ELEMENTARY PARTICLES
Introduction
E. particles in the exact meaning of this term are primary, indecomposable particles, of which, by assumption, all matter consists. In the concept of "E. h." in modern Physics finds expression in the idea of primitive entities that determine all the observable properties of the material world, 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 on a microscopic level. 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 observed substances as combinations of a finite, albeit large, number of structural components - atoms. The subsequent identification of the constituent parts of atoms - electrons and nuclei, the establishment of the complex nature of the nuclei themselves, which turned out to be built from only two particles (nucleons): protons and neutrons, significantly reduced the number of discrete elements that form the properties of matter, and gave reason to assume that the chain the constituent parts of matter culminate in discrete structureless formations - E. h. Revealed in the beginning. 20th century possibility of interpretation of el-magn. fields as a collection of special particles - photons - further strengthened the conviction of the correctness of this approach.
However, the formulated 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. It is also possible 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.
As a rule, the term "E. h." used in modern physics not in its exact meaning, but less strictly - to name a large group of the smallest observable particles of matter, subject to the condition that they are not atoms or atomic nuclei, i.e. objects of a obviously composite nature (the exception is the proton - the nucleus of the hydrogen atom). Research has shown that this group of particles is unusually broad. Besides proton(R), neutron(n), electron(f) and photon(g) it includes: pi mesons(p), muons(m), tau leptons(T), neutrino three types ( v e, v m, v t), so-called strange particles ( K-mesons And hyperons), charmed particles and lovely (beautiful) particles (D- and B-mesons and the corresponding baryons),varied resonances, incl. mesons with hidden charm and charm ( ncu-frequently, upsilon-particles) and finally opened at the beginning. 80s intermediate vector bosons (W, Z)- more than 350 particles in total, mainly unstable. The number of particles included in this group as they are discovered is constantly growing, and we can confidently say that it will continue to grow. It is obvious that such a huge number of particles cannot act as elementary components of matter, and indeed, in the 70s. it was shown that most of the listed particles (all mesons and baryons) are composite systems. The particles included in this last group should more accurately be called “subnuclear” particles, since they represent specific forms of existence of matter that is not aggregated into nuclei. Use of the name "E.h." in relation to all the particles mentioned, it is mainly history, reasons and is associated with the period of research (early 30s), when the only known representatives of this group were the proton, neutron, electron and electron-magnetic particle. fields - photon. Then these particles, with a certain right, could lay claim to the role of E. particles.
Discovery of new microscopy. particles gradually destroyed this simple picture of the structure of matter. However, the newly discovered particles in their properties were in a number of respects close to the first four known particles: either the proton and neutron, or the electron, or the photon. As long as the number of such particles was not very large, the belief remained that they all played the fundam. role in the structure of matter, and they were included in the category of E. particles. With the increase in the number of particles, this belief had to be abandoned, but traditionally. name "Eh." was reserved for them.
In accordance with established practice, the term "E. h." will be used below as a general name for all the smallest particles of matter. In cases where we are talking about particles that claim to be the primary elements of matter, the term “true” will be used if necessary elementary particles".
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 in the late 1960s. 19th century It was prepared by detailed studies of the spectra of atoms, the study of electricity. phenomena in liquids and gases, the discovery of photoelectricity, X-rays. rays, natural radioactivity, indicating the existence of a complex structure of matter.
Historically, the first element discovered was the electron, the carrier of negative elementary electricity. charge in atoms. In 1897, J. J. Thomson convincingly showed that the so-called. cathode rays represent a stream of charges. particles, which were later called electrons. In 1911 E. Rutherford, passing alpha particles from nature radioact. source through thin foil decomposition. substances, came to the conclusion that he would put. the charge in atoms is concentrated in compact formations - nuclei, and in 1919 he discovered protons - particles with a unit positive - among particles knocked out of atomic nuclei. charge and mass 1840 times greater than the mass of the 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 the mass of a proton, but does not have electricity. charge. The discovery of the neutron completed the identification of particles that are the structural elements of atoms and their nuclei.
Conclusion about the existence of an el-magnetic particle. fields - the photon - originates from the work of M. Planck (M. Planck, 1900). To obtain a correct description of the radiation spectrum of an absolutely black body, Planck was forced to assume that the radiation energy is divided into parts. portions (quanta). Developing Planck's idea, A. Einstein in 1905 suggested that el-magn. radiation is a flow of quanta (photons) and on this basis explained the laws of the photoelectric effect. Direct experiments. evidence of the existence of the photon was given by R. Millikan in 1912-15 when studying the photoelectric effect and by A. Compton in 1922 when studying the scattering of gamma quanta by electrons (see. Compton effect).
The idea of the existence of a neutrino, a particle that interacts extremely weakly with matter, belongs to W. Pauli (W. Pauli, 1930), who pointed out that such a hypothesis eliminates difficulties with the law of conservation of energy in the beta decay processes of radioacts. cores. The existence of neutrinos was experimentally confirmed by studying the process of inverse beta decay only in 1956 [F. F. Reines and C. Cowan].
From the 30s to the beginning. 50s the study of E. h. was closely related to the study cosmic rays. In 1932, as part of the space mission. rays by C. Anderson was discovered positron(e +) - a particle with the mass of an electron, but with a positive electric. charge. The positron was the first to be discovered antiparticle. The existence of the positron follows directly from the relativistic theory of the electron, developed by P. Dirac in 1928-31 shortly before the discovery of the positron. In 1936, Anderson and S. Neddermeyer discovered during space exploration. rays, muons (both signs of electric charge) are particles with a mass of approximately 200 masses of an electron, but otherwise surprisingly close to it in properties.
In 1947 also in space. rays by S. Powell's group were 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 proposed by H. Yukawa in 1935.
Con. 40s-early 50s 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-hyperons - were discovered in space. rays, subsequent discoveries of strange particles were made on charged particle accelerators- installations that create intense flows of high-energy protons and electrons. When accelerated protons and electrons collide with matter, they give birth to new electron particles, which are then recorded using complex detectors.
From the beginning 50s accelerators have become the main tool for studying E. h. In the 90s. Max. The energies of particles accelerated at accelerators amounted to hundreds of billions of electronvolts (GeV), and the process of increasing energies continues. The desire to increase the energies of accelerated particles is due to the fact that this path opens up opportunities for studying the structure of matter at shorter distances, the higher the energy of colliding particles, as well as the possibility of the birth of increasingly heavier 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 commissioning of proton accelerators with energies of billions of eV made it possible to discover heavy antiparticles: antiproton (1955), antineutron(1956), anti-sigmagi-peron (I960). In 1964, the heaviest particle from the group of hyperons - W - (with a mass of about two times the mass of a proton) was discovered.
Since the 60s. With the help of accelerators, a large number of extremely unstable (compared to other unstable electron particles) particles, called particles, have been identified. resonances. Most masses exceed the mass of a proton. [The first of them, D (1232), which decays into a p-meson and a nucleon, has been known since 1953.] It turned out that resonances are the main component. part of E. h.
In 1974, massive (3-4 proton masses) and at the same time relatively stable psi particles were discovered, with a lifetime approximately 10 3 times longer than the lifetime typical of resonances. They turned out to be closely related to the new family of E. charmed particles, the first representatives of which (D-mesons, L With-baryons) were discovered in 1976.
In 1977, even heavier (about 10 proton masses) upsilon particles were discovered, as well as psi particles, which were anomalously stable for particles of such large masses. They heralded the existence of another unusual family of lovely, or beautiful, particles. Its representatives - B-mesons - were discovered in 1981-83, L b-baryons - in 1992.
In 1962 it was found that in nature there is not one type of neutrino, but at least two: electron v e and muon v m. 1975 brought the discovery of the t-lepton, a particle almost 2 times heavier than the proton, but otherwise replicating the properties of the electron and muon. It soon became clear that another type of neutrino was associated with it v T.
Finally, in 1983, during experiments at the proton-antiproton collider (an installation for carrying out colliding beams of accelerated particles), the heaviest known electron particles were discovered: charged intermediate bosons W b (m W 80 GeV) and a neutral intermediate boson Z 0 (m Z = 91 GeV).
Thus, in almost 100 years since the discovery of the electron, a huge number of different microparticles of matter have been discovered. The world of E. h. turned out to be quite complex. Unexpected in plural. relations turned out to be the properties of the discovered E. parts. To describe them, in addition to the characteristics borrowed from the classical. physics, such as electrical charge, mass, angular momentum, it was necessary to introduce many new specials. characteristics, in particular to describe strange, enchanted and charming (beautiful) E. h.- weirdness[TO. Nishijima (K. Nishijima), M. Gell-Mann (M. Gell-Mann), 1953], Charm[J. Bjorken (J. Bjorken), Sh. Glashow (Sh. Glashow), 1964], beauty. The names of the given characteristics already reflect the unusual nature of the properties they describe.
Studying internal From its first steps, the 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 classical laws. mechanics and that they required completely new theoretical theories for their description. constructions. Such new theories were, first of all, particular (special) relativity theory(Einstein, 1905) and quantum mechanics(H. Bohr, L. de Broglie, W. Heisenberg, E. Schrödinger, M. Born; 1924-27). 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, it turned out to be insufficient to describe the processes occurring with E. h. The next step was needed - quantization of classical. fields (so-called secondary quantization) and development quantum field theory. The most important stages on the path of its development were: formulation quantum electrodynamics(Dirac, 1929), quantum theory of beta decay [E. Fermi (E. Fermi), 1934] - predecessors of modern. phenomenological theory of weak interactions, quantum mesodynamics (X. Yukawa, 1935). This period ended with the creation of a succession. will calculate. apparatus of quantum electrodynamics [S. Tomona-ga (S. Tomonaga), P. Feynman (R. Feynman), J. Schwinger (J. Schwinger); 1944-49], based on the use of technology renormalization This technique was later generalized to other variants of quantum field theory.
A significant stage in the subsequent development of quantum field theory was associated with the development of ideas about the so-called. calibration fields or Young - Mills fields(C. Young, P. Mills, 1954), which made it possible to establish the relationship between properties symmetry interactions with fields. The quantum theory of gauge fields is currently the basis for describing the interactions of electron particles. This theory has a number of serious successes, and yet it is still very far from complete and cannot yet claim to be a comprehensive theory of electron particles. More may be needed more than one restructuring of all ideas and a much deeper understanding of the relationship between the properties of microparticles and the properties of space-time before such a theory is built.
Basic properties of elementary particles. Interaction classes
All E. h are objects of exceptionally small masses and sizes. For most of them, the masses m are of 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, p- and K-mesons are equal in order of magnitude to 10 -13 cm (see. "Size" of an elementary particle). It was not possible to determine the sizes of the electron and muon; it is only known that they are less than 10 -16 cm. Microscopic. The masses and dimensions of electron particles underlie the quantum specificity of their behavior. Characteristic wavelengths that should be attributed to electron particles in quantum theory (= /tc-Compton wavelength), in order of magnitude are close to the typical sizes on which their interaction occurs (for example, for the p-meson /ts 1.4 10 -13 cm). This leads to the fact that quantum laws are decisive in the behavior of electron particles.
