and binds these particles to a weak field.
To represent the weak interaction field as a gauge field, it is first necessary to establish the exact form of the corresponding gauge symmetry. The fact is that the symmetry of the weak interaction is much more complex than the electromagnetic one. After all, the mechanism of this interaction itself turns out to be more complex. First, in the decay of a neutron, for example, the weak interaction involves particles of at least four different types (neutron, proton, electron and neutrino). Secondly, the action of weak forces leads to a change in their nature (the transformation of some particles into others due to weak interaction). On the contrary, electromagnetic interaction does not change the nature of the particles participating in it.
This determines the fact that the weak interaction corresponds to a more complex gauge symmetry associated with a change in the nature of the particles. It turned out that to maintain symmetry, three new force fields are needed here, as opposed to a single electromagnetic field. A quantum description of these three fields was also obtained: there should be three new types of particles - carriers of interaction, one for each field. Collectively they are called spin-1 heavy vector bosons and are carriers of the weak force.
W+ and W- particles are carriers of two of the three fields associated with the weak interaction. The third field corresponds to an electrically neutral carrier particle, called the Z particle. The existence of a Z particle means that the weak interaction may not be accompanied by electric charge transfer.
In the creation of the theory of electroweak interaction, the concept of spontaneous symmetry breaking played a key role: not every solution to a problem must have all the properties of its original level. Thus, particles that are completely different at low energies may actually turn out to be one and the same particle at high energies, but in different states. Based on the idea of spontaneous symmetry breaking, the authors of the theory of electroweak interaction, Weinberg and Salam, were able to solve a great theoretical problem - they combined seemingly incompatible things: a significant mass of weak interaction carriers, on the one hand, and the idea of gauge invariance, which assumes the long-range nature of the gauge field, and means zero rest mass of carrier particles, on the other hand. Thus, electromagnetism and the weak interaction were combined into a unified theory of the gauge field.
This theory presents only four fields: the electromagnetic field and three fields corresponding to weak interactions. In addition, a constant scalar field (a type of Higgs field) has been introduced throughout space, with which particles interact differently, which determines the difference in their masses. Scalar field quanta are new elementary particles with zero spin. They are called Higgs (named after the physicist P. Higgs, who suggested their existence). The number of such Higgs bosons can reach several dozen. Such bosons have not yet been experimentally discovered. Moreover, a number of physicists consider their existence unnecessary, but a perfect theoretical model without Higgs bosons has not yet been found. Initially, W and Z quanta have no mass, but symmetry breaking causes some Higgs particles to merge with W and Z particles, giving them mass.
The theory explains the differences in the properties of electromagnetic and weak interactions by breaking symmetry. If the symmetry were not broken, then both interactions would be comparable in magnitude. Symmetry breaking entails a sharp decrease in the weak interaction. We can say that the weak interaction is so small because the W and Z particles are very massive. Leptons rarely come together at such short distances (r 10n cm, where n = -16). But at high energies ( > 100 GeV), when W and Z particles can be freely produced, the exchange of W and Z bosons occurs as easily as the exchange of photons (massless particles). The difference between photons and bosons is erased. Under these conditions, there should be complete symmetry between the electromagnetic and weak interactions - the electroweak interaction.
Testing the new theory consisted of confirming the existence of the hypothetical W and Z particles. Their discovery became possible only with the creation of very large accelerators of the latest type. The discovery of W and Z particles in 1983 meant the triumph of the theory of electroweak interaction. There was no longer any need to talk about the four fundamental interactions. There are three of them left.
3.3. Quantum chromodynamics.
The next step on the path to the Great Unification of fundamental interactions is the merging of the strong interaction with the electroweak interaction. To do this, it is necessary to give the features of a gauge field to the strong interaction and introduce a generalized idea of isotopic symmetry. The strong interaction can be thought of as the result of the exchange of gluons, which ensure the binding of quarks (in pairs or triplets) into hadrons.
The idea here is as follows. Each quark has an analogue of electric charge, which serves as a source of the gluon field. It was called a color (of course, this name has nothing to do with ordinary color). If the electromagnetic field is generated by a charge of only one type, then three different color charges were required to create a more complex gluon field. Each quark is “colored” in one of three possible colors, which were quite arbitrarily called red, green and blue. And accordingly, antiques are anti-red, anti-green and anti-blue.
