The weak force is one of the four fundamental forces that govern all matter in the Universe. The other three are gravity, electromagnetism and the strong force. While other forces hold things together, the weak force plays a big role in breaking them apart.
The weak force is stronger than gravity, but it is only effective at very small distances. The force operates at the subatomic level and plays a critical role in powering stars and creating the elements. It is also responsible for most of the natural radiation in the Universe.
Fermi theory
Italian physicist Enrico Fermi developed a theory in 1933 to explain beta decay, the process of a neutron turning into a proton and displacing an electron, often called a beta particle in this context. He identified a new type of force, the so-called weak force, which was responsible for decay, the fundamental process of turning a neutron into a proton, a neutrino and an electron, which was later defined as an antineutrino.
Fermi originally assumed that there was zero distance and zero cohesion. The two particles had to touch for the force to work. It has since been discovered that the weak force is actually a force that manifests itself over an extremely short distance, equal to 0.1% of the proton diameter.
Electroweak force
The first step in hydrogen fusion is the collision of two protons with sufficient force to overcome the mutual repulsion they experience due to their electromagnetic interaction.
If both particles are placed close to each other, a strong force can bind them together. This creates an unstable form of helium (2 He), which has a nucleus with two protons, as opposed to a stable form (4 He), which has two neutrons and two protons.
At the next stage, weak interaction comes into play. Due to an overabundance of protons, one of them undergoes beta decay. After this, other reactions, including intermediate formation and fusion of 3He, eventually form stable 4He.
The Feynman diagram of the beta decay of a neutron into a proton, electron and electron antineutrino through the intermediate W boson is one of the four fundamental physical interactions between elementary particles, along with gravitational, electromagnetic and strong. Its most famous manifestation is beta decay and the radioactivity associated with it. Interaction named weak, since the strength of the field corresponding to it is 10 13 less than in the fields that hold nuclear particles (nucleons and quarks) together and 10 10 less than the Coulomb one on these scales, but much stronger than the gravitational one. The interaction has a short range and appears only at distances on the order of the size of the atomic nucleus.The first theory of weak interaction was proposed by Enrico Fermi in 1930. When developing the theory, he used Wolfgang Pauli's hypothesis about the existence of a new elementary particle, the neutrino, at that time.
The weak interaction describes those processes in nuclear and particle physics that occur relatively slowly, in contrast to the fast processes caused by the strong interaction. For example, the half-life of a neutron is approximately 16 minutes. – Eternity compared to nuclear processes, which are characterized by a time of 10 -23 s.
For comparison, charged pions? ± decay through weak interaction and have a lifetime of 2.6033 ± 0.0005 x 10 -8 s, whereas the neutral pion? 0 decays into two gamma rays through electromagnetic interaction and has a lifetime of 8.4 ± 0.6 x 10 -17 s.
Another characteristic of interaction is the free path of particles in a substance. Particles that interact through electromagnetic interaction - charged particles, gamma quanta - can be detained by an iron plate several tens of centimeters thick. Whereas a neutrino, which interacts only weakly, passes through a layer of metal a billion kilometers thick without ever colliding.
The weak interaction involves quarks and leptons, including neutrinos. In this case, the aroma of the particles changes, i.e. their type. For example, as a result of the decay of a neutron, one of its d-quarks turns into a u-quark. Neutrinos are unique in that they interact with other particles only through weak, and even weaker, gravitational interactions.
According to modern concepts, formulated in the Standard Model, the weak interaction is carried by gauge W- and Z-bosons, which were discovered at accelerators in 1982. Their masses are 80 and 90 times the mass of a proton. The exchange of virtual W-bosons is called a charged current, the exchange of Z-bosons is called a neutral current.
The vertices of Feynman diagrams describing possible processes involving gauge W- and Z-bosons can be divided into three types:
A lepton can viprominite or absorb a W boson and turn into a neutrino;
a quark can viprominite or absorb a W boson, and change its flavor, becoming a superposition of other quarks;
a lepton or quark can absorb or viprominite a Z-boson
The ability of a particle to weakly interact is described by a quantum number called weak isospin. Possible isospin values for particles that can exchange W and Z bosons are ± 1 / 2. It is these particles that interact through the weak interaction. Particles with zero weak isospin, for which the processes of exchange of W and Z bosons are impossible, do not interact through weak mutualism. Weak isospin is conserved in reactions between elementary particles. This means that the total weak isospin of all particles participating in the reaction remains unchanged, although the types of particles may change.
A feature of the weak interaction is that it violates parity, since only fermions with left-handed chirality and antiparticles of fermions with right-handed chirality have the ability to weakly interact through charged currents. Parity nonconservation in weak interactions was discovered by Yang Zhenning and Li Zhengdao, for which they received the Nobel Prize in Physics for 1957. The reason for parity non-conservation is seen in spontaneous symmetry breaking. In the Standard Model, symmetry breaking corresponds to a hypothetical particle, the Higgs boson. This is the only particle of the ordinary model that has not yet been discovered experimentally.