Naib. An important quantum property of all electrons is their ability to be born and destroyed (emitted and absorbed) when interacting with other particles. In this respect they are completely analogous to photons. E. h. is specific. quanta of matter, more precisely - quanta of the corresponding physical fields. 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+pp+ n + p +) or the process of an electron and a positron, when instead of disappeared particles, for example, two g-quanta appear (e + +e - g+ g). But also the processes of elastic scattering of particles, for example. e - +p- >
e - + p, are also associated with the absorption of the beginning. 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 is similar to the decay of an excited atom into a base. state and photon. Examples of decays of electron particles include (the “tilde” sign above the particle symbol here and in what follows corresponds to the antiparticle).
Diff. processes with electron particles at relatively low energies [up to 10 GeV in the center of mass system (c.m.)] differ noticeably in the intensity of their occurrence. In accordance with this, the interactions of the E. particles that generate them can be phenomenologically divided into several. classes: strong force, electromagnetic force And weak interaction All E. h. have, in addition, gravitational interaction.
Strong interaction is distinguished as an interaction that is responsible for processes involving electron particles that occur with the greatest intensity compared to other processes. It leads to the strongest bond of the electron element. It is the strong interaction that determines the bond of protons and neutrons in the nuclei of atoms and ensures exclusion. the strength of these formations, which underlies the stability of matter under terrestrial conditions.
El-magn. interaction is characterized as interaction, the basis of which is the connection with the electric magnet. field. The processes caused by it are less intense than the processes of strong interaction, and the connection between the electron forces generated by it is noticeably weaker. El-magn. interaction, in particular, is responsible for the processes of photon emission, for the connection of atomic electrons with nuclei and the connection of atoms in molecules.
Weak interaction, as the name itself shows, weakly affects the behavior of electron particles or causes very slowly occurring processes of change in their state. This statement can be illustrated, for example, by the fact that neutrinos, participating only in weak interactions, freely penetrate, for example, the thickness of the Earth and the Sun. Weak interaction is responsible for the relatively slow decays of the so-called. quasi-stable electron particles. As a rule, the lifetimes of these particles lie in the range of 10 -8 -10 -12 s, while typical transition times for the strong interaction of electron particles are 10 -23 s.
Gravity interactions that are well known for their macroscopic nature. manifestations, in the case of E. particles, due to the extreme smallness of their masses at characteristic distances of ~10 -13 cm, give exceptionally small effects. They will not be discussed further (except for Section 7).
"Strength" decomp. classes of interactions can be approximately characterized by dimensionless parameters associated with the squares of the corresponding interaction constants. For strong, el-magnetic, weak and gravitational. interactions of protons at process energies of ~ 1 GeV BC. c. m. these parameters correlate as 1:10 -2:10 -10:10 -38. The need to indicate cf. energy of the process is associated with the fact that in phenomenological. theory of weak interaction, the dimensionless parameter depends on energy. In addition, the intensity of decomposition processes depend very differently on energy, and the phenomenological theory of weak interaction at high energies M W in the village c. m. ceases to be fair. All this leads to what relates. role diff. interactions, generally speaking, changes with increasing energy of interacting particles, and the division of interactions into classes, based on a comparison of the intensities of processes, is reliably carried out at not too high energies.
According to modern ideas, at energies higher M W(i.e. 80 GeV in c.m.) weak and el-magnetic. interactions are compared in strength and act as a manifestation of a single electroweak interaction. An attractive assumption has also been put forward about the possible alignment of the constants of all three types of interactions, including strong ones, at ultra-high energies greater than 10 16 GeV (the so-called model). Great Unification).
Depending on their participation in certain types of interactions, all electron particles studied, with the exception of the photon, W- and Z-bosons are divided into two main ones. groups: hadrons And leptons. Hadrons are characterized primarily by the fact that they participate in the strong interaction, along with the electromagnetic and weak interactions, while leptons participate only in the electromagnetic and weak interactions. (The presence of gravitational interaction common to both groups is implied.) The hadron masses are close in order of magnitude to the proton mass ( T R )
, sometimes exceeding it by several. once; min. The p-meson has mass among hadrons: T p 1 /
7 m p , . The masses of leptons known before 1975-76 were small (0.1 m p) - hence their name. However, more recent data indicate the existence of heavy t-leptons with a mass of ca. two proton masses.
Hadrons are the most extensive group of known electron particles. It includes all baryons and mesons, as well as the so-called. resonances (i.e., most of the mentioned 350 E. hours). As already indicated, these particles have a complex structure and in fact cannot be considered as elementary. Leptons are represented by three charged (e, m, m) and three neutral particles ( v e, v m, v T). Photon, W +
and Z 0 -bosons together form an important group of gauge bosons that carry out the transfer of the electron-weak interaction. The elementarity of particles from these last two groups has not yet been seriously doubted.
Characteristics of elementary particles
Each element, along with the specificity of its inherent interactions, is described by a set of discrete values of the definition. physical quantities or their characteristics. In a number of cases, these discrete values are expressed through integer or fractional numbers and a certain common factor - a unit of measurement; these numbers are spoken of as quantum numbers E. h. and set only them, omitting the units of measurement.
General characteristics of all E. h - mass ( T), lifetime (t), spin ( J) and electric charge ( Q).
Depending on the lifetime, electron particles are divided into stable, quasi-stable, and unstable (resonances). Stable, within the limits of modern accuracy. measurements are electron (t>2 · 10 22 years), proton (t>5 · 10 32 years), photon and all types of neutrinos. Quasi-stable particles include particles that disintegrate due to electric magnetism. and weak interactions. Their lifetimes range from 900 s for a free neutron to 10 -20 s for an S 0 hyperon. Resonances are called Electron particles that decay due to strong interactions. Their characteristic lifetimes are 10 -22 -10 -24 s. In table 1 they are marked with * and instead of m a more convenient value is given: resonance width Г=/т.
Spin E. h. J 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 all leptons J= 1/2, at the photon, W b- and Z-bosons J= 1. There are particles with high spin. The magnitude of the spin of an electron particle determines the behavior of an ensemble of identical (identical) particles or their statistics (Pauli, 1940). The particles of the half-integer spin obey 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 such permutations) and, therefore, “prohibits” two particles of half-integer spin from being in the same state ( Pauli's principle). Particles of the whole spin obey Baze - Einstein statistics(hence the name bosons), which requires a wave function with respect to permutations of particles and allows any number of particles of a whole spin to be in the same state. Statistical The properties of E. particles turn out to be significant in cases where several particles are formed during birth or decay. identical particles.
Note: Particles are marked with * on the left (as a rule, resonances), for which instead of time life t the width Г=/t is given. True NeutralThese particles are placed in the middle between the particles and antiparticles. Members of one isotopic multibraids are located on one line (in those cases, when the characteristics of each member of the multi are knownbraid - with a slight vertical displacement). Izmemissing parity sign P for antibaryons is not indicated, equalbut like changing signs S, C, b y all antiparticles. For leptons and intermediate bosons, the internal parity is not exact (conserving) quantumnumber and therefore not indicated. Numbers in brackets at the end of the given physical quantities they denote existing error in the meaning of these quantities, relating to the last of the given figures.
Electric the charges of the studied electron particles (except for ) are integer multiples of e= 1.6 10 -19 C (4.8 10 -10 CGS), called. elementary electric charge. In known E. h. Q = 0, +
1, b2.
In addition to the indicated quantities, electron particles are additionally characterized by a number of quantum numbers, called. "internal". Leptons carry specific lepton number (L)three types: electronic L e, equal to +1 for e - And v e, muonic L m equal to +1 for m - and v m, and L t equal to +1 for t - and v t.
For hadrons L= 0, and this is another manifestation of their difference from leptons. In turn, that means. parts of hadrons should be attributed to the so-called. baryon number B (|B| = I )
. Hadrons with B=+ 1 form a subgroup of baryons (this includes the proton, neutron, hyperons; charmed and lovely baryons; baryon resonances), and hadrons with B= 0 - a subgroup of mesons (p-mesons, K-mesons, charmed and charming mesons, bosonic resonances). Name subgroups of hadrons come from the Greek. words baruV - heavy and mEsoV - medium, which is at the beginning. stage of research E. h. reflected comparison. the mass values of the then known baryons and mesons. Later data showed that the masses of baryons and mesons are comparable. For leptons B=0. For a photon, W b- and Z-bosons B= 0 and L= 0.
The studied baryons and mesons are divided into the already mentioned aggregates: ordinary (non-strange) particles (proton, neutron, p-mesons), strange particles (hyperons, K-mesons), charmed and charming particles. This division corresponds to the presence of special quantum numbers in hadrons: strangeness S, charms C and charms (beauty) b with acceptable values (modulo) 0, 1, 2, 3. For ordinary particles S=C= b=0, for strange particles S 0,C= b= 0, for charmed particles C0, b= 0, and for the lovely ones b O. Along with these quantum numbers, the quantum number is also often used hypercharge Y=B+S+C + b, which apparently has more funds. meaning.
Already the first studies of ordinary hadrons revealed the presence among them of families of particles that are similar in mass and with very similar properties with respect to the strong interaction, but with different characteristics. electrical values charge. The proton and neutron (nucleons) were the first example of such a family. Such families were later discovered among the strange, enchanted and lovely hadrons. The commonality of the properties of particles included in such families is a reflection of the existence of the same quantum number in them - isotopic spin I, which, like an ordinary spin, accepts integer and half-integer values. The families themselves are usually called isotopic multiplets. Number of particles in a multiplet n associated with I ratio n = 2I+1. Particles of the same isotopic multiplets differ from each other in the value of the “projection” of the isotopic. back I 3 and corresponding values Q are given by the expression
An important characteristic of hadrons is internal parity P, associated with the operation of spaces. inversions: P takes values +
1.
For all electron numbers with non-zero values of at least one of the quantum numbers Q, L, B, S, C, b there are antiparticles with the same mass values T, lifetime t, spin J and for hadrons isotopic. back I, but with opposite signs of the indicated quantum numbers, and for baryons with the opposite sign internal. parity R. Particles that do not have antiparticles are called. true neutral particles. Truly neutral hadrons have special properties. - charge parity(i.e. parity with respect to the charge conjugation operation) C with values +
1; examples of such particles are p 0 - and h-mesons (C = +1), r 0 - and f-mesons (C = -1), etc.