At the next stage, the theory of strong interaction is developed according to the same scheme as the theory of weak interaction. The requirement of local gauge symmetry (i.e., invariance with respect to color changes at each point in space) leads to the need to introduce compensating force fields. A total of eight new compensating force fields are required. The carrier particles of these fields are gluons, and thus the theory implies that there must be as many as eight different types of gluons, while the carrier of the electromagnetic force is only one (photon), and the carriers of the weak force are three. Gluons have zero rest mass and spin 1. Gluons also have different colors, but not pure, but mixed (for example, blue-anti-green). Therefore, the emission or absorption of a gluon is accompanied by a change in the color of the quark (“play of colors”). So, for example, a red quark, losing a red-anti-blue gluon, turns into a blue quark, and a green quark, absorbing a blue-anti-green gluon, turns into a blue quark. In a proton, for example, three quarks constantly exchange gluons, changing their color. However, such changes are not arbitrary in nature, but are subject to a strict rule: at any moment of time, the “total” color of three quarks must be white light, i.e. the sum "red + green + blue". This also applies to mesons consisting of a quark-antiquark pair. Since an antiquark is characterized by an anticolor, such a combination is obviously colorless (“white”), for example, a red quark in combination with an antired quark forms a colorless meson.
From the point of view of quantum chromodynamics (quantum color theory), strong interaction is nothing more than the desire to maintain a certain abstract symmetry of nature: maintaining the white color of all hadrons while changing the color of their constituent parts. Quantum chromodynamics perfectly explains the rules that govern all combinations of quarks, the interaction of gluons with each other, the complex structure of a hadron consisting of quarks “dressed” in clouds, etc.
It may be premature to evaluate quantum chromodynamics as the final and complete theory of the strong interaction, but its achievements are nonetheless promising.
3.4. On the way to... The Great Unification.
With the creation of quantum chromodynamics, hope arose for the creation of a unified theory of all (or at least three out of four) fundamental interactions. Models that describe at least three of the four fundamental interactions in a unified way are called Grand Unified models. Theoretical schemes that combine all known types of interactions (strong, weak, electromagnetic and gravitational) are called supergravity models.
The experience of successfully combining weak and electromagnetic interactions based on the idea of gauge fields suggested possible ways for further development of the principle of the unity of physics and the unification of fundamental physical interactions. One of them is based on the amazing fact that the interaction constants of the electromagnetic, weak and strong interactions become equal to each other at the same energy. This energy was called the energy of unification. At energies above 10n GeV, where n = 14, or at distances r 10n cm, where n = -29, strong and weak interactions are described by a single constant, i.e., they have a common nature. Quarks and leptons are practically indistinguishable here.
In the 70-90s, several competing theories of the Grand Unification were developed. They are all based on the same idea. If the electroweak and strong forces are really just two sides of the grand unified force, then the latter should also have an associated gauge field with some complex symmetry. It (symmetry) must be sufficiently general, capable of covering all gauge symmetries contained in both quantum chromodynamics and the theory of electroweak interaction. Finding such symmetry is the main task towards creating a unified theory of strong and electroweak interactions. There are different approaches that give rise to competing versions of Grand Unification theories.
However, all of these hypothetical versions of the Great Unification have a number of common features:
Firstly, in all hypotheses, quarks and leptons - carriers of the strong and electroweak interactions - are included in a single theoretical scheme. Until now they have been considered as completely different objects.
Secondly, the use of abstract gauge symmetries leads to the discovery of new types of fields that have new properties, for example, the ability to transform quarks into leptons. In the simplest version of the Grand Unified Theory, twenty-four fields are required to transform quarks into leptons. Twelve of the quanta of these fields are already known: a photon, two W particles, a Z particle and eight gluons. The remaining twelve quanta are new superheavy intermediate bosons, united under the common name X and Y - particles (with an electric charge of 1/3 and 4/3). These quanta correspond to fields that maintain broader gauge symmetry and mix quarks with leptons. Consequently, quanta of these fields (i.e. X and Y particles) can transform quarks into leptons (and vice versa).
Based on Grand Unified theories, at least two important patterns are predicted that can and should be tested experimentally: proton instability and the existence of magnetic monopoles. Experimental detection of proton decay and magnetic monopoles could provide a strong argument in favor of Grand Unified theories. Experimental efforts are aimed at testing these predictions. But there is still no firmly established experimental data on this matter. The fact is that Grand Unified theories deal with particle energies above 10n GeV, where n = 14. This is very high energy. It is difficult to say when it will be possible to obtain particles of such high energies in accelerators. This explains, in particular, the difficulty of detecting the X and Y bosons. And therefore, the main area of application and testing of Grand Unified theories is cosmology. Without these theories, it is impossible to describe the early stage of the evolution of the Universe, when the temperature of the primary plasma reached 10n K, where n = 27. It was under such conditions that superheavy particles could be born and annihilated.
