Weak interaction.TO To reveal the existence of the weak interaction, physics progressed slowly. 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 exhibited a highly bizarre feature. Studies led to the conclusion that this decay seems to violate one of the fundamental laws of physics - the law of conservation of energy. It seemed that part of the energy disappeared somewhere. In order to "save" the law of conservation of energy, V. Pauli suggested that during beta decay, along with an electron, another particle flies out, taking with it the missing energy. It is neutral and has an unusually high penetrating power, as a result of which it could not be observed. E. Fermi called the invisible particle "neutrino".
But the prediction of the neutrino is only the beginning of the problem, its formulation. It was necessary to explain the nature of the neutrino, but there remained a lot of mystery. 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. It has been 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 studies showed that the neutrons that make up the nucleus, left to themselves, after a few minutes decay into a proton, an electron and a neutrino, i.e. instead of one particle, three new ones appear. The analysis led to the conclusion that known forces cannot cause such a disintegration. He, apparently, was generated by some other, unknown force. Studies have shown that this force corresponds to some weak interaction.
The weak interaction is much smaller than all

interactions other than gravitational, and in systems where it is present, its effects are overshadowed by electromagnetic and strong interactions. In addition, the weak force propagates over very small distances. The weak interaction radius is very small. Weak interaction stops at a distance greater than 10-16 cm from the source, and therefore it cannot affect macroscopic objects, but is limited to the microcosm, subatomic particles. When the avalanche-like discovery of many unstable subnuclear particles began, it was found that most of them participate in weak interaction.

Strong interaction.Last among the fundamental interactions is the strong interaction, which is a source of enormous energy. The most characteristic example of the energy released by the strong force is the Sun. In the depths of the Sun and stars, thermonuclear reactions are continuously taking place, caused by strong interactions. But man has also learned to release the strong interaction: created H-bomb, technologies for controlled thermonuclear reaction have been designed and are being improved.
Physics came to the idea of ​​the existence of a strong interaction in the course of studying the structure of the atomic nucleus. Some force must hold the positively charged protons in the nucleus, preventing them from flying apart under the action of electrostatic repulsion. Gravity is too weak to provide this; Obviously, some kind of interaction is needed, moreover, stronger than electromagnetic. It was subsequently discovered. It turned out that although the strong interaction significantly exceeds all other fundamental interactions in its magnitude, it is not felt outside the core. As in the case of the weak interaction, the radius of action new strength turned out to be very small: the strong interaction manifests itself at a distance determined by the size of the nucleus, i.e. about 10-13 cm. In addition, it turned out that not all particles experience strong interaction. So, it is experienced by protons and neutrons, but electrons, neutrinos and photons are not subject to it. Usually only heavy particles participate in the strong interaction. 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 was outlined only in the early 1960s, 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.



Gravitational interaction exists between all elementary particles and determines the gravitational attraction of all bodies to each other at any distance (see gravity law); it is negligible in physical processes in the microcosm, but plays a major role, for example, in cosmogony. Weak interaction manifests itself only at distances of about 10-18 m and causes decay processes (for example, beta decay of some elementary particles and

nuclei). Electromagnetic interaction exists at any distance between elementary particles that have an electric charge or a magnetic moment; in particular, it determines the bond between electrons and nuclei in atoms, and is also responsible for all kinds of electromagnetic radiation. Strong interaction manifests itself at distances of about 10-15 m and determines the existence of atomic nuclei.

WEAK INTERACTION- one of the four known funds. interactions between . S. v. much weaker than strong and e-magn. interactions, but much stronger than gravitational. In the 80s. found that weak and e-magn. interactions - dec. manifestations of a single electroweak interaction.

The intensity of interactions can be judged by the speed of the processes it causes. Usually, the rates of processes are compared with each other at GeV energies, which are characteristic of elementary particle physics. At such energies, the process due to the strong interaction takes place in s, e-magn. process in time s, while the characteristic time of processes occurring due to S. v. (weak processes), much more: c, so that in the world of elementary particles, weak processes proceed extremely slowly.