Quantum numbers of quantum numbers are divided into precise (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 a number of processes). Spin J is associated with a strict conservation law and is therefore an exact quantum number. Another exact quantum number is electric. charge Q. Within the limits of the accuracy of the measurements, quantum numbers are also preserved B And L, although there are no serious theoretical theories for this. prerequisites. Moreover, the observed baryon asymmetry of the Universe max. can naturally be interpreted under the assumption of violation of baryon number conservation IN(A.D. Sakharov, 1967). Nevertheless, the observed stability of the proton is a reflection of the high degree of conservation accuracy B And L(no, for example, decay pe + + p 0). The decays m - e - +g, m - m - +g, etc. are also not observed. However, most hadron quantum numbers are inaccurate. Isotopic spin, while conserved in the strong interaction, is not conserved in the el-magn. and weak interactions. Strangeness, charm and charm are preserved in the strong and el-magnetic. interactions, but are not conserved in weak interactions. The weak interaction also changes the internal and charge parity of the set of particles participating in the process. Combined parity is preserved with a much greater degree of accuracy CP (CP parity), however, it is also violated in certain processes caused by. Reasons causing non-preservation of plurals. quantum numbers of hadrons are not clear and, apparently, are related both to the nature of these quantum numbers and to the deep structure of the weak interaction.
In table 1 shows the maximum well-studied electron particles from groups of leptons and hadrons and their quantum numbers. In special group, gauge bosons are identified. Particles and antiparticles are given separately (change P not indicated for antibaryons). True neutral particles are placed in the center of the first column. Members of one isotopic multiplets are located in one line, sometimes with a slight offset (in cases where the characteristics of each member of the multiplet are given).
As already noted, the group of leptons is very small, and the particle masses are mainly. small. There are quite strict upper limits for the masses of all types of neutrinos, but what their true values are remains to be seen.
Basic part of the electron particles are hadrons. An increase in the number of known E. h. in the 60-70s. occurred solely due to the expansion of this group. Hadrons are mostly represented by resonances. Noteworthy is the tendency for spin to increase as the resonance mass increases; it can be clearly seen in different directions. groups of mesons and baryons with given I,
S and C. It should also be noted that strange particles are somewhat more massive than normal particles, charmed particles are more massive than strange particles, and charming particles are more massive than charmed particles.
Classification of elementary particles. Quark model of hadrons
If the classification of gauge bosons and leptons does not cause any special problems, then a large number of hadrons already in the beginning. 50s was 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. Isotopic selection hadron multiplets was the first step on this path. With math. point of view, grouping of hadrons into isotopes. multiplets reflect the presence of symmetry in the strong interaction associated with group rotation, more formally, with a unitary group S.U.(2) - a group of transformations in a complex two-dimensional space [see. Symmetry SU ( 2
)]
. It is assumed that these transformations act in some specific way. internal space - so-called isotopic space different from normal. Existence of isotopic space manifests itself only in the observable properties of symmetry. On math. isotopic language multiplets are irreducible group submissions symmetry S.U. (2).
The concept of symmetry as a factor determining the existence of various. groups and families of E. h. 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 combine certain groups of particles, are associated with special. types of symmetry arising due to the freedom of transformations into special internal ones. spaces. This is where the name comes from. "internal quantum numbers".
A careful examination shows that strange and ordinary hadrons together form broader associations of particles with similar properties than isotopic ones. multiplets. They are usually called supermultiplets. The number of particles included in the observed supermultiplets is 8 and 10. From the point of view of symmetry, the emergence of supermultiplets is interpreted as a manifestation of the existence of a symmetry group for the strong interaction wider than the group SU( 2)
, namely the unitary group S.U.(3) - transformation groups in three-dimensional complex space [Gell-Man, Y. Neeman, 1961]; cm. SU(3) symmetry. The corresponding symmetry is called unitary symmetry. Group S.U.(3) has, in particular, irreducible representations with the number of components 8 and 10, which can be compared to the observable supermultiplets: octet and decuplet. Examples of supermultiplets are the following groups of particles with the same values JP(i.e. with the same pairs of values J And P):
Unitary symmetry is less accurate than isotopic symmetry. 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 large masses. At large masses, when there are many different types. particles with similar masses, this division is more difficult to implement.
Detection of selected supermultiplets of fixed dimensions among hadrons, corresponding to the definition. representations of a unitary group S.U.(3), was the key to the most important conclusion about the existence of special structural elements in hadrons - quarks.
The hypothesis that the observed hadrons are built from particles of an unusual nature - quarks carrying spin 1 /
2, which have a strong interaction, but at the same time do not belong to the class of hadrons, was put forward by G. Zweig and independently by Gell-Mann in 1964 (see. Quark models). The idea of quarks was suggested by Mathematics. structure of representations of unitary groups. Ma-them. formalism opens up the possibility of describing all representations of a group SU(n) (and, consequently, all hadron multiplets associated with it) based on multiplication of the simplest (fundamental) representation of the group containing n component. It is only necessary to assume the existence of special particles associated with these components, which was done by Zweig and Gell-Mann for the special case of the group SU( 3)
. These particles were called quarks.
The specific quark composition of mesons and baryons was deduced from the fact that mesons, as a rule, are included in supermultiplets with the number of particles equal to 8, and baryons - 8 and 10. This pattern is easily reproduced if we assume that mesons are composed of quarks and antique, symbolically: M=(q)
, and the baryon is made of three quarks, symbolically: B = (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.
Thus, revealed by experiments in the 60s. the existence of supermultiplets composed of ordinary and strange hadrons led to the conclusion that all these hadrons are built from 3 quarks, usually denoted u, d, s(Table 2). The entire set of facts known at that time was in perfect agreement with this proposal.
Table 2.-Characteristics of quarks
*Preliminary experimental evaluation.
The subsequent discovery of psi particles, and then upsilon particles, charmed and lovely hadrons showed that to explain their properties three quarks are not enough and it is necessary to admit the existence of two more types of quarks c And b, carrying new quantum numbers: charm and beauty. This circumstance did not, however, shake the basic principles of the quark model. In particular, the center was preserved. point in her diagram of the structure of hadrons: M=(q), B = (qqq). Moreover, it was precisely on the basis of the assumption of the quark structure of psi- and upsilon particles that it was possible to give physical results. interpretation of their largely unusual properties.
Historically, the discovery of psi- and upsilon particles, as well as new types of charmed and charming hadrons, was an important stage in the establishment of ideas about the quark structure of all strongly interacting particles. According to modern theoretical models (see below), one should expect the existence of one more - sixth t-quark, which was discovered in 1995.
The above quark structure of hadrons and math. properties of quarks as objects associated with foundations. presentation of the group SU(n), lead to the following quantum numbers of quarks (Table 2). The unusual (fractional) electrical values are noteworthy. charge Q, and IN, not found in any of the studied electron particles. With index a for each type of quark q i (i= 1, 2, 3, 4, 5, 6) a special characteristic of quarks is associated - color, which is not present in the observed hadrons. Index a takes values 1, 2, 3, i.e., each type of quark ( q i)represented in three varieties q a i. The quantum numbers of each type of quark do not change when the color changes, so table. 2 applies to quarks of any color. As was shown later, the quantities q a (for each i) when a changes from the point of view of their transformation. properties should be considered as components of the fund. presentation of another group S.U.(3), color, operating in three-dimensional color space [see. SU color symmetry(3)].
The need to introduce color follows from the requirement of antisymmetry of the wave function of the system of quarks forming baryons. Quarks, as particles with spin 1/2, must obey Fermi-Dirac statistics. Meanwhile, there are baryons composed of three identical quarks with the same spin orientation: D ++ (), W - (), which are clearly symmetrical with respect to the permutations of quarks, if the latter do not have complementarity. degree of freedom. This will complement. the degree of freedom is color. Taking color into account, the required antisymmetry is easily restored. The refined parameters of the structural composition of mesons and baryons look like this:
where e abg is a completely antisymmetric tensor ( Levi-Chi-vita symbol)(1/
1/
-normalization factors). It is important to note that neither mesons nor baryons carry color indices (have no color) and are, as is sometimes said, “white” particles.
In table 2 shows only the “effective” quark masses. This is due to the fact that quarks in a free state, despite numerous careful searches for them, have not been observed. This, by the way, reveals another feature of quarks as particles of a completely new, unusual nature. Therefore, there is no direct data on the masses of quarks. There are only indirect estimates of the masses of quarks, which can be extracted from their decomposition. dynamic manifestations in the characteristics of hadrons (including the masses of the latter), as well as in decomp. processes occurring with hadrons (decays, etc.). For the mass t-quark is given a preliminary experiment. grade.
All the diversity of hadrons arises due to decomposition. combinations i-, d-, s-, s- And b-quarks forming bound states. Ordinary hadrons correspond to bound states constructed only from And- And d-quarks [for mesons with the possible participation of combinations ( s.), (With) And ( b)]. Presence in a bound state, along with u- And d-quarks, one s-, s- or b-quark means that the corresponding hadron is strange ( S= - 1), enchanted (C= + 1) or charming ( b= - 1). A baryon may contain two or three s-quark (respectively With- And b-quark), i.e., double and triple strange (charmed, charming) baryons are possible. Combinations of various types are also acceptable. numbers s- And With-, b-quarks (especially in baryons), which correspond to “hybrid” forms of hadrons (strangely charmed, strangely charming). Obviously, the more s-, s- or b-quarks the hadron contains, the more massive it is. If we compare the ground (non-excited) states of hadrons, this is exactly the picture that is observed (Table 1).
Since the spin of quarks is 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 basic. states, the meson spin values should be 0 or 1 (for antiparallel and parallel orientation of quark spins), and the baryon spin: 1 /
2
or 3/2 (for spin configurations
And
). Taking into account the fact that internal the 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 + . It is these values that are observed for hadrons that have the smallest mass at given values I And S, WITH, b.
As an illustration in table. 3 and 4 show the quark composition of mesons with JP= 0 - and baryons J P = 1 / 2 +
(the necessary summation over quark colors is assumed throughout).
Table 3.- Quark composition of the studied mesons With JP=0 - ()
Table 4.- Quark composition of the studied baryons With JP= 1/2 + ()
Note: The symbol () means symmetrization with respect to variable particles; symbol -antisymmetrization.
Thus, the quark model of natural explains the origin of the main groups of hadrons and their observed quantum numbers. A more detailed dynamic consideration also allows one to draw a number of useful conclusions regarding the relationship of masses within the decomposition. families of hadrons.
Correctly conveying the specificity of hadrons with the smallest masses and spins, the quark model of naturals. also 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 different types. excited states of quark systems. All excited states of quark systems are unstable relative to fast transitions due to strong interactions in underlying states. They form the basis. part 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 rotation. movement 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 P system, in the second case the increase in mass occurs without change JP .