Thus, it becomes clear that proving the Grand Unified theory is the main task of physicists today, because this theory will not only help connect disparate fragments of human knowledge into a single picture, but also take a step towards understanding the origin of the Universe.
Bibliography.
School Student's Handbook. 5-11 grades. 2004
Computer encyclopedia of Cyril and Methodius. 2005
I. L. Rosenthal “Elementary particles and the structure of the Universe.” 1984
Page 8
In nature, not one, but sometimes several types of mutual influence and properties act between elementary particles, and the structure of particles is determined by the commonality of all types of mutual influence taking part. For example, the proton, which is part of the hadronic type of elementary particles, takes part in strong mutual influence, and in electromagnetic mutual influence due to the fact that it is an electrically charged particle. On the other hand, a proton can be generated in the b decay process of a neutron, that is, in weak mutual influences, thus it is associated with weak mutual influences. And finally, the proton, as a material formation with mass, takes part in gravitational mutual influences. Unlike the proton, a number of elementary particles take part in all types of mutual influence, but only in some of their types. For example, a neutron, due to the fact that it is an uncharged particle, does not take part in electromagnetic mutual influences, and the electron and mu-mesons do not participate in strong mutual influences. Fundamental mutual influences are the reason for the transformation of particles - their destruction and generation. For example, the collision of a neutron and a proton produces two neutrons and one positive pimeson.
The period of transformation of elementary particles depends on the mutually influencing force. Nuclear reactions associated with strong mutual influences occur in 10-24 - 10-23 seconds. This is the period when an elementary particle transforms into a high-energy particle and acquires a speed close to the speed of light, dimensions of the order of 10-13 cm. Changes caused by electromagnetic mutual influences occur in 10-21 - 10-19 seconds, due to weak mutual influences of the change (for example , the process of decay of elementary particles) – in 10-10 seconds.
The period of various changes occurring in the microcosm can be approached from the point of view of reasoning about the creating mutual influences.
Quanta of mutual influence of elementary particles are realized through the physical fields corresponding to these particles. In modern quantum theory, a field is understood as a system of particles that change in number (sex quanta). The state when the field, and in general, field quanta exist with the lowest energy, is called vacuum. Particles of the electromagnetic field (photons) in a vacuum in a state of excitation lose the mechanical properties that they contain and which are inherent in corpuscular matter (for example, during movement the body does not feel friction).
Vacuum does not contain simple types of matter, however, despite this, it is not emptiness in the true sense of the word, so in vacuum excitation quanta of the electromagnetic field arise - photons, which realize electromagnetic mutual influence. In a vacuum, in addition to the electromagnetic field, there are other physical fields, including the gravitational field, which has not yet been noted in the so-called graviton experiments.
A quantum field is a collection of quanta and is discrete in nature. Thus, the mutual influence of elementary particles, their mutual transformations, emission and absorption of photons is discrete in nature and occurs only in a situation of quantization. As a result, the following question arises: in what exactly is the continuity of the field, its continuity, manifested? In both quantum electrodynamics and quantum mechanics, the field state is described unambiguously not by observable real phenomena, but only by a wave function associated with the reciprocal concept. The square of the modulus of this function shows the ability to observe the physical phenomena under consideration.
The main problem of quantum field theory is the description of various types of mutual influences of particles in the corresponding equations. This problem has so far found its solution only in quantum electrodynamics, which describes the mutual influences of electrons, positrons and photons. A quantum field theory has not yet been created for strong and weak mutual influences. Currently, these types of mutual influence are not described using strict methods. Although it is known that it is impossible to understand elementary particles if they are not in the corresponding physical theory, it is impossible to understand their structure, determined by the structure of these theories. Therefore, the problem of the structure of elementary particles has not yet been fully resolved.1 Modern physics at the present time proves the existence of complex particles that have the internal structure of particles considered “elementary”. It became known that the proton and neutron, as a result of the virtual processes occurring in them, undergo internal transformations. As a result of experiments carried out to study the structure of protons, it was determined that the proton, which until recently was considered indivisible, the simplest and most structureless, is in fact a complex particle. At its center there is a dense core called the “core”, it is surrounded by positive pi mesons.