Another characteristic of interaction is particles in matter. Strongly interacting particles (hadrons) can be stopped by an iron plate several times thick. tens of cm, while a neutrino with only SV would pass without experiencing a single collision through an iron plate about a billion kilometers thick. Gravity is even weaker. interaction, the strength of which at an energy of ~1 GeV is 10 33 times less than that of S. v. However, usually the role of gravitational interactions are much more noticeable than the role of S. in. This is due to the fact that gravitational interaction, like electromagnetic, has an infinitely large radius of action; therefore, for example, gravity acts on bodies located on the surface of the Earth. the attraction of all the atoms that make up the earth. The weak interaction has a very small radius of action: approx. 2 * 10 -16 cm (which is three orders of magnitude smaller than the radius of strong interaction). As a result, for example, S. century. between the nuclei of two neighboring atoms, located at a distance of 10 -8 cm, is negligible, incomparably weaker not only electromagnetic, but also gravitational. interactions between them.

However, despite the small size and short-acting, S. century. plays a very important role in nature. So, if it were possible to “turn off” the S. century, then the Sun would go out, since the process of converting a proton into a neutron, a positron and a neutrino would be impossible, as a result of which four protons turn into 4 He, two positrons and two neutrinos. This process serves as the source of energy for the sun and most stars Hydrogen cycle). Processes of S. in. with the emission of neutrinos in general are exceptionally important in evolution of stars, since they cause energy losses by very hot stars, in supernova explosions with the formation of pulsars, etc. If there were no S. v., muons, mesons, strange and charmed, would be stable and widespread in ordinary matter particles, to-rye disintegrate as a result of S. century. Such a large role of S. E. is connected with the fact that it does not obey a number of prohibitions characteristic of a strong and el-magn. interactions. In particular, S. century. turns charged leptons into neutrinos, and one type (flavor) into quarks of other types.

The intensity of weak processes grows rapidly with increasing energy. So, neutron beta decay, the energy release in Krom is small (~ 1 MeV), lasts approx. 10 3 s, which is 10 13 times longer than the lifetime of a hyperon, the energy release during the decay of which is ~100 MeV. The cross section of interaction with nucleons for neutrinos with an energy of ~100 GeV is approx. a million times more than for neutrinos with an energy of ~1 MeV. According to the theoretical representations, the growth of the cross section will last up to energies of the order of several. hundreds of GeV (in the system of the center of mass of colliding particles). At these energies and at large momentum transfers, effects associated with the existence intermediate vector bosons. At distances between colliding particles much smaller than 2 x 10 -16 cm (the Compton wavelength of intermediate bosons), the S. v. and el-magn. interactions have almost the same intensity.

Naib. a common process due to S. century, - beta decay radioactive atomic nuclei. In 1934, E. Fermi (E. Fermi) built a theory of decay, to-paradise with certain creatures. modifications formed the basis of the subsequent theory of the so-called. universal local four-fermion S. v. (Fermi interactions). According to Fermi's theory, an electron and a neutrino (more precisely, ), emitted from a radioactive nucleus, were not in it before, but arose at the moment of decay. This phenomenon is analogous to the emission of low-energy photons (visible light) from excited atoms or high-energy photons (-quanta) from excited nuclei. The reason for such processes is the interaction of electric. particles with e-magn. field: a moving charged particle creates an electromagnetic current, which perturbs the e-mag. field; as a result of the interaction, the particle transfers energy to the quanta of this field - photons. Interaction of photons with e-magn. current is described by the expression A. Here e- elementary electric charge, which is a constant e-magn. interactions (see Interaction constant), A- operator of the photon field (i.e., the operator of the creation and destruction of a photon), j em - the operator of the density of the e-mag. current. (Often, the expression for the electric magnetic current also includes a multiplier e.) In j em all the charges contribute. particles. For example, the term corresponding to an electron has the form: [Above, for simplification, it is not shown that j uh, as well as A, is a four-dimensional vector. More precisely, instead, you should write a set of four expressions where - Dirac matrix,= 0, 1, 2, 3. Each of these expressions is multiplied by the corresponding component of the four-dimensional vector.]