When formulating the quark model, quarks were considered as hypothetical. structural elements that open up the possibility of a very convenient description of hadrons. In subsequent years, 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 on nucleons at very large angles. These experiments (1968), reminiscent of the classic. Rutherford's experiments on the scattering of alpha particles on atoms revealed the presence of point charges inside the nucleon. formations (see The Partons). Comparison of the data from these experiments with similar data on neutrino scattering on nucleons (1973-75) allowed us to draw a conclusion about cf. the size of the square of the electrical charge of these point formations. The result was close to the expected fractional values (2 / 3) 2 e 2 and (1/3)2 e 2. The study of the process of hadron production during the annihilation of an electron and a positron, which supposedly goes through the following stages:
indicated the presence of two groups of hadrons, the so-called. jets (see Hadron jet), 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 in the intermediate state each type of quark is represented by three varieties, i.e., quarks are three-colored.
Thus, the quantum numbers of quarks, given on the basis of theoretical considerations, received a comprehensive experiment. confirmation. Quarks have actually acquired the status of new electron particles and are serious contenders for the role of true electron particles for strongly interacting forms of matter. The number of known types of quarks is small. Up to length<=10 -16
см кварки выступают как точечные бесструктурные образования. Бесструктурность
кварков, конечно, может отражать лишь достигнутый уровень исследования этих
материальных образований. Однако ряд специфич. особенностей кварков даёт известные
основания предполагать, что кварки являются частицами, замыкающими цепь структурных
составляющих сильновзаимодействующей материи.
Quarks differ from all other electron particles in that they apparently do not exist in a free state, although there is clear evidence of their existence in a bound state. This feature of quarks is most likely associated with the specifics of their interaction, generated by the exchange of special particles - gluons, leading to the fact that the forces of attraction between them do not weaken with distance. As a consequence, infinite energy is required to separate quarks from each other, which is obviously impossible (the theory of the so-called confinement or trapping of quarks; see Color retention).In reality, when trying to separate quarks from each other, a complement formation occurs. hadrons (so-called hadronization of quarks). The impossibility of observing quarks in a free state makes them a completely new type of structural units of matter. It is unclear, for example, whether in this case it is possible to raise the question of the constituent parts of quarks and whether the sequence of structural components of matter is thereby interrupted. All of the above leads to the conclusion that quarks, along with leptons and gauge bosons, which also have no observable signs of structure, form a group of electron particles, which have the greatest grounds to claim the role of true electron particles.
Elementary particles and quantum field theory. Standard interaction model
To describe the properties and interactions of E. h. in modern times. creature theories. What matters is the concept of a physical field, which is assigned to each particle. The field is specific. the form of matter distributed in space; it is described by a function that is specified at all points of space-time and has a definition. transformation properties with respect to transformations Lorenz group(scalar, spinor, vector, etc.) and "internal" groups. symmetries (isotopic scalar, isotopic spinor, etc.). El-magn. field having the properties of a four-dimensional vector A m ( x)(m= 1, 2, 3, 4) is historically the first example of physical. fields. The fields that are compared by E. particles are of a quantum nature, that is, their energy and momentum are composed of many separate parts. portions - quanta, and the total energy e k and momentum p k quantum are related by the relation special. theory of relativity: e 2 k =p 2 k s 2 + t 2 With 4 . Each such quantum is an electron particle with mass T, with a given energy e k and impulse p k. El-magnetic quanta fields are photons, quanta of other fields correspond to all other known electron particles Ma-themes. The apparatus of quantum field theory (QFT) makes it possible to describe the birth and destruction of a particle at each space-time point.
Transformation the properties of the field determine the main quantum numbers of E. particles. Transformation properties in relation to transformations of the Lorentz group are determined by the spin of particles: the scalar corresponds to the spin J= 0, spinor- spin J= 1 /
2, vector - spin J= 1, etc. Transformation properties of fields in relation to "internal" transformations spaces (“charge space”, “isotopic space”, “unitary space”, “color space”) determine the existence of such quantum numbers as L, B, I, S, WITH, b, and for quarks and gluons also colors. Introduction "internal" spaces in the apparatus of theory 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. particle, is actually more than four - i.e. greater than the dimension of space-time, characteristic of all macroscopic. physical processes.
The mass of E. particles is not directly related to transformation. properties of fields. This is an additional characteristic of them, the origin of the cut is not fully understood.
To describe the processes occurring with electron particles, QFT uses Lagrangian formalism.IN Lagrangians, constructed from the fields involved in the interaction of particles, contains all the information about the properties of particles and the dynamics of their behavior. The Lagrangian includes two chapters. terms: Lagrangian, which describes the behavior of free fields, and the interaction Lagrangian, which reflects the interrelationship of decomp. fields and the possibility of converting E. h. Knowledge of the exact form allows, in principle, using the apparatus scattering matrices (S-matrices), calculate the probabilities of transitions from the initial set of particles to a given final set of particles, occurring under the influence of the interaction existing between them. Thus, the establishment of a structure that opens up the possibility of quantities. descriptions of processes with E. h. is one of the centers. CTP problems.
Creatures progress in solving this problem was achieved in the 50-70s. based on the development of the idea of vector gauge fields formulated in the already mentioned work of Yang and Mills. Based on the well-known proposition that every experimentally observed conservation law is associated with the invariance of the Lagrangian describing the system with respect to transformations of a certain symmetry group ( Noether's theorem), Yang and Mills demanded that this invariance be carried out locally, that is, take place for an arbitrary dependence of the transformations on a point in space-time. It turned out that the fulfillment of this requirement, which is physically related to the fact that the interaction cannot be instantly transmitted from point to point, is possible only by introducing a special type into the structure of the Lagrangian. gauge fields of vector nature, def. transforming under transformations of the symmetry group. Moreover, the structures of the free Lagrangian turned out to be closely related in this approach: knowledge in means. to a certain extent predetermined the appearance
The latter circumstance is due to the fact that the requirement of local gauge invariance can be performed only if in all derivatives acting on free fields in , the replacement is made 
Here g- interaction constant; V a m - gauge fields; T a - generators of the symmetry group in the matrix representation corresponding to the free field; r- group size.
Due to the above, strictly defined terms automatically appear in the modified Lagrangian. structures that describe the interaction of the fields originally included in , with the newly introduced gauge fields. In this case, gauge fields act as carriers of interaction between the original fields. Of course, since new gauge fields have appeared in the Lagrangian, the free Lagrangian must be supplemented with a term associated with them and undergo the modification procedure described above. If gauge invariance is strictly observed, gauge fields correspond to bosons with zero mass. When symmetry is broken, the boson mass is nonzero.
In this approach, the task of constructing a Lagrangian reflecting the dynamics of interacting fields essentially comes down to the correct selection of the system of fields that make up the initial free Lagrangian and fixing its form. The latter, however, with given transformation properties with respect to the Lorentz group, is uniquely determined by the requirement of relativistic invariance and the obvious requirement of the inclusion of only structures that are quadratic in the fields.
Thus, the main question for describing the dynamics is the choice of the system of primary fields that form, i.e., in fact, the same center. question of physics E. ch.: “Which particles (and, accordingly, fields) should be considered the most fundamental (elementary) when describing observable particles of matter?”
Modern the theory, as already noted, identifies structureless particles with spin 1/2 as such particles: quarks and leptons. This choice allows, based on the principle of local gauge invariance, to construct a very successful scheme for describing the strong and weak interactions of electron particles, which is called. STANDARD MODEL.
The model is based primarily on the assumption that for the strong interaction there is exact symmetry SU c(3), corresponding to transformations in the “color” three-dimensional space. In this case, it is assumed that quarks are transformed according to the funds. representation of the group SU c(3). Fulfillment of the requirement of local gauge invariance for the quark Lagrangian leads to the appearance in the structure of the theory of eight massless gauge bosons, called gluons, interacting with quarks (and each other) in a strictly defined manner. way (Fritzsch, Goell-Man, 1972). The scheme for describing the strong interaction developed on this basis was called quantum chromodynamics. The correctness of her predictions has been confirmed numerous times. experiments, including convincing evidence of the existence of gluons. There are also serious reasons to believe that the apparatus of quantum chromodynamics contains an explanation of the phenomenon of confinement.
When constructing the theory of el-weak interaction, the fact was used that the existence of pairs of leptons with the same lepton number ( L e , L v , L t), but with different electrical charge (e - , v e; m - , v m; T - , v r) can be interpreted as a manifestation of symmetry associated with the so-called group. weak isospin S.U. sl (2), and the pairs themselves are considered as spinor (doublet) representations of this group. A similar interpretation is possible in relation to pairs of quarks participating in weak interaction. Note that consideration within the framework of this scheme of weak interaction with the participation of a quark b necessarily leads to the conclusion that it has an isotopic partner quark t, making up a pair ( t, b). Isolation by weak interaction is defined. helicity(left) for the fermions participating in it can additionally be considered as a manifestation of the existence of symmetry U cl (1), associated with weak hypercharge Y sl. In this case, the left and right fermions should be assigned different hypercharge values Y sl, and right-handed fermions should be considered as isotopic scalars. In the adopted construction, the relation naturally arises Q = I 3 cl + 1/2 Y sl, which we have already encountered among hadrons.
Thus, a careful analysis of the el-weak interaction of leptons and quarks makes it possible to reveal that they have a symmetry (noticeably broken, however), corresponding to the group S.U. sl (2) U cl (
1)
. If we ignore the violation of this symmetry and use the strict condition of local gauge invariance, then a theory of the weak interaction of quarks and leptons will arise, which involves four massless bosons (two charged and two neutral) and two interaction constants corresponding to the groups S.U. sl (2) and U sl (1). In this theory, the terms of the Lagrangian corresponding to the interaction with the charge. bosons, correctly reproduce the known structure charged currents, but do not provide the short-range action observed in weak processes, which is not surprising, since the zero mass of intermediate bosons leads to long-range action. It only follows that in realism. weak interaction theories, the masses of intermediate bosons must be finite. This is also in accordance with the fact that symmetry is broken S.U. sl (2) U sl (1).