The complexity of the structure of “elementary” particles was proven by the quark hypothesis put forward in 1964 by the American scientist Hel-Mann and independently by the Swedish scientist Zweig. According to this hypothesis, elementary particles with relationships characterized by strong mutual influences (hadrons: proton, neutron, hyperons) should be formed from quark particles whose charge is equal to one-third or two-thirds of the electron charge. Thus, the theory shows that the electric and baryon charges of the marked quarks that form the particles should be expressed as a fractional number. Indeed, particles called quarks have not yet been discovered and remain hypothetical inhabitants of the microworld at the current level of scientific development.
Thus, on the one hand, it is clear that elementary particles have a special structure, on the other hand, the nature of this structure still remains unclear. From the above data it becomes clear that elementary particles are not elementary at all, they have an internal structure and can be divided and transformed into each other. We still know very little about both structures. Thus, today, based on a number of facts, we can claim that the matter of elementary particles is a new type, qualitatively different from more complex particles (nucleus, atom, molecule). At the same time, this difference is so significant that the categories and expressions we use when studying nuclei, atoms, molecules, macroscopic bodies (“simple” and “complex”, “internal structure”, “formed”) can also be applied to elementary particles. The concepts “simple and complex”, “component parts”, “structure”, “whole” are, in general, relative concepts. For example, despite the fact that an atom has a complex structure, and its structure consists of nuclear and electronic tiers, it is simpler in comparison with its constituent molecule.
All currently known elementary particles can be divided into groups according to their general properties and relationship to interaction. There are four known such interactions in nature: strong, electromagnetic, weak and gravitational.
Strong the interaction has the highest intensity compared to other interactions. It determines the connection between protons and neutrons in the nuclei of atoms (through the exchange of virtual n-mesons), which ensures the exceptional strength of these formations.
Electromagnetic interaction characterizes less intense processes. It determines the connection of atomic electrons with nuclei, the connection of atoms in molecules, as well as the interaction of matter with electromagnetic fields.
Weak interaction characterizes processes associated with the particles themselves, in particular with (β-decay, as well as with the decays of μ, π, K-mesons and hyperons. It turned out that the weak interaction is universal in nature, all particles participate in it. The lifetime of most of these particles lies in the range of 10 -8 - 10 -10 s, while the typical time of strong interactions is 10 -23 -10 -24 s. An illustration of such interaction is the fact that neutrinos, capable of only weak interactions, can pass unhindered in substance distance ~10 14 km.
Gravitational the interaction, so well known for its macroscopic manifestations, in the case of elementary particles produces extremely insignificant effects due to the small size of their masses. However, these effects also increase significantly in the microcosm at distances of the order of 10 -33 cm, since the mass of generated particles increases. These interactions play a dominant role in the megaworld.
Comparison of these four interactions by dimensionless parameters associated with the squares of the corresponding interaction constants gives the following ratios for strong, electromagnetic, weak and gravitational: 1:10 -3:10 -10:10 -38. Generally speaking, the intensity of various processes depends differently on energy, so as the energy of interacting particles increases, the relative role of various interactions changes.
Depending on their participation in certain types of interactions, all particles, as we have already indicated, can be divided into four groups.
Group I: e, μ, τ, ν e, ν μ, ν τ - leptons participate in weak and electromagnetic interactions; II group are made up of strongly interacting particles (there are now more than 300 of them), called hadrons(they also participate in weak and electromagnetic interactions).
The study of hadrons led to the conclusion that they have something in common in their structure. In 1964, M. Gell-Mann and J. Zweig hypothesized that the structure of all hadrons includes objects exotic in their characteristics, called quarks. It was assumed that there are three types of quarks u, d, s, the charges of which are fractional e u = + 2 / 3, e d = e s = - 1 / 3 of the electron charge, and the masses m u = m d ~300 MeV, m s ~450 MeV. Subsequently, as the logic of the development of the theory required, to describe the weak interactions of hadrons (weak decays) it was necessary to introduce another type of quarks, the so-called c-quarks with a charge e c = e u = + 2 / 3 electron charge. This quark is characterized by a new quantum number called charm.
In November 1974, a new J/ψ particle was discovered with unusual properties (mass 3.1 GeV, approximately three times the mass of a proton), lifetime ~10 -20 s (i.e. 1000 times longer than any known previously particles with such a large mass). It breaks up into pairs e + + e - or μ + + μ - . Soon a particle was also discovered, called ψ" (mass 3.7 GeV).
Experiments have shown that J/ψ, ψ" particles belong to a whole family of mesons, which well corresponds to the spectrum of charmonium with an effective mass corresponding to the predicted mass of the c-quark (m c ≈1.6 GeV). For final confirmation of the existence of the c-quark it is necessary It was possible to discover hadrons with obvious “charm.” Phenomena have now been discovered that indicate the birth of charmed particles.