The interaction describes not only the emission and absorption of photons by electrons and positrons, but also processes such as the production of electron-positron pairs by photons (see Fig. Birth of couples)or annihilation these pairs into photons. The exchange of a photon between two charges. particles leads to their interaction with each other. The result is, for example, the scattering of an electron by a proton, which is schematically depicted Feynman diagram shown in fig. 1. During the transition of a proton in the nucleus from one level to another, the same interaction can lead to the creation of an electron-positron pair (Fig. 2).

Fermi's decay theory is essentially analogous to the el-magn theory. processes. Fermi based his theory on the interaction of two "weak currents" (see Fig. Current in quantum field theory), but interacting with each other not at a distance by exchanging a particle - a field quantum (a photon in the case of an el-magnet interaction), but by contact. This is the interaction between four fermion fields (four fermions p, n, e and neutrino v) in modern. notation looks like: . Here G F- Fermi constant, or weak four-fermion interaction constant, experiment. swarm erg * cm 3 (the value has the dimension of the square of the length, and in units the constant , Where M- proton mass), - proton creation operator (antiproton annihilation), - neutron annihilation operator (antineutron production), - electron creation operator (positron annihilation), v - neutrino annihilation operator (antineutrino generation). (Here and in what follows, the operators for the creation and annihilation of particles are denoted by the symbols of the corresponding particles in bold type.) The current converting a neutron into a proton was later called nucleon, and the current - lepton. Fermi postulated that, like e-magn. current, weak currents are also four-dimensional vectors: Therefore, the Fermi interaction is called. vector.

Like the creation of an electron-positron pair (Fig. 2), the decay of a neutron can be described by a similar diagram (Fig. 3) [antiparticles are marked with a tilde above the symbols of the corresponding particles]. The interaction of lepton and nucleon currents should also lead to other processes, for example. to reaction (Fig. 4), k steam (Fig. 5) and etc.

Creatures. the difference between weak currents and electromagnetic is that a weak current changes the charge of particles, while e-magn. the current does not change: a weak current turns a neutron into a proton, an electron into a neutrino, and an electromagnetic current turns a proton into a proton, and an electron into an electron. Therefore, weak currents ev are called. charged currents. According to such a term of logic, ordinary e-mag. her current is neutral current.

Fermi's theory was based on the results of research in three different ways. areas: 1) experimental. researches actually S. of century. (-decay), which led to the hypothesis of the existence of neutrinos; 2) experiment. studies of the strong force (), which led to the discovery of protons and neutrons and to the understanding that nuclei are composed of these particles; 3) experiment. and theoretical research e-magn. interactions, as a result of which the foundation of quantum field theory was laid. The further development of elementary particle physics has repeatedly confirmed the fruitful interdependence of studies of the strong, weak, and e-magn. interactions.

The theory of universal four-fermion S. v. differs from Fermi's theory in a number of beings, items. These differences, established in subsequent years as a result of the study of elementary particles, were reduced to the following.

The hypothesis that S. in. does not preserve parity, was put forward by Lee Tsung-Dao and Yang Chen Ning in 1956 at the theoretical decay research K-mesons; soon non-conservation R- and C-parities were found experimentally in the decay of nuclei [Wu Chien-Shiung et al], in the decay of the muon [R. Garvin (R. Garwin), L. Lederman (L. Lederman), V. Telegdi (V. Telegdi), J. Friedman (J. Friedman) and others] and in the decays of other particles.

Summarizing a huge experiment. material, M. Gell-Mann (M. Gell-Mann), P. Feynman (R. Feynman), P. Marshak (R. Marshak) and E. Sudarshan (E. Sudarshan) in 1957 proposed the theory of universal S. in. - so-called. V- A-theory. In a formulation based on the quark structure of hadrons, this theory is that the total weak charged current j u is the sum of lepton and quark currents, each of these elementary currents containing the same combination of Dirac matrices:

As it turned out later, the charge. the lepton current, represented in the Fermi theory by one term, is the sum of three terms: and each of the known charges. leptons (electron, muon and heavy lepton) is included in the charge. current with his neutrino.

Charge the hadronic current, represented in the Fermi theory by the term, is the sum of the quark currents. By 1992 five types of quarks were known , from which all known hadrons are built, and the existence of a sixth quark is assumed ( t With Q=+ 2 / 3). Charged quark currents, like lepton currents, are usually written as the sum of three terms:

However, here are linear combinations of operators d, s, b, so the quark charged current consists of nine terms. Each of the currents is the sum of the vector and axial currents with coefficients equal to one.