However, the direct introduction of finite masses of intermediate bosons into the Lagrangian constructed in the manner described above is impossible, since it contradicts the requirement of local gauge invariance. It was possible to take into account symmetry breaking in a consistent manner and achieve the appearance of intermediate bosons in the theory of finite masses with the help of an important assumption about the existence in nature of special scalar fields F ( Higgs fields), interacting with fermionic and gauge fields and having a specific self-interaction leading to the phenomenon spontaneous symmetry breaking[P. Higgs (P. Higgs), 1964]. The introduction of one doublet (in the weak isospin group) of Higgs fields into the Lagrangian theory in the simplest version leads to the fact that the entire system of fields passes to a new, lower energy vacuum state corresponding to broken symmetry. If initially vacuum average from field F was equal to zero<Ф>0 = 0, then in a new state<Ф>0 = Ф 0 0. Violation of symmetry and the appearance in the theory of finite F 0 results due to Higgs mechanism to the non-vanishing mass of charge. intermediate bosons W +
and to the emergence of mixing (linear combination) of two neutral bosons appearing in the theory. As a result of mixing, a massless electric magnet arises. field interacting with electric magnet. current of quarks and leptons, and the field of a massive neutral boson Z 0 interacting with neutral current strictly defined structure. Blending parameter (angle) ( Weinberg corner)neutral bosons in this scheme is given by the ratio of group interaction constants U sl (l) and S.U. sl (2) :
tgq W =g"/g. The same parameter determines the mass connection mW And m Z (m Z = m W / cosq W) and electrical communication charge e s weak isospin group constant g:e = g sinq W. The discovery in 1973, while studying neutrino scattering, of neutral weak currents predicted by the scheme described above, and the subsequent discovery in 1983 W- and Z-bosons with masses of 80 GeV and 91 GeV, respectively, brilliantly confirmed the entire concept of a unified description of el-magn. and weak interactions. Let's experiment. determining the value of sin 2 q W= 0.23 showed that the constant g and electric charge e are close in size. It became clear that the “weakness” of the weak interaction at energies noticeably lower mW And m Z, mainly due to the large mass of intermediate bosons. Indeed, the constant of the phenomenological four-fermion theory of the weak Fermi interaction G F in the above diagram it is equal to G F =g 2 /8m 2 W. This means that eff. weak interaction constant at energy in s. c. m. ~t r equal to G F m p 2
10 -5, and its square is close to 10 -10, i.e. to the value given above. At energies in cm, large or of the order mW, the only parameter characterizing weak interaction becomes the quantity g 2 /
4p or e 2 /
4p, i.e. weak and el-magn. interactions become comparable in intensity and must be considered together.
Construction of a unified description of el-magn. and weak interactions is an important achievement of the theory of gauge fields, comparable in importance to the development of Maxwell in the end. 19th century unified theory of el-magn. phenomena. Quantity The predictions of the theory of weak interaction in all measurements carried out were justified with an accuracy of 1%. Important physical a consequence of this construction is the conclusion about the existence in nature of a particle of a new type - neutral Higgs boson. At the beginning 90s no such particle was found. The searches showed that its mass exceeds 60 GeV. The theory does not, however, give an exact prediction for the mass of the Higgs boson. We can only say that its mass does not exceed 1 TeV. The estimated mass of this particle lies in the range of 300-400 GeV.
So, the “standard model” selects as a fund lady. particles three pairs of quarks ( and, d)(With, s) (t, b) and three pairs of leptons ( v e,e -)(v m ,m -) ( v t, m -), usually grouped according to the magnitude of their masses into families (or generations) as follows:
and postulates that their interactions satisfy the symmetry S.U. sl (3) S.U. sl (2) U sl (l). As a consequence, a theory is obtained in which the interaction carriers are gauge bosons: gluons, photons, W b and Z. And although the “standard model” copes very successfully with the description of all known facts related to E.H., nevertheless, most likely, it is an intermediate stage in the construction of a more perfect and comprehensive theory of E.H. In the structure of the “standard model” there are still quite a lot of arbitrary, empirically determined parameters (the values of the masses of quarks and leptons, the values of interaction constants, mixing angles, etc.). The number of fermion generations in the model is also not determined. So far, the experiment only confidently asserts that the number of generations does not exceed three, unless heavy neutrinos with masses of several exist in nature. tens of GeV.
From the point of view of the symmetry properties of interactions, it would be more natural to expect that in the comprehensive theory of E.H. instead of the direct product of symmetry groups, one symmetry group will appear G with one interaction constant corresponding to it. The symmetry groups of the “standard model” in this case could be interpreted as products of reduction of a large group when the symmetry associated with it is broken. On this path, in principle, the possibility of a Great Unification of Interactions could arise. The formal basis for such a combination can be the property of change with energy eff. interaction constants of gauge fields g i 2 /4p = a i (i=1, 2, 3), which arises when taking into account the higher orders of the theory (the so-called running constants). In this case, the constant a 1 is associated with the group U(I); a 2 - with group SU( 2);
a 3 -with group SU( 3)
. The very slow (logarithmic) changes mentioned are described by the expression
connecting the values of eff. constants a i(M) and a i(m) at two different energy values: M and m( M> m). The nature of these changes is different for different types. symmetry groups (and, therefore, various interactions) and is given by the coefficients b i, incorporating information both about the structure of symmetry groups and about the particles participating in the interaction. Because the b 1 , b 2 and b 3 are different, it is possible that, despite noticeable discrepancies in the values of a i-1 (m) at the studied energies m, at very high energies M all three values of a i -1 (M)will coincide, i.e. the Great Unification of interactions will be realized. Careful analysis, however, showed that within the standard model, using known values of a i-1 (m), match all three values of a i -1 (M)at some large M impossible, i.e. The version of the theory with the Great Unification is not feasible in this model. At the same time, it was found that in schemes different from the standard model, with a changed composition of the basic. (fund.) fields or particles, the Great Unification may take place. Changes in the composition of the main particles lead to changes in the values of the coefficients " b i" and thus provide the possibility of matching a i (M) at large M.
The guiding idea when choosing a modified base composition. particle theory was the idea of the possible existence of E. particles in the world. supersymmetry, the edge establishes a definition. relationships between whole and half-integer spin particles that appear in the theory. To meet the requirements of supersymmetry, e.g. in the case of the standard model, each particle must be associated with a particle with a spin shifted by 1/2 - Moreover, in the case of exact supersymmetry, all these particles must have the same masses. Thus, quarks and leptons of spin 1/2 should be associated with their supersymmetric partners (superpartners) with spin zero, all gauge bosons with spin 1 with superpartners with spin 1/2, and the Higgs boson of spin zero with a superpartner with spin 1/ 2. Since superpartners of quarks, leptons and gauge bosons are certainly not observed in the energy region studied, supersymmetry, if it exists, should be noticeably broken, and the masses of superpartners should have values significantly exceeding the masses of known fermions and bosons.
A consistent expression of the requirements of supersymmetry is found in the minimal supersymmetric model (MCCM), in which, in addition to the already listed changes in the composition of particles of the standard model, the number of Higgs bosons increases to five (of which two are charged and three are neutral particles). Accordingly, five superpartners of Higgs bosons with spin 1/2 appear in the model - MCCM is the simplest generalization of the standard model to the case of supersymmetry. Meaning M, when a coincidence occurs i (M)(Grand Unification), in MCCM is approximately equal to 10 16 GeV.
One of the promising possibilities for the development of the theory of gauge fields is associated with the hypothesis of the existence of supersymmetry, which also resolves a number of its internal problems. problems associated with the stability of the parameters appearing in it. Supersymmetry, as noted, makes it possible to preserve in the theory of electron particles the attractive possibility of the Grand Unification of interactions. A decisive confirmation of the existence of supersymmetry would be the discovery of superpartners of known particles. Their masses are estimated to range from hundreds of GeV to 1 TeV. Particles of such masses will be available for study at the next generation of proton colliders.
Testing the hypothesis of the existence of supersymmetry and the search for supersymmetric particles is undoubtedly one of the most important tasks in the physics of elementary particles, which will undoubtedly receive priority attention in the near future.
Some general problems of the theory of elementary particles
The latest development of particle physics has clearly identified from all the microcomponents of matter a group of particles that play a special role and have the greatest grounds (at the beginning of the 90s) to be called truly electron particles. This includes foundations. spin 1 fermions /
2 - leptons and quarks, which make up three generations, and gauge bosons of spin 1 (gluons, photons and intermediate bosons), which are carriers of strong and weak interactions. A particle with spin 2 should most likely be added to this group, graviton, as a carrier of gravity. interaction that connects all particles. A special group consists of spin 0 particles, Higgs bosons, which, however, have not yet been discovered.
Many questions nevertheless remain unanswered. Thus, it remains unclear whether physical exists. a criterion that fixes the number of generations of elementary fermions. It is not clear how fundamental the difference in the properties of quarks and leptons is, associated with the presence of color in the former, or whether this difference is specific only to the energy region studied. Related to this question is the question of physical the nature of the Grand Unification, since in its formalism quarks and leptons are considered as objects with similar properties.
It is important to understand whether the existence of different "intrins." quantum numbers of quarks and leptons ( B, L, I, S, C, b etc.) to a more complex geometry of the microworld, corresponding to a greater number of dimensions than the four-dimensional geometry of the macroscopic world that is familiar to us. space-time. Closely related to this question is the question of what is the max. symmetry group G, which satisfy the interactions of elementary particles and into which are embedded symmetry groups that manifest themselves in the studied energy region. The answer to this question would help determine the limiting number of carriers of interaction between E. h. and clarify their properties. It is possible that the max. group G actually reflects the symmetry properties of a certain multidimensional space. This range of ideas is well-known reflected in the theory superstrings, which are analogues of ordinary strings in spaces with more than four dimensions (usually in a space of 10 dimensions). Superstring theory interprets electron particles as manifestations of specific excitations of superstrings, corresponding to various types. backs. It is believed that extra (beyond four) dimensions do not reveal themselves in observations due to the so-called. compactification, i.e. the formation of closed subspaces with characteristic dimensions of ~10 -33 cm. Ext. the manifestation of the existence of these subspaces are the observable “internal.” quantum numbers of electron particles. There is no data yet confirming the correctness of the approach to the interpretation of the properties of electron particles associated with the idea of superstrings.
As can be seen from the above, ideally, a complete theory of electron particles should not only correctly describe the interactions of a given set of particles selected as fundamental, but also contain an explanation of what factors determine the number of these particles, their quantum numbers, interaction constants, the values of their masses, etc. The reasons for the prominence of the most important ones must also be understood. wide symmetry group G and at the same time the nature of the mechanisms that cause the violation of symmetry as we move to lower energies. In this regard, clarification of the role of Higgs bosons in E.H. physics is of paramount importance. Models offered by modern The theory of E. h. is still far from satisfying all of the listed criteria.
The description of the interactions of electron particles, as already noted, is associated with gauge field theories. These theories have developed mathematics. a device that allows you to carry out calculations of processes with E.H. at the same level of rigor as in quantum electrodynamics. However, in the apparatus of gauge field theories, in its modern form. formulation, there is one being present. A flaw common to quantum electrodynamics is that in the process of calculations, meaningless infinitely large expressions appear in it. With the help of special method of redefining observable quantities (mass and interaction constants) - renormalization- manages to eliminate infinities from the endings. calculation results. However, the renormalization procedure is a purely formal bypass of the difficulties existing in the theoretical apparatus, although at some level of accuracy it can affect the degree of agreement between the predictions of the theory and measurements.
The appearance of infinities in calculations is due to the fact that in the interaction Lagrangians 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 in several ways. reasons:
a) true E. h., as carriers of finite mass, it is most natural to attribute, albeit very small, but finite dimensions if we want to avoid the infinite density of matter;
b) the properties of space-time at small distances are most likely radically different from its macroscopic properties. properties (starting from a certain characteristic distance, usually called fundamental length);
c) at the smallest distances (~ 10 -33 cm) geometric changes are affected. properties of space-time due to the influence of quantum gravitational effects (metric fluctuations; see Quantum theory of gravity).