Physicists believe that the existence of the c-quark has been experimentally confirmed. But since the existence of c-quarks was based on the assumption of the existence of light quarks - u, d, s, the discovery of charmed hadrons is of fundamental importance for confirming the truth of the entire quark hypothesis.
Theoretical physicists have come to the conclusion that quarks of each type must be in one of three states, which are now usually characterized by three flowers(for example, yellow, blue, red); they suggest that the strong interaction of quarks is the interaction of their color with a new field, the so-called. gluonic (from the English glue - glue, because this field seems to “glue” the quarks in the hadron). Gluon field quanta - gluons- do not participate in electromagnetic and weak interactions. They not only change the color state of the quark, but also carry color themselves and interact with the gluon field. All this gave rise, by analogy to quantum electrodynamics, to a new branch of physics - the so-called quantum chromodynamics.
It is important to emphasize that quarks and gluons are not observed in a free state; they do not “fly out” from hadrons.
There are special studies that prove the fundamental impossibility of the existence of quarks in a free state.
Physicists have long been trying to create a consistent theory of weak interactions. In 1967, S. Weinberg and A. Salam proposed a version of such a theory - they built a model based on the use of general principles of symmetry. This theory predicted the existence of previously unknown particles - quanta of special vector fields responsible for the transfer of both weak and electromagnetic interactions.
Two of these W ± particles must have charges and can actually be observed, since, in their opinion, it is the exchange of charged W ± mesons that gives rise to the weak interaction of the so-called charged currents. As for the two neutral particles W°, B°-quanta of neutron fields, quanta of any linear combination of them can be physically observable:
where Θ W is the so-called Weinberg angle.
It was shown that one of their combinations - the so-called field A - is identified with the electromagnetic field, and the exchange of neutral Z° mesons gives rise to a new type of weak interactions - the so-called neutral currents, which were discovered in 1973. They became the first confirmation of the relative truth of the Weinberg-Salam model. Currently, W ± and Z° particles are open.
It is also necessary to pay attention to the discovery of new leptons. This is an extremely rare event. Suffice it to recall that the electron (e) was discovered in 1897, and the muon (μ) in 1936-1938. In 1975-1976 evidence appeared in favor of the existence of τ ±, the so-called heavy lepton with a mass of 1.8 GeV (2 Mr). Studying the τ lepton provides another argument in favor of three states of quarks. It was also suggested that there is a new lepton (v τ - a new neutrino), the τ-lepton has a new lepton quantum number, which was called sequaleptone(from the English sequential - sequential).
Further research led to the conclusion that to restore symmetry it would be necessary to increase the number of quarks. Four were no longer enough to describe the objects of the microworld; it was necessary to introduce two more quarks. The fact is that in May - June 1977, L. Lederman's group obtained important results, namely, the discovery of a new family of heavy particles with masses of ~10 GeV.
The discovery of these particles (they were called γ-mesons) brought to life the need for the existence of an even heavier “b” quark with an effective mass m b ~5 GeV with a new quantum number, called “beauty”.
The new γ mesons are particles with a hidden charm. Thus, the study of hadrons and leptons has enriched science with knowledge about new objects, their quantitative and qualitative characteristics, and their interactions. All this indicates the advent of a new era in the study of the inexhaustible properties of micro-objects, which, together with various fields, constitute a fragment of the integral material world.
Now there is hope for the creation of a unified theory of interaction. At one time, A. Einstein tried to create such a field theory. W. Heisenberg also made a lot of effort to build a unified (so-called spinor) theory of “primordial matter”. Now we have witnessed the emergence of another version of the unified theory of interaction, called the Great Unification.
It has already been possible to create a unified electroweak interaction, and encouraging results have been obtained in combining the strong and electroweak interactions; Moreover, strong and weak interactions themselves are its manifestation. Outside the unification, gravitational interaction still remains, but there are already approaches to including it (supersymmetry) in a unified theory of interaction.
The modern development of elementary particle physics has made it possible to show that known particles (leptons, hadrons, quarks, gluons, photons) significantly determine the specifics of the processes of the microworld. Apparently, this list is far from complete, like the theory of elementary particles itself.
As noted, elementary particle physics has a wealth of empirical material, and the theory already provides a rational explanation for a significant part of it. However, it still lags significantly behind experiment and is not an internally closed system of certain principles and concepts, although its conceptual apparatus is much more capacious and differs from the apparatus of previously existing theories.