With weak interaction, CP symmetry is also broken. This violation was discovered experimentally in 1964 in experiments with kaon. The authors of the discovery, James Cronin and Val Fitch, were awarded the Nobel Prize in 1980. CP symmetry violation occurs much less frequently than parity violation. It also means, since the conservation of CPT symmetry is based on fundamental physical principles - Lorentz transformations and short-range interaction, the possibility of breaking T-symmetry, i.e. non-invariance of physical processes with respect to changes in the direction of time.
In 1969, a unified theory of electromagnetic and weak nuclear interaction was constructed, according to which at energies of 100 GeV, which corresponds to a temperature of 10 15 K, the difference between electromagnetic and weak processes disappears. Experimental verification of the unified theory of electroweak and strong nuclear interaction requires an increase in accelerator energy by a hundred billion times.
The theory of electroweak interaction is based on the SU(2) symmetry group.
Despite its small size and short duration, the weak interaction plays a very important role in nature. If it were possible to “turn off” the weak interaction, then the Sun would go out, since the process of converting a proton into a neutron, a positron and a neutrino, as a result of which 4 protons turn into 4 He, two positrons and two neutrinos, would become impossible. This process serves as the main source of energy for the Sun and most stars (see Hydrogen cycle). Weak interaction processes are important for the evolution of stars, since they cause the energy loss of very hot stars in supernova explosions with the formation of pulsars, etc. If there were no weak interaction in nature, muons, pi-mesons and other particles would be stable and widespread in ordinary matter. Such an important role of the weak interaction is due to the fact that it does not obey a number of prohibitions characteristic of the strong and electromagnetic interactions. In particular, the weak interaction turns charged leptons into neutrinos, and quarks of one flavor into quarks of another.
Weak interaction
Physics has moved slowly towards identifying the existence of the weak interaction. The weak force is responsible for particle decays; and therefore its manifestation was confronted with the discovery of radioactivity and the study of beta decay.
Beta decay has revealed an extremely strange feature. Research led to the conclusion that this decay seemed to violate one of the fundamental laws of physics - the law of conservation of energy. It seemed that part of the energy was disappearing somewhere. To “save” the law of conservation of energy, W. Pauli suggested that during beta decay, another particle flies out along with the electron, taking with it the missing energy. It is neutral and has an unusually high penetrating ability, as a result of which it could not be observed. E. Fermi called the invisible particle “neutrino”.
But predicting neutrinos is only the beginning of the problem, its formulation. It was necessary to explain the nature of neutrinos, but there remained a lot of mystery here. The fact is that electrons and neutrinos were emitted by unstable nuclei. But it has been irrefutably proven that there are no such particles inside nuclei. About their occurrence, it was suggested that electrons and neutrinos do not exist in the nucleus in a “ready-made form”, but are somehow formed from the energy of the radioactive nucleus. Further research showed that the neutrons included in the nucleus, left to their own devices, after a few minutes decay into a proton, electron and neutrino, i.e. instead of one particle, three new ones appear. The analysis led to the conclusion that known forces could not cause such a disintegration. It was apparently generated by some other, unknown force. Research has shown that this force corresponds to some weak interaction.
The weak interaction is significantly smaller in magnitude than all interactions except gravitational interaction, and in systems where it is present, its effects are overshadowed by the electromagnetic and strong interactions. In addition, the weak interaction propagates over very small distances. The radius of the weak interaction is very small. The weak interaction stops at a distance greater than 10-16 cm from the source, and therefore it cannot influence macroscopic objects, but is limited to the microcosm, subatomic particles. When the avalanche-like discovery of many unstable subnuclear particles began, it was discovered that most of them participate in weak interactions.
Strong interaction
The last in the series of fundamental interactions is the strong interaction, which is a source of enormous energy. The most typical example of energy released by the strong interaction is the Sun. In the depths of the Sun and stars, thermonuclear reactions continuously occur, caused by strong interactions. But man has also learned to release strong interactions: a hydrogen bomb has been created, technologies for controlled thermonuclear reactions have been designed and improved.
Physics came to the idea of the existence of strong interaction during the study of the structure of the atomic nucleus. Some force must hold the positively charged protons in the nucleus, preventing them from flying away under the influence of electrostatic repulsion. Gravity is too weak to provide this; Obviously, some kind of interaction is necessary, moreover, stronger than electromagnetic. It was subsequently discovered. It turned out that although the strong interaction significantly exceeds all other fundamental interactions in its magnitude, it is not felt outside the nucleus. As in the case of the weak interaction, the radius of action of the new force turned out to be very small: the strong interaction manifests itself at a distance determined by the size of the nucleus, i.e. approximately 10-13 cm. In addition, it turned out that not all particles experience strong interaction. Thus, protons and neutrons experience it, but electrons, neutrinos and photons are not subject to it. Usually only heavy particles participate in strong interactions. It is responsible for the formation of nuclei and many interactions of elementary particles.
The theoretical explanation of the nature of the strong interaction has been difficult to develop. A breakthrough appeared only in the early 60s, when the quark model was proposed. In this theory, neutrons and protons are considered not as elementary particles, but as composite systems built from quarks.