Perhaps these reasons are closely related. So, it is taking into account gravitational effects max. naturally leads to the size of true E.h. about 10 -33 cm, and fundam. the length may actually coincide with the so-called. Planck length l Pl = 10 -33 cm, where x-gravity constant (M. Markov, 1966). Any of these reasons should lead to a modification of the theory and the elimination of infinities, although the practical implementation of this modification may be very difficult.
One of the interesting possibilities for consistently taking into account the effects of gravity is associated with the extension of the ideas of supersymmetry to gravitation. interaction (theory supergravity, especially extended supergravity). Joint accounting of gravitational and other types of interactions leads to a noticeable reduction in the number of divergent expressions in the theory, but whether supergravity leads to the complete elimination of divergences in calculations has not been strictly proven.
Thus, the logical conclusion of the ideas of the Great Unification will most likely be the inclusion of gravitational forces in the general scheme of considering the interactions of E. ch. interactions, taking into account which can be fundamental at very short distances. It is on the basis of simultaneous accounting of all types of interactions that the most It is likely to expect the creation of a future theory of E. h.
Lit.: Elementary particles and compensating fields. Sat. Art., trans. from English, M., 1964; Kokkede Ya., Theory of quarks, trans. from English, M.. 1971; Markov M. A., On the nature of matter, M., 1976; Gla-show Sh., Quarks with color and aroma, trans. from English. "UFN", 1976, vol. 119, v. 4, p. 715; Bernstein J., Spontaneous symmetry breaking, gauge theories, the Higgs mechanism, etc., in the book: Quantum theory of gauge fields. Sat. Art., trans. from English, M., 1977 (News of fundamental physics, v. 8); Bogolyubov N. N., Shirkov D. V., Quantum fields, 2nd ed., M., 1993; Okun L. B., Leptons and quarks, 2nd ed., M., 1990.
– material objects that cannot be divided into their component parts. In accordance with this definition, molecules, atoms and atomic nuclei that can be divided into component parts cannot be classified as elementary particles - an atom is divided into a nucleus and orbital electrons, a nucleus into nucleons. At the same time, nucleons, consisting of smaller and more fundamental particles - quarks, cannot be divided into these quarks. Therefore, nucleons are classified as elementary particles. Considering the fact that the nucleon and other hadrons have a complex internal structure consisting of more fundamental particles - quarks, it is more appropriate to call hadrons not elementary particles, but simply particles.
Particles are smaller in size than atomic nuclei. The dimensions of the nuclei are 10 -13 − 10 -12 cm. The largest particles (including nucleons) consist of quarks (two or three) and are called hadrons. Their dimensions are ≈ 10 -13 cm. There are also structureless (at the current level of knowledge) point-like (< 10 -17 см) частицы, которые
называют фундаментальными. Это кварки, лептоны, фотон и некоторые другие.
Всего известно несколько сот частиц. Это в подавляющем большинстве адроны.
Table 1
|
Fundamental fermions |
|||||||
|---|---|---|---|---|---|---|---|
|
Interactions |
Generations | Charge Q/e |
|||||
| leptons | ν e | ν μ | ν τ | ||||
| e | μ | τ | |||||
| quarks | c | t | +2/3 | ||||
| s | b | -1/3 | |||||
The fundamental particles are 6 quarks and 6 leptons (Table 1), having spin 1/2 (these are fundamental fermions) and several particles with spin 1 (gluon, photon, W ± and Z bosons), as well as a graviton (spin 2), called fundamental bosons (Table 2). Fundamental fermions are divided into three groups (generations), each of which contains 2 quarks and 2 leptons. All observable matter consists of particles of the first generation (quarks u, d, electron e -): nucleons are made of quarks u and d, nuclei are made of nucleons. Nuclei with electrons in orbits form atoms, etc.
table 2
| Fundamental Interactions | ||||
|---|---|---|---|---|
| Interaction | Field quantum | Radius, cm | Interaction constant (order of magnitude) |
Example manifestations |
| strong | gluon | 10 -13 | 1 | nucleus, hadrons |
| electromagnetic | γ-quantum | 10 -2 | atom | |
| weak | W ± , Z | 10 -16 | 10 -6 | γ decay |
| gravitational | graviton | ∞ | 10 -38 | gravity |
The role of fundamental bosons is that they realize the interaction between particles, being “carriers” of interactions. During various interactions, particles exchange fundamental bosons. Particles participate in four fundamental interactions - strong (1), electromagnetic (10 -2), weak (10 -6) and gravitational (10 -38). The numbers in parentheses characterize the relative strength of each interaction in the energy region less than 1 GeV. Quarks (and hadrons) participate in all interactions. Leptons do not participate in the strong interaction. The carrier of the strong interaction is the gluon (8 types), the electromagnetic interaction is the photon, the weak interaction is the W ± and Z bosons, and the gravitational interaction is the graviton.
The overwhelming number of particles in a free state is unstable, i.e. disintegrates. The characteristic lifetimes of particles are 10 -24 –10 -6 sec. The lifetime of a free neutron is about 900 seconds. The electron, photon, electron neutrino and possibly the proton (and their antiparticles) are stable.
The basis for the theoretical description of particles is quantum field theory. To describe electromagnetic interactions, quantum electrodynamics (QED) is used, weak and electromagnetic interactions are jointly described by a unified theory - the electroweak model (ESM), strong interaction - quantum chromodynamics (QCD). QCD and ESM, which together describe the strong, electromagnetic and weak interactions of quarks and leptons, form a theoretical framework called the Standard Model.
Further penetration into the depths of the microworld is associated with the transition from the level of atoms to the level of elementary particles. As the first elementary particle at the end of the 19th century. the electron was discovered, and then in the first decades of the 20th century. – photon, proton, positron and neutron.
After the Second World War, thanks to the use of modern experimental technology, and above all powerful accelerators, in which conditions of high energies and enormous speeds are created, the existence of a large number of elementary particles was established - over 300. Among them there are both experimentally discovered and theoretically calculated, including resonances, quarks and virtual particles.
Term elementary particle originally meant the simplest, further indecomposable particles that underlie any material formations. Later, physicists realized the entire convention of the term “elementary” in relation to micro-objects. Now there is no doubt that particles have one structure or another, but, nevertheless, the historically established name continues to exist.
The main characteristics of elementary particles are mass, charge, average lifetime, spin and quantum numbers.
Resting mass elementary particles are determined in relation to the rest mass of the electron. There are elementary particles that do not have a rest mass - photons. The remaining particles according to this criterion are divided into leptons– light particles (electron and neutrino); mesons– medium-sized particles with a mass ranging from one to a thousand electron masses; baryons– heavy particles whose mass exceeds a thousand electron masses and which includes protons, neutrons, hyperons and many resonances.
Electric charge is another important characteristic of elementary particles. All known particles have a positive, negative or zero charge. Each particle, except the photon and two mesons, corresponds to antiparticles with opposite charges. Around 1963–1964 a hypothesis was put forward about the existence quarks– particles with a fractional electric charge. This hypothesis has not yet been confirmed experimentally.
By lifetime particles are divided into stable And unstable . There are five stable particles: the photon, two types of neutrinos, the electron and the proton. It is stable particles that play the most important role in the structure of macrobodies. All other particles are unstable, they exist for about 10 -10 -10 -24 s, after which they decay. Elementary particles with an average lifetime of 10–23–10–22 s are called resonances. Due to their short lifetime, they decay before they even leave the atom or atomic nucleus. Resonant states were calculated theoretically; they could not be detected in real experiments.
In addition to charge, mass and lifetime, elementary particles are also described by concepts that have no analogues in classical physics: the concept back . Spin is the intrinsic angular momentum of a particle that is not associated with its movement. Spin is characterized by spin quantum number s, which can take integer (±1) or half-integer (±1/2) values. Particles with integer spin – bosons, with a half-integer – fermions. Electrons are classified as fermions. According to the Pauli principle, an atom cannot have more than one electron with the same set of quantum numbers n,m,l,s. Electrons, which correspond to wave functions with the same number n, are very close in energy and form an electron shell in the atom. Differences in the number l determine the “subshell”, the remaining quantum numbers determine its filling, as mentioned above.
In the characteristics of elementary particles there is another important idea interaction. As noted earlier, four types of interactions between elementary particles are known: gravitational,weak,electromagnetic And strong(nuclear).
All particles having a rest mass ( m 0), participate in gravitational interaction, and charged ones also participate in electromagnetic interaction. Leptons also participate in weak interactions. Hadrons participate in all four fundamental interactions.
According to quantum field theory, all interactions are carried out due to the exchange virtual particles , that is, particles whose existence can only be judged indirectly, by some of their manifestations through some secondary effects ( real particles can be directly recorded using instruments).
It turns out that all four known types of interactions - gravitational, electromagnetic, strong and weak - have a gauge nature and are described by gauge symmetries. That is, all interactions are, as it were, made “from the same blank.” This gives us hope that it will be possible to find “the only key to all known locks” and describe the evolution of the Universe from a state represented by a single supersymmetric superfield, from a state in which the differences between the types of interactions, between all kinds of particles of matter and field quanta have not yet appeared.
There are a huge number of ways to classify elementary particles. For example, particles are divided into fermions (Fermi particles) - particles of matter and bosons (Bose particles) - field quanta.
According to another approach, particles are divided into 4 classes: photons, leptons, mesons, baryons.
Photons (electromagnetic field quanta) participate in electromagnetic interactions, but do not have strong, weak, or gravitational interactions.
Leptons got their name from the Greek word leptos- easy. These include particles that do not have strong interaction: muons (μ – , μ +), electrons (е – , у +), electron neutrinos (v e – ,v e +) and muon neutrinos (v – m, v + m). All leptons have a spin of ½ and are therefore fermions. All leptons have a weak interaction. Those that have an electrical charge (that is, muons and electrons) also have an electromagnetic force.
Mesons – strongly interacting unstable particles that do not carry the so-called baryon charge. Among them is R-mesons, or pions (π + , π – , π 0), TO-mesons, or kaons (K +, K –, K 0), and this-mesons (η) . Weight TO-mesons is ~970me (494 MeV for charged and 498 MeV for neutral TO-mesons). Lifetime TO-mesons has a magnitude of the order of 10 –8 s. They disintegrate to form I-mesons and leptons or only leptons. Weight this-mesons is 549 MeV (1074me), the lifetime is about 10–19 s. This-mesons decay to form π-mesons and γ-photons. Unlike leptons, mesons have not only a weak (and, if they are charged, electromagnetic) interaction, but also a strong interaction, which manifests itself when they interact with each other, as well as during the interaction between mesons and baryons. All mesons have zero spin, so they are bosons.