Let us now consider in retrospect some attempts to construct a unified theory covering all particles and fields. There are two main trends here, ultimately related to each other. The first of them originates from the idea of Louis de Broglie, which consists in basing the simplest wave function of the spinor type, describing a particle with minimal non-vanishing angular momentum, i.e. spin S = 1 / 2 (in fractions of h / 2π) . Then, combining these wave functions (ultimately multiplying), under some additional conditions we obtain through a similar “merger” all other possible wave functions of particles with spins 0.1; 3/2; 2... Combining two angular momentum + 1/2 and - 1/2, we get 0, combining two angular momentum + 1/2 and + 1/2, we get 1 (since the spins + 1/2 can only be oriented parallel or antiparallel). Using the fusion method, it is possible, by combining two Dirac equations that describe spin particles ("fermions"), to obtain the Klein-Gordon and Prock equations, and in the special case of vanishing rest mass - the Maxwell equations of electrodynamics. In this way, it is in principle possible to construct photons from neutrino-antineutrino pairs. The ideas of the neutrino theory of light by Louis de Broglie were developed by Kronig, Jordan, and A. Sokolov.
The weak point of the merging method is the absence of any forces that determine the merging itself. It remains unclear what causes, for example, neutrinos to turn into electromagnetic field quanta. The so-called nonlinear unified spinor theory of matter by W. Heisenberg tried to answer this question. The name of this theory is clearly unfortunate. The discussion was about creating a unified theory of elementary particles and fields, and not about the theory of matter, because the only theory of matter as an objective reality existing outside and independently of the cognizing subject is dialectical materialism. If we take some unified spinor field as the basis of the new theory, then it is capable of interacting only with itself. This leads to the appearance of so-called nonlinear terms in the Dirac equations (which were first introduced by D. Ivanenko back in 1938), and then considered in more detail by W. Heisenberg (193, 441-485; 34).
This theory does not give exact values for particle masses and coupling constants, but it is undoubtedly one of the attempts that deserves attention, although it is not without its shortcomings. This is only a research program that should not be overestimated, as has already been the case in individual articles published in our press.
It must be borne in mind that several years ago the incorrectness of the mathematical interpretation of Heisenberg’s spinor theory was revealed, and it was also shown that the indefinite metric introduced by Heisenberg leads to a violation of microcausality. One can with good reason believe that Heisenberg’s concrete attempt to create a unified theory of elementary particles has so far failed, but his chosen direction of research should not be discounted. In recent years, there has been a peculiar return to the ideas of W. Heisenberg.
In 1958 in the USA, when Pauli reported on Heisenberg’s theory, N. Bohr, who was present at the discussion, made a remark: “For a new theory, Heisenberg’s theory is not crazy enough” (crasy) (23, 20). N. Bohr meant the absence of an unusual, outlandish idea in this theory. In our opinion, physicists do not yet have such an idea. Academician I. Tamm considered the most promising direction in the development of the theory of elementary particles to be attempts to radically revise our space-time concepts as applied to ultra-small scales. He refers to the statements of academician L. T. Mandelstam about the inapplicability of ordinary concepts of space and time to nuclear scales, as well as to the work of X. Snyder (1947), who proposed a method for quantizing space and time, leading to the conclusion that space is discrete. Snyder showed that quantized space, that is, the space of coordinates that do not commute with each other, is discrete and at the same time isotropic. However, Snyder’s ideas received almost no further development with the exception of the works of Golfand and Kadyshevsky.
V. G. Kadyshevsky (50. 1961. 136. (1)) proposed introducing a universal length “l” into the theory of elementary particles based on changes in the geometry of space-time. He believed that the new geometry must satisfy the following conditions:
a) the form S 2 = X 2 0 - X 2 2 is non-invariant to the coordinate transformation, and the group of motions would allow a lower degree of isotropy of the 4-space than the Lorentz group;
b) non-invariance of the interval and the presence of a universal length would be the reasons for non-conservation of parity;
c) there must be a subgroup for which S 2 is an invariant, so that the symmetries of large regions of 4-space - large compared to the elementary length "l" - can be described. The author connects the length "l" with the value C - the universal constant of weak interaction. After selecting the multipliers " h" and "C" for "l" follows the value 7 * 10 -17 cm. This and the work that followed it are very interesting, but so far the possibilities of this theory remain unclear.