Thus, in fundamental physical interactions the difference between long-range and short-range forces is clearly visible. On the one hand, interactions of an unlimited radius (gravity, electromagnetism), and on the other, of a small radius (strong and weak). The world of physical processes unfolds within the boundaries of these two polarities and is the embodiment of the unity of the extremely small and the extremely large - short-range action in the microworld and long-range action throughout the Universe.
The weak force, or weak nuclear force, is one of the four fundamental forces in nature. It is responsible, in particular, for the beta decay of the nucleus. This interaction is called weak, since the other two interactions that are significant for nuclear physics (strong and electromagnetic) are characterized by much greater intensity. However, it is much stronger than the fourth of the fundamental interactions, gravitational. This interaction is the weakest of the fundamental interactions experimentally observed in the decays of elementary particles, where quantum effects are fundamentally significant. Quantum manifestations of gravitational interaction have never been observed. Weak interaction is distinguished using the following rule: if an elementary particle called a neutrino (or antineutrino) participates in the interaction process, then this interaction is weak.
A typical example of the weak interaction is the beta decay of a neutron
where n is a neutron, p is a proton, e- is an electron, e is an electron antineutrino.
It should, however, be borne in mind that the above rule does not mean at all that any act of weak interaction must be accompanied by a neutrino or antineutrino. It is known that a large number of neutrinoless decays occur. As an example, we can note the process of decay of a lambda hyperon into a p proton and a negatively charged pion. According to modern concepts, the neutron and proton are not truly elementary particles, but consist of elementary particles called quarks.
The intensity of the weak interaction is characterized by the Fermi coupling constant GF. The GF constant is dimensional. To form a dimensionless quantity, it is necessary to use some reference mass, for example the proton mass mp. Then the dimensionless coupling constant will be
It can be seen that the weak interaction is much more intense than the gravitational interaction.
The weak interaction, unlike the gravitational interaction, is short-range. This means that the weak force between particles only comes into play if the particles are close enough to each other. If the distance between particles exceeds a certain value called the characteristic radius of interaction, the weak interaction does not manifest itself. It has been experimentally established that the characteristic radius of weak interaction is about 10-15 cm, that is, weak interaction is concentrated at distances smaller than the size of the atomic nucleus. Although the weak interaction is significantly concentrated within the nucleus, it has certain macroscopic manifestations. In addition, the weak interaction plays an important role in the so-called thermonuclear reactions responsible for the mechanism of energy release in stars. The most amazing property of the weak interaction is the existence of processes in which mirror asymmetry is manifested. At first glance, it seems obvious that the difference between the concepts left and right is arbitrary. Indeed, the processes of gravitational, electromagnetic and strong interaction are invariant with respect to spatial inversion, which carries out mirror reflection. It is said that in such processes the spatial parity P is conserved. However, it has been experimentally established that weak processes can proceed with non-conservation of spatial parity and, therefore, seem to sense the difference between left and right. Currently, there is solid experimental evidence that parity nonconservation in weak interactions is universal in nature; it manifests itself not only in the decays of elementary particles, but also in nuclear and even atomic phenomena. It should be recognized that mirror asymmetry is a property of Nature at the most fundamental level.
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Weak interaction.
Physics has moved slowly towards identifying the existence of the weak interaction. The weak interaction is responsible for particle decays. Therefore, its manifestation was encountered during the discovery of radioactivity and the study of beta decay (see 8.1.5).
Beta decay has revealed an extremely strange feature. It seemed that in this decay the law of conservation of energy was violated, that part of the energy disappeared somewhere. To “save” the law of conservation of energy, W. Pauli suggested that during beta decay, another particle flies out along with the electron, taking with it the missing energy. It is neutral and has an unusually high penetrating ability, as a result of which it could not be observed. E. Fermi called the invisible particle “neutrino”.
But predicting neutrinos is only the beginning of the problem, its formulation. It was necessary to explain the nature of neutrinos; there remained a lot of mystery here. The fact is that electrons and neutrinos were emitted by unstable nuclei, but it was known that there were no such particles inside the nuclei. How did they arise? It turned out that the neutrons included in the nucleus, left to their own devices, after a few minutes decay into a proton, electron and neutrino. What forces cause such disintegration? The analysis showed that known forces cannot cause such a disintegration. It was apparently generated by some other, unknown force, which corresponds to some “weak interaction”.
The weak interaction is much smaller in magnitude than all interactions except gravitational interaction. Where it is present, its effects are overshadowed by the electromagnetic and strong interactions. In addition, the weak interaction extends over very small distances. The radius of weak interaction is very small (10-16 cm). Therefore, it cannot influence not only macroscopic, but even atomic objects and is limited to subatomic particles. In addition, compared to the electromagnetic and strong interactions, the weak interaction is extremely slow.
When the avalanche-like discovery of many unstable subnuclear particles began, it was discovered that most of them participate in weak interactions. The weak interaction plays a very important role in nature. It is an integral part of thermonuclear reactions in the Sun and stars, providing the synthesis of pulsars, supernova explosions, the synthesis of chemical elements in stars, etc.