Class baryons combines nucleons (p,n) and unstable particles with a mass greater than the mass of nucleons, called hyperons. All baryons have a strong interaction and, therefore, actively interact with atomic nuclei. The spin of all baryons is ½, so the baryons are fermions. With the exception of the proton, all baryons are unstable. During the decay of baryons, along with other particles, a baryon is necessarily formed. This pattern is one of the manifestations baryon charge conservation law.
In addition to the particles listed above, a large number of strongly interacting short-lived particles have been discovered, which are called resonances . These particles are resonant states formed by two or more elementary particles. The resonance lifetime is only ~ 10 –23 –10 –22 s.
Elementary particles, as well as complex microparticles, can be observed thanks to the traces that they leave as they pass through matter. The nature of the traces allows us to judge the sign of the particle’s charge, its energy, momentum, etc. Charged particles cause ionization of molecules along their path. Neutral particles do not leave traces, but they can reveal themselves at the moment of decay into charged particles or at the moment of collision with any nucleus. Consequently, neutral particles are ultimately also detected by the ionization caused by the charged particles they generate.
Particles and antiparticles. In 1928, the English physicist P. Dirac managed to find a relativistic quantum mechanical equation for the electron, from which a number of remarkable consequences follow. First of all, from this equation the spin and numerical value of the electron’s own magnetic moment are obtained naturally, without any additional assumptions. Thus, it turned out that spin is both a quantum and a relativistic quantity. But this does not exhaust the significance of the Dirac equation. It also made it possible to predict the existence of the electron’s antiparticle – positron. From the Dirac equation, not only positive but also negative values are obtained for the total energy of a free electron. Studies of the equation show that for a given particle momentum, there are solutions to the equation corresponding to the energies: .
Between the greatest negative energy (– m e With 2) and the least positive energy (+ m e c 2) there is an interval of energy values that cannot be realized. The width of this interval is 2 m e With 2. Consequently, two regions of energy eigenvalues are obtained: one begins with + m e With 2 and extends to +∞, the other starts from – m e With 2 and extends to –∞.
A particle with negative energy must have very strange properties. Transitioning into states with less and less energy (that is, with negative energy increasing in magnitude), it could release energy, say, in the form of radiation, and, since | E| unconstrained, a particle with negative energy could emit an infinitely large amount of energy. A similar conclusion can be reached in the following way: from the relation E=m e With 2 it follows that a particle with negative energy will also have a negative mass. Under the influence of a braking force, a particle with a negative mass should not slow down, but accelerate, performing an infinitely large amount of work on the source of the braking force. In view of these difficulties, it would seem that it would be necessary to admit that the state with negative energy should be excluded from consideration as leading to absurd results. This, however, would contradict some general principles of quantum mechanics. Therefore, Dirac chose a different path. He proposed that transitions of electrons to states with negative energy are usually not observed for the reason that all available levels with negative energy are already occupied by electrons. 
According to Dirac, a vacuum is a state in which all levels of negative energy are occupied by electrons, and levels with positive energy are free. Since all levels lying below the forbidden band are occupied without exception, electrons at these levels do not reveal themselves in any way. If one of the electrons located at negative levels is given energy E≥ 2m e With 2, then this electron will go into a state with positive energy and will behave in the usual way, like a particle with positive mass and negative charge. This first theoretically predicted particle was called the positron. When a positron meets an electron, they annihilate (disappear) - the electron moves from a positive level to a vacant negative one. The energy corresponding to the difference between these levels is released in the form of radiation. In Fig. 4, arrow 1 depicts the process of creation of an electron-positron pair, and arrow 2 – their annihilation. The term “annihilation” should not be taken literally. Essentially, what occurs is not a disappearance, but a transformation of some particles (electron and positron) into others (γ-photons).
There are particles that are identical with their antiparticles (that is, they do not have antiparticles). Such particles are called absolutely neutral. These include the photon, π 0 meson and η meson. Particles identical with their antiparticles are not capable of annihilation. This, however, does not mean that they cannot be transformed into other particles at all.
If baryons (that is, nucleons and hyperons) are assigned a baryon charge (or baryon number) IN= +1, antibaryons – baryon charge IN= –1, and all other particles have a baryon charge IN= 0, then all processes occurring with the participation of baryons and antibaryons will be characterized by conservation of charge baryons, just as processes are characterized by conservation of electric charge. The law of conservation of baryon charge determines the stability of the softest baryon, the proton. The transformation of all quantities that describe a physical system, in which all particles are replaced by antiparticles (for example, electrons with protons, and protons with electrons, etc.), is called the conjugation charge.
Strange particles.TO-mesons and hyperons were discovered as part of cosmic rays in the early 50s of the XX century. Since 1953, they have been produced at accelerators. The behavior of these particles turned out to be so unusual that they were called strange. The unusual behavior of the strange particles was that they were clearly born due to strong interactions with a characteristic time of the order of 10–23 s, and their lifetimes turned out to be of the order of 10–8–10–10 s. The latter circumstance indicated that the decay of particles occurs as a result of weak interactions. It was completely unclear why the strange particles lived for so long. Since the same particles (π-mesons and protons) are involved in both the creation and decay of the λ-hyperon, it was surprising that the rate (that is, the probability) of both processes was so different. Further research showed that strange particles are born in pairs. This led to the idea that strong interactions cannot play a role in particle decay due to the fact that the presence of two strange particles is necessary for their manifestation. For the same reason, the single creation of strange particles turns out to be impossible.
To explain the prohibition of the single production of strange particles, M. Gell-Mann and K. Nishijima introduced a new quantum number, the total value of which, according to their assumption, should be conserved under strong interactions. This is a quantum number S was named the strangeness of the particle. In weak interactions, the strangeness may not be preserved. Therefore, it is attributed only to strongly interacting particles - mesons and baryons.
Neutrino. Neutrino is the only particle that does not participate in either strong or electromagnetic interactions. Excluding the gravitational interaction, in which all particles participate, neutrinos can only take part in weak interactions.
For a long time, it remained unclear how a neutrino differs from an antineutrino. The discovery of the law of conservation of combined parity made it possible to answer this question: they differ in helicity. Under helicity a certain relationship between the directions of the impulse is understood R and back S particles. Helicity is considered positive if spin and momentum are in the same direction. In this case, the direction of particle motion ( R) and the direction of “rotation” corresponding to the spin form a right-handed screw. When the spin and momentum are oppositely directed, the helicity will be negative (the translational movement and “rotation” form a left-handed screw). According to the theory of longitudinal neutrinos developed by Yang, Lee, Landau and Salam, all neutrinos existing in nature, regardless of the method of their origin, are always completely longitudinally polarized (that is, their spin is directed parallel or antiparallel to the momentum R). Neutrino has negative(left) helicity (corresponding to the ratio of directions S And R, shown in Fig. 5 (b), antineutrino – positive (right-handed) helicity (a). Thus, helicity is what distinguishes neutrinos from antineutrinos.
Rice. 5. Scheme of helicity of elementary particles
Systematics of elementary particles. The patterns observed in the world of elementary particles can be formulated in the form of conservation laws. Quite a lot of such laws have already accumulated. Some of them turn out to be not exact, but only approximate. Each conservation law expresses a certain symmetry of the system. Laws of conservation of momentum R, angular momentum L and energy E reflect the properties of symmetry of space and time: conservation E is a consequence of the homogeneity of time, the preservation R due to the homogeneity of space, and the preservation L– its isotropy. The law of conservation of parity is associated with the symmetry between right and left ( R-invariance). Symmetry with respect to charge conjugation (symmetry of particles and antiparticles) leads to the conservation of charge parity ( WITH-invariance). The laws of conservation of electric, baryon and lepton charges express a special symmetry WITH-functions. Finally, the law of conservation of isotopic spin reflects the isotropy of isotopic space. Failure to comply with one of the conservation laws means a violation of the corresponding type of symmetry in this interaction.
In the world of elementary particles the following rule applies: everything that is not prohibited by conservation laws is permitted. The latter play the role of exclusion rules governing the interconversion of particles. First of all, let us note the laws of conservation of energy, momentum and electric charge. These three laws explain the stability of the electron. From the conservation of energy and momentum it follows that the total rest mass of the decay products must be less than the rest mass of the decaying particle. This means that an electron could only decay into neutrinos and photons. But these particles are electrically neutral. So it turns out that the electron simply has no one to transfer its electric charge to, so it is stable.
Quarks. There have become so many particles called elementary that serious doubts have arisen about their elementary nature. Each of the strongly interacting particles is characterized by three independent additive quantum numbers: charge Q, hypercharge U and baryon charge IN. In this regard, a hypothesis arose that all particles are built from three fundamental particles - carriers of these charges. In 1964, Gell-Mann and, independently of him, the Swiss physicist Zweig put forward a hypothesis according to which all elementary particles are built from three particles called quarks. These particles are assigned fractional quantum numbers, in particular, an electric charge equal to +⅔; –⅓; +⅓ respectively for each of the three quarks. These quarks are usually designated by the letters U,D,S. In addition to quarks, antiquarks are considered ( u,d,s). To date, 12 quarks are known - 6 quarks and 6 antiquarks. Mesons are formed from a quark-antiquark pair, and baryons are formed from three quarks. For example, a proton and a neutron are composed of three quarks, which makes the proton or neutron colorless. Accordingly, three charges of strong interactions are distinguished - red ( R), yellow ( Y) and green ( G).
Each quark is assigned the same magnetic moment (μV), the value of which is not determined from theory. Calculations made on the basis of this assumption give the value of the magnetic moment μ p for the proton = μ kv, and for a neutron μ n = – ⅔μ sq.
Thus, for the ratio of magnetic moments the value μ p is obtained / μ n = –⅔, in excellent agreement with the experimental value.
Basically, the color of the quark (like the sign of the electric charge) began to express the difference in the property that determines the mutual attraction and repulsion of quarks. By analogy with quanta of fields of various interactions (photons in electromagnetic interactions, R-mesons in strong interactions, etc.) particles that carried the interaction between quarks were introduced. These particles were called gluons. They transfer color from one quark to another, causing the quarks to be held together. In quark physics, the confinement hypothesis was formulated (from the English. confinements– capture) of quarks, according to which it is impossible to subtract a quark from the whole. It can only exist as an element of the whole. The existence of quarks as real particles in physics is reliably substantiated.
The idea of quarks turned out to be very fruitful. It made it possible not only to systematize already known particles, but also to predict a whole series of new ones. The situation that has developed in the physics of elementary particles is reminiscent of the situation created in atomic physics after the discovery of the periodic law in 1869 by D. I. Mendelev. Although the essence of this law was clarified only about 60 years after the creation of quantum mechanics, it made it possible to systematize the chemical elements known by that time and, in addition, led to the prediction of the existence of new elements and their properties. In the same way, physicists have learned to systematize elementary particles, and the developed taxonomy has, in rare cases, made it possible to predict the existence of new particles and anticipate their properties.