In 1959, the Canadian physicist H. Coish and the Soviet physicist I. S. Shapiro in their research considered a discrete space consisting of a finite number of elements and showed a good agreement of a number of conclusions with experimental data. This is also one of the possible search paths that brings us closer to the creation of a systematics of elementary particles, to a new generalizing physical theory. However, I. S. Shapiro, speaking in 1962 at the Meeting on Philosophical Problems of Elementary Particle Physics, assessed his work as an initial stage, very distant from the creation of a theory that allows comparison with experience. A philosophical analysis of this problem was given by R. A. Aronov (31.1957.3).
In physics, questions about the so-called spectral representations and dispersion relations were considered. According to a number of physicists, this was a kind of new stage in its development, when the analytical properties of physical quantities (for example, scattering amplitude) were studied as they extended from real values into the complex region. The application of the theory of functions of a complex variable to these quantities yielded extremely important results. Mandelstam (99) introduced double dispersion relations, considering complex values of not only energy, but also momentum. Regge proposed a generalization of the S-matrix formalism and dispersion relations into complex values of angular momentum. As a result of the application of "registry", the relationships between the probability amplitudes of various scattering processes were determined: ππ, πN, NN, etc. at high energies. However, there is data (in the field of ultra-high energy physics) that limits the claims of the “regists” to the comprehensiveness of their ideas.
Academician I. Tamm considered the dispersion theory to be to a certain extent phenomenological, since it, without going into the mechanism of elementary physical phenomena, extracts from experimental data the numerical values of a number of parameters included in it and then correctly predicts the results of a much wider range of experiments than those on on the basis of which these parameters were determined. In the second edition of this book, we wrote (p. 194) that although at first glance there is a close unity of theory and practice, it seems to us that the theory itself is of a prescription nature. We agreed with I. Tamm’s conclusion that “the successes of dispersion theory (both present and future) do not at all solve the main problem of creating a new physical theory based on a limited number of general principles and postulates” (23, 21). Subsequent developments in physics confirmed these assumptions. There have been many other attempts to construct a theory of elementary particles. Let's briefly look at some of them.
Fermi and Young proposed to consider the n-meson as formed from a nucleon and an antinucleon with the help of some still unknown forces acting at extremely short distances p+¯p = π. The enormous potential binding energy “eats up” almost the entire mass of both nucleons, leaving only the mass of the pion. The proposal of S. Sakata, who based the theory on p, π, λ and three corresponding antiparticles, aroused interest. Then, by combining these fundamental particles, you can get all the pions, K-mesons and hyperons. “This model,” wrote S. Sakata, “attracted attention, since it not only served as a “substantial” basis for the structure of the strong interaction, but also made it possible to explain the mass spectrum of composite particles and predicted the existence of resonant particles that were then being discovered” (74, 168). However, the nature of the adhesion forces remained unclear. A minimum of three fundamental particles are required to ensure the presence of such fundamental properties as charge, isospin, strangeness (represented by the λ-hyperon). It is clear, again, that the basis should be on “rotating” spinor particles, fermions, since in the absence of “rotation” there would be nowhere to get it from. We see here a kind of revival of the theory of Helmholtz and Kelvin, who tried in the middle of the 19th century. build matter from hypothetical ethereal vortices.
When constructing the “composite” model, Sakata proceeded from the following view of elementary particles: “... I consider elementary particles as one of an endless number of levels of the structure of matter, qualitatively different from each other and collectively forming nature. My point of view is based on the provisions of materialistic dialectics... it is necessary first of all to establish whether the thirty-odd types of elementary particles discovered to date belong to one or several different levels of the structure of matter" (31. 1962. 6, 134). Sakata and his collaborators tried to include leptons in their scheme. The basis is taken by leptons e - , v, μ and some “baryon” field B (the so-called B-matter). By combining one of the leptons with the B field, they obtain fundamental particles. Thus, the similarity noted by Marshak - Gamba - Okuba (203) between baryons (р, π, λ and leptons v, e -, μ -) is realized. The same symmetry is realized in the nonlinear spinor theory of particles.
Marshak called his ideas about symmetry “Kiev symmetry”, since they were born at the symposiums of the Kiev Conference on High Energy Physics in the summer of 1959. We are talking (as we have already mentioned) about some analogy that existed between triplets of baryons (p, π, λ) and leptons (v, e - , μ -). Any term of the four-fermion interaction, with the participation of the operators of these particles, can be contrasted with a similar term obtained from the first by replacing λ with μ -, π with e -, p with v. Then, if a process is allowed/forbidden before the replacement, then it remains allowed/forbidden after replacing one particle from the baryon/lepton triad with a “symmetrofactor” from the leptonic/baryon triad. Marshak points out that he carefully analyzed all the experimental data and did not find a single case that contradicts the specified “symmetry,” but the nature of this symmetry remains unclear. Now that the quark model has already been created, it has become possible to interpret the Kiev symmetry as the correspondence of four quarks - u, c, d, s to four leptons - v e, v μ, e, μ, but the nature of this symmetry is still not well known.