The coefficients of nine charged quark currents are usually presented as a 3x3 matrix, which is parameterized by three angles and a phase factor characterizing the violation CP invariance in weak decays. This matrix is ​​called matrices Kobayashi - Maskawa (M. Kobayashi, T. Maskawa).

Lagrangian S. v. charged currents has the form:

Edetok, conjugate, etc.). Such an interaction of charged currents quantitatively describes a huge number of weak processes: leptonic, semileptonic ( etc.) and non-leptonic ( ,, etc.). Many of these processes were discovered after 1957. During this period, two fundamentally new phenomena were also discovered: CP violation and neutral currents.

Violation of CP invariance was discovered in 1964 in the experiment of J. Christepson, J. Cronin, V. Fitch, and R. Turley, who observed decay of long-lived K° mesons into two mesons. Later, CP violation was also observed in semileptonic decays. To elucidate the nature of the CP-noninvariant interaction, it would be extremely important to find a k-l. CP-noninvariant process in decays or interactions of other particles. In particular, of great interest are the searches for the dipole moment of the neutron (the presence of which would mean the violation of invariance with respect to reversal of time, and therefore, according to the theorem SRT, and CP-invariance).

The existence of neutral currents was predicted by a unified theory of weak and el-magn. interactions, created in the 60s. Sh. Glashow, S. Weinberg, A. Salam and others, and later called. standard theory of the electroweak interaction. According to this theory, S. century. is not a contact interaction of currents, but occurs through the exchange of intermediate vector bosons ( W + , W - , Z 0) - massive particles with spin 1. In this case, bosons carry out the interaction of the charge. currents (Fig. 6), and Z0-bosons - neutral (Fig. 7). In the standard theory, three intermediate bosons and a photon are vector quanta, the so-called. calibration fields, appearing at asymptotically large transfers of the four-dimensional momentum ( , mz, Where m w , m z- masses W- and Z-bosons in energetic. units) is completely equal. Neutral currents were discovered in 1973 in the interaction of neutrinos and antineutrinos with nucleons. Later, the processes of scattering of a muon neutrino by an electron were found, as well as the effects of parity nonconservation in the interaction of electrons with nucleons, due to the electron neutral current (these effects were first observed in experiments on parity nonconservation during atomic transitions, carried out in Novosibirsk by L.M. Barkov and M. S. Zolotorev, as well as in experiments on the scattering of electrons by protons and deuterons in the USA).

The interaction of neutral currents is described by the corresponding term in the S. v. Lagrangian:

where is a dimensionless parameter. In the standard theory (the experimental value of p coincides with 1 within one percent of the experimental accuracy and the accuracy of the calculation radiative corrections). The total weak neutral current contains contributions from all leptons and all quarks:

A very important property of neutral currents is that they are diagonal, that is, they transfer leptons (and quarks) into themselves, and not into other leptons (quarks), as in the case of charged currents. Each of the 12 quark and lepton neutral currents is a linear combination of the axial current with a coefficient. I 3 and vector current with coefficient. , Where I 3- the third projection of the so-called. weak isotopic spin, Q is the charge of the particle, and - Weinberg angle.