So, at present, quarks and leptons can be considered truly elementary; There are 12 of them, or together with anti-chatits - 24. In addition, there are particles that provide four fundamental interactions (interaction quanta). There are 13 of these particles: graviton, photon, W± - and Z-particles and 8 gluons.
Existing theories of elementary particles cannot indicate what is the beginning of the series: atoms, nuclei, hadrons, quarksIn this series, each more complex material structure includes a simpler one as a component. Apparently, this cannot continue indefinitely. It was assumed that the described chain of material structures is based on objects of a fundamentally different nature. It is shown that such objects may not be pointlike, but extended, albeit extremely small (~10‑33 cm) formations, called superstrings. The described idea is not realizable in our four-dimensional space. This area of physics is generally extremely abstract, and it is very difficult to find visual models that help simplify the perception of the ideas inherent in the theories of elementary particles. Nevertheless, these theories allow physicists to express the mutual transformation and interdependence of the “most elementary” micro-objects, their connection with the properties of four-dimensional space-time. The most promising is the so-called M-theory (M – from mystery- riddle, secret). She's operating twelve-dimensional space . Ultimately, during the transition to the four-dimensional world that we directly perceive, all “extra” dimensions are “collapsed.” M-theory is so far the only theory that makes it possible to reduce four fundamental interactions to one - the so-called Superpower. It is also important that M-theory allows for the existence of different worlds and establishes the conditions that ensure the emergence of our world. M-theory is not yet sufficiently developed. It is believed that the final "theory of everything" based on M-theory will be built in the 21st century.
ELEMENTARY PARTICLES- primary, further indecomposable particles, of which all matter is believed to consist. In modern physics, the term "elementary particles" is usually used to designate a large group of tiny particles of matter that are not atoms (see Atom) or atomic nuclei (see Atomic nucleus); The exception is the nucleus of the hydrogen atom - the proton.
By the 80s of the 20th century, science knew more than 500 elementary particles, most of which were unstable. Elementary particles include proton (p), neutron (n), electron (e), photon (γ), pi-mesons (π), muons (μ), heavy leptons (τ +, τ -), neutrinos of three types - electronic (V e), muonic (V μ) and associated with the so-called heavy depton (V τ), as well as “strange” particles (K-mesons and hyperons), various resonances, mesons with hidden charm, “charmed” particles, upsilon particles (Υ), “beautiful” particles, intermediate vector bosons, etc. An independent branch of physics has emerged - the physics of elementary particles.
The history of particle physics dates back to 1897, when J. J. Thomson discovered the electron (see Electron radiation); in 1911, R. Millikan measured the magnitude of its electric charge. The concept of “photon” - quantum of light - was introduced by M. Planck in 1900. Direct experimental evidence of the existence of the photon was obtained by Millikan (1912-1915) and Compton (A. N. Compton, 1922). In the process of studying the atomic nucleus, E. Rutherford discovered the proton (see Proton radiation), and in 1932, J. Chadwick discovered the neutron (see Neutron radiation). In 1953, the existence of neutrinos, which W. Pauli had predicted back in 1930, was experimentally proven.
Elementary particles are divided into three groups. The first is represented by a single elementary particle - a photon, γ-quantum, or quantum of electromagnetic radiation. The second group is leptons (Greek leptos small, light), participating, in addition to electromagnetic ones, also in weak interactions. There are 6 known leptons: electron and electron neutrino, muon and muon neutrino, heavy τ-lepton and the corresponding neutrino. The third - main group of elementary particles are hadrons (Greek hadros large, strong), which participate in all types of interactions, including strong interactions (see below). Hadrons include particles of two types: baryons (Greek barys heavy) - particles with half-integer spin and a mass no less than the mass of a proton, and mesons (Greek mesos medium) - particles with zero or integer spin (see Electron paramagnetic resonance). Baryons include the proton and neutron, hyperons, some resonances and “charmed” particles and some other elementary particles. The only stable baryon is the proton, the rest of the baryons are unstable (a neutron in a free state is an unstable particle, but in a bound state inside stable atomic nuclei it is stable. Mesons got their name because the masses of the first discovered mesons - the pi-meson and the K-meson - had values intermediate between the masses of a proton and an electron. Later, mesons were discovered, the mass of which exceeds the mass of a proton. Hadrons are also characterized by strangeness (S) - zero, positive or negative quantum number. Hadrons with zero strangeness are called ordinary, and with S ≠ 0 - strange In 1964, G. Zweig and M. Gell-Mann independently proposed the quark structure of hadrons.The results of a number of experiments indicate that quarks are real material formations inside hadrons.Quarks have a number of unusual properties, for example, fractional electric charge, etc. Quarks have not been observed in a free state. It is believed that all hadrons are formed due to various combinations of quarks.
Initially, elementary particles were studied in the study of radioactive decay (see Radioactivity) and cosmic radiation (see). However, since the 50s of the 20th century, studies of elementary particles have been carried out on charged particle accelerators (see), in which accelerated particles bombard a target or collide with particles flying towards them. In this case, the particles interact with each other, resulting in their interconversion. This is how most elementary particles were discovered.
Each elementary particle, along with the specifics of its inherent interactions, is described by a set of discrete values of certain physical quantities, expressed in integer or fractional numbers (quantum numbers). The common characteristics of all elementary particles are mass (m), lifetime (t), spin (J) - the intrinsic angular momentum of elementary particles, which has a quantum nature and is not associated with the movement of the particle as a whole, electric charge (Ω) and magnetic moment ( μ). The electric charges of the studied elementary particles in absolute value are integer multiples of the electron charge (e≈1.6*10 -10 k). Known elementary particles have electric charges equal to 0, ±1 and ±2.
All elementary particles have corresponding antiparticles, the mass and spin of which are equal to the mass and spin of the particle, and the electric charge, magnetic moment and other characteristics are equal in absolute value and opposite in sign. For example, the antiparticle of an electron is a positron - an electron with a positive electrical charge. An elementary particle that is identical to its antiparticle is called truly neutral, for example, a neutron and an antineutron, a neutrino and an antineutrino, etc. When antiparticles interact with each other, their annihilation occurs (see).
When an elementary particle enters a material environment, it interacts with it. There are strong, electromagnetic, weak and gravitational interactions. Strong interaction (stronger than electromagnetic interaction) occurs between elementary particles located at a distance of less than 10 -15 m (1 Fermi). At distances greater than 1.5 Fermi, the interaction force between particles is close to zero. It is the strong interactions between elementary particles that provide the exceptional strength of atomic nuclei, which underlies the stability of matter under terrestrial conditions. A characteristic feature of the strong interaction is its independence of electric charge. Hadrons are capable of strong interactions. Strong interactions cause the decay of short-lived particles (lifetime of the order of 10 -23 - 10 -24 sec.), which are called resonances.
All charged elementary particles, photons and neutral particles with a magnetic moment (for example, neutrons) are subject to electromagnetic interaction. The basis of electromagnetic interactions is the connection with the electromagnetic field. The forces of electromagnetic interaction are approximately 100 times weaker than the forces of strong interaction. The main scope of electromagnetic interaction is atoms and molecules (see Molecule). This interaction determines the structure of solids and the nature of the chemical. processes. It is not limited by the distance between elementary particles, so the size of an atom is approximately 10 4 times the size of the atomic nucleus.
Weak interactions underlie extremely slow processes involving elementary particles. For example, neutrinos, which have a weak interaction, can easily penetrate the thickness of the Earth and the Sun. Weak interactions also cause slow decays of so-called quasi-stable elementary particles, the lifetime of which is in the range of 10 8 - 10 -10 sec. Elementary particles born during strong interaction (in a time of 10 -23 -10 -24 sec.), but decaying slowly (10 -10 sec.), are called strange.
Gravitational interactions between elementary particles produce extremely small effects due to the insignificance of the particle masses. This type of interaction has been well studied on macro-objects with large masses.
The diversity of elementary particles with different physical characteristics explains the difficulty of their systematization. Of all the elementary particles, only photons, electrons, neutrinos, protons and their antiparticles are actually stable, since they have a long lifetime. These particles are the end products of the spontaneous transformation of other elementary particles. The birth of elementary particles can occur as a result of the first three types of interactions. For strongly interacting particles, the source of creation is strong interaction reactions. Leptons, most likely, arise from the decay of other elementary particles or are born in pairs (particle + antiparticle) under the influence of photons.
Flows of elementary particles form ionizing radiation (see), causing ionization of neutral molecules of the medium. The biological effect of elementary particles is associated with the formation of substances with high chemical activity in irradiated tissues and body fluids. These substances include free radicals (see Free radicals), peroxides (see) and others. Elementary particles can also have a direct effect on biomolecules and supramolecular structures, cause the rupture of intramolecular bonds, depolymerization of high-molecular compounds, etc. The processes of energy migration and the formation of metastable compounds resulting from long-term preservation of the state of excitation in some macromolecular substrates. In cells, the activity of enzyme systems is suppressed or distorted, the structure of cell membranes and surface cell receptors changes, which leads to an increase in membrane permeability and a change in diffusion processes, accompanied by the phenomena of protein denaturation, tissue dehydration, and disruption of the internal environment of the cell. The susceptibility of cells largely depends on the intensity of their mitotic division (see Mitosis) and metabolism: with an increase in this intensity, the radiosusceptibility of tissues increases (see Radiosensitivity). Their use for radiation therapy (see), especially in the treatment of malignant neoplasms, is based on this property of flows of elementary particles - ionizing radiation. The penetrating ability of charged elementary particles depends to a large extent on the linear transfer of energy (see), that is, on the average energy absorbed by the medium at the point of passage of the charged particle, per unit of its path.
The damaging effect of the flow of elementary particles especially affects the stem cells of hematopoietic tissue, epithelium of the testicles, small intestine, and skin (see Radiation sickness, Radiation damage). First of all, systems that are in a state of active organogenesis and differentiation during irradiation are affected (see Critical organ).
The biological and therapeutic effect of elementary particles depends on their type and dose of radiation (see Doses of ionizing radiation). For example, when exposed to X-ray radiation (see X-ray therapy), gamma radiation (see Gamma therapy) and proton radiation (see Proton therapy) on the entire human body at a dose of about 100 rad, a temporary change in hematopoiesis is observed; external influence of neutron radiation (see Neutron radiation) leads to the formation in the body of various radioactive substances, for example, radionuclides of sodium, phosphorus, etc. When radionuclides that are sources of beta particles (electrons or positrons) or gamma quanta enter the body, this happens called internal irradiation of the body (see Incorporation of radioactive substances). Especially dangerous in this regard are rapidly resorbing radionuclides with a uniform distribution in the body, for example. tritium (3H) and polonium-210.
Radionuclides, which are sources of elementary particles and participate in metabolism, are used in radioisotope diagnostics (see).
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R. V. Stavntsky.