We know that any, even the most successful attempt to create a unified theory of matter and field will inevitably be temporary, transitory. Further theoretical and experimental penetration into the depths of the microcosm and increasingly broader studies of phenomena in space, inevitably disturbing any single picture, will lead to its disintegration into individual elements, until tendencies to unify again arise at a higher level.
The introduction of various concepts reflecting the real properties of particles (isotopic spin, strangeness, baryon charge, etc.) brought us closer to the correct classification of particles. A huge role in the classification of microparticles belongs to the principle of symmetry. It is easy to notice that elementary particles of each class (photons, leptons, mesons, hyperons) have certain symmetry properties common to them, but we will consider this issue in more detail in the course of further presentation.
J. Chu, M. Gell-Mann and I. Neeman (21, 5E) proposed a new classification of strongly interacting particles of matter, in which the division of particles into elementary and complex (composite) loses its meaning. These authors proposed to consider particles united in groups (supermultiplets) so that particles with different rest masses in each group can be considered as different excited states of the same system. The mass spectrum of particles in this scheme has a close analogy with the spectrum of energy states of the atom. Each of the particles can be considered with equal grounds both as simple and as complex. To find the mass spectrum, two methods are proposed: one of them is based on the properties of symmetry and group theory, the other is based on the use of so-called Regge trajectories, i.e. curves connecting the mass of a particle with its internal angular momentum (spin) in each group.
Many physicists currently believe that the Gell-Mann octet scheme is the most successful. It is based on the principle S.U.(3) symmetry. The eight known baryons are considered to be a supermultiplet corresponding to higher symmetry; this symmetry is broken, and the supermultiplet splits into isotopic spin multiplets. Strongly interacting particles are described in the space of "unitary spin", which has eight components: the first three are isospin components, the next four act as strangeness-modifying operators, and the last is proportional to the hypercharge. When higher symmetry ("unitary") is broken, isospin and hypercharge are preserved, and the components of the unitary spin corresponding to the strangeness change; As a result, the supermultiplet splits into isotopic spin multiplets. Thus, the Gell-Mann theory to some extent takes into account the deep dialectical unity of symmetry and asymmetry in the world of elementary particles. This is what allowed this theory to unite strongly interacting particles according to a harmonious scheme and at the same time reflect their specificity (asymmetry of properties). The Gell-Mann octet scheme once again demonstrates the enormous heuristic power of the symmetry principle. Within the framework of the “eight-fold path” hypothesis, based on the concepts of symmetry and conservation laws, the existence of the Ω-hyperon was predicted, which was discovered at the Brookhaven accelerator in the USA (214). At one time, we wrote that the successes that resulted from taking into account the property of unitary symmetry in the theory give us hope that experimental studies will lead to the discovery of other particles with a fractional electric charge predicted by the theory (± 1/3 and ± 2/3 of the electron charge) , the so-called quarks. The subsequent development of physics justified these hopes.
Let us point out some more attempts to systematize elementary particles. Thus, several years ago M.A. Markov (204) proposed an original model Maximonov. Based on the ideas of the general theory of relativity, he showed that the macro- and microworlds can closely intersect with each other. The formal basis for the introduction of new hypothetical elements was the fact that of the most important world constants of modern physical theory, two combinations with the dimension of mass can be made. One of these quantities has a numerical value of one millionth of a gram, and the other has a numerical value ten times greater. The maximons introduced in this way are 10 19 times larger in mass than real hadrons (strongly interacting particles). Maximons are so heavy for their spatial dimensions that “these particles cannot be detected in any vessel on the surface of the Earth. They fall to the center of the planet under the influence of gravity... Since the birth of maximons requires an energy of 10 28 eV, the possibility of the birth of maximons is even at accelerators of the distant future are excluded" (53.1966.51, 878).
Analysis of existing models shows some differences in the approach of their authors to the problem of systematization of micro-objects. Some proceed from certain properties of elementary particles and fields and try to solve the problem of the structure of micro-objects by introducing new properties of space-time symmetry, others, on the contrary, retain the known properties of space and time, but to explain the structure of micro-particles they introduce new characteristics of the properties of material micro-objects and fields. Such a difference in approaches to solving the same problem is completely justified.
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