Necessity for the existence of four vector fields of intermediate bosons W+, W-, Z0 and photon A can explain the following. manner. As you know, in e-magn. electrical interaction. charge plays a dual role: on the one hand, it is a conserved quantity, and on the other hand, it is a source of e-mag. field that carries out the interaction between charged particles (interaction constant e). Such a role is electric. charge is provided by the gauge, which consists in the fact that the equations of the theory do not change when the wave functions of charged particles are multiplied by an arbitrary phase factor depending on the space-time point [local symmetry U(1)], and at the same time e-magn. the field, which is the gauge field, undergoes a transformation . Local group transformations U(1) with one type of charge and one gauge field commute with each other (such a group is called Abelian). The specified property of the electric. charge served as a starting point for constructing theories and other types of interactions. In these theories, conserved quantities (for example, isotopic spin) are simultaneously sources of certain gauge fields that transfer the interaction between particles. In the case of several types of "charges" (eg, different projections of isotopic. spin), when separate. transformations do not commute with each other (a non-Abelian group of transformations), it turns out that it is necessary to introduce several calibration fields. (Multiplets of gauge fields corresponding to local non-Abelian symmetries, called Young - Mills fields.) In particular, to isotopic. spin [to which the local group corresponds SU(2)] acted as the interaction constant, three gauge fields with charges 1 and 0 are needed. charged currents of pairs of particles are involved etc., then it is assumed that these pairs are doublets of the weak isospin group, i.e., the group SU(2). Invariance of the theory under local group transformations SU(2) requires, as noted, the existence of a triplet of massless gauge fields W+, W - , W 0, the source of which is weak isospin (the interaction constant g). By analogy with the strong interaction, in which hypercharge Y particles included in the isotopic. multiplet, determined by f-loy Q = I 3 + Y/2(Where I 3- third isospin projection, a Q- electric charge), along with a weak isospin, a weak hypercharge is introduced. Then the conservation of electric. charge and weak isospin corresponds to the conservation of a weak hypercharge [group [ U(1)]. A weak hypercharge is a source of a neutral gauge field At 0(interaction constant g"). Two Mutually Orthogonal Linear Superpositions of Fields В° And W° describe the photon field A and the Z-boson field:

Where . It is the magnitude of the angle that determines the structure of neutral currents. It also defines the connection between the constant g characterizing the interaction of bosons with a weak current, and the constant e characterizing the interaction of a photon with an electric. current:

In order for S. to. had a short-range character, the intermediate bosons must be massive, while the quanta of the initial gauge fields - - massless. According to the standard theory, the formation of mass in intermediate bosons occurs when spontaneous symmetry breaking SU(2) X U(1)before U(1) uh. In this case, one of the superpositions of fields At 0 And W0- photon ( A) remains massless, and the a- and Z-bosons acquire masses:

Experiment. data on neutral currents gave . This corresponded to the expected masses W- and Z-bosons, respectively, and

To discover W- and Z-bosons created special. installations in which these bosons are produced in collisions of high-energy colliding beams. The first installation went into operation in 1981 at CERN. In 1983, reports appeared on the detection at CERN of the first cases of the production of intermediate vector bosons. Birth data published in 1989 W- And Z-bosons at the American Proton-Antiproton Collider - Tevatron, at the Fermi National Accelerator Laboratory (FNAL). To con. 1980s total number W- and Z-bosons observed at the proton-antiproton colliders at CERN and FNAL numbered in the hundreds.

In 1989, the LEP electron-positroin colliders at CERN and the SLC at the Stanford Linear Accelerator Center (SLAC) became operational. The work of the LEP was especially successful, where by the beginning of 1991 more than half a million cases of the creation and decay of Z-bosons had been registered. The study of the decays of Z-bosons showed that no other neutrinos, except for the previously known ones, exist in nature. The mass of the Z-boson was measured with high accuracy: t z = 91.173 0.020 GeV (the mass of the W boson is known with much worse accuracy: mw= 80.220.26 GeV). Exploring properties W- and Z-bosons confirmed the correctness of the basic (gauge) idea of ​​the standard theory of the electroweak interaction. However, to test the theory in full, it is also necessary to experimentally investigate the mechanism of spontaneous symmetry breaking. In the framework of the standard theory, the source of spontaneous symmetry breaking is a special iso-doublet scalar field with a specific. self-action , where is a dimensionless constant, and the constant h has the dimension of mass . The minimum interaction energy is achieved at, and, t, o., the lowest energy. the state - vacuum - contains a non-zero vacuum value of the field. If this mechanism of symmetry breaking really occurs in nature, then there must be elementary scalar bosons - the so-called. Higgs boson(quanta of the Higgs field). The standard theory predicts the existence of at least one scalar boson (it must be neutral). In more complex versions of the theory, there are several. such particles, and some of them are charged (it is possible). Unlike the intermediate bosons, the masses of the Higgs bosons are not predicted by theory.

The gauge theory of the electroweak interaction is renormalizable: this means, in particular, that the amplitudes of the weak and e-magn. processes can be calculated using perturbation theory, and the higher corrections are small, as in ordinary quantum theory (see Fig. Renormalizability). (In contrast to this, the four-fermion theory of S. V. is non-renormalizable and is not an internally consistent theory.)

There are theoretical models Grand Unification, in which as a group electroweak interaction, and the group SU(3) of a strong interaction are subgroups of a single group characterized by a single gauge interaction constant. In even more funds. models, these interactions are combined with gravitational (so-called. superunification).

Lit.: In at Ts. S., Moshkovsky S. A., Beta decay, trans. from English, M., 1970; Weinberg S., Unified theories of interaction of elementary particles, trans. from English, UFN, 1976, v. 118, c. 3, p. 505; Taylor, J., Gauge theories of weak interactions, trans. from English, M., 1978; Towards a unified field theory. Sat. Art., translations, M., 1980; Okun L. B., Leptons and Quarks, 2nd ed., M., 1990. L. B. Okun.

Time is like a river carrying events passing by, and its current is strong; only something will seem to your eyes - and it has already been carried away, and something else is visible, which will also soon be carried away.

Marcus Aurelius

Each of us strives to create a complete picture of the world, including a picture of the Universe, from the smallest subatomic particles to the greatest scales. But the laws of physics are sometimes so strange and counterintuitive that this task can become overwhelming for those who have not become professional theoretical physicists.

The reader asks:

Although this is not astronomy, but maybe you will tell me. The strong force is carried by gluons and binds quarks and gluons together. Electromagnetic is carried by photons and binds electrically charged particles. Gravity is supposedly carried by gravitons and binds all particles to mass. The weak is carried by the W and Z particles, and … is due to decay? Why is the weak force described in this way? Is the weak force responsible for the attraction and/or repulsion of any particles? And what? And if not, why then is this one of the fundamental interactions, if it is not associated with any forces? Thank you.

Let's take a look at the basics. There are four fundamental forces in the universe - gravity, electromagnetism, strong nuclear force and weak nuclear force.


And all these are interactions, forces. For particles whose state can be measured, the application of a force changes its momentum - in ordinary life in such cases we speak of acceleration. And for three of these forces, this is true.

In the case of gravity, total amount energy (mostly mass, but this includes all energy) warps space-time, and the motion of all other particles changes in the presence of anything that has energy. This is how it works in the classical (not quantum) theory of gravity. Maybe there is a more general theory, quantum gravity, where there is an exchange of gravitons, leading to what we observe as a gravitational interaction.

Before proceeding, please understand:

  1. Particles have a property, or something inherent in them, that allows them to feel (or not feel) a certain type of force.
  2. Other interaction-carrying particles interact with the first
  3. As a result of interactions, particles change momentum, or accelerate

In electromagnetism, the main property is electric charge. Unlike gravity, it can be positive or negative. A photon, a particle that carries an interaction associated with a charge, leads to the fact that the same charges repel, and the different ones attract.

It is worth noting that moving charges, or electric currents, experience another manifestation of electromagnetism - magnetism. The same thing happens with gravity, and is called gravitomagnetism (or gravitoelectromagnetism). We will not go deep - the point is that there is not only a charge and a carrier of force, but also currents.

There is also a strong nuclear force, which has three types of charges. Although all particles have energy and are all subject to gravity, and although quarks, half of the leptons and a couple of bosons contain electrical charges, only quarks and gluons have a color charge and can experience the strong nuclear force.

There are a lot of masses everywhere, so gravity is easy to observe. And since the strong force and electromagnetism are quite strong, they are also easy to observe.

But what about the last one? Weak interaction?

We usually talk about it in the context of radioactive decay. A heavy quark or lepton decays into lighter and more stable ones. Yes, the weak force has something to do with it. But in this example it is somehow different from the rest of the forces.

It turns out that the weak force is also a force, just not often talked about. She's weak! 10,000,000 times weaker than electromagnetism at a distance as long as the diameter of a proton.

A charged particle always has a charge, whether it is moving or not. But electricity, created by it, depends on its movement relative to other particles. Current determines magnetism, which is just as important as the electrical part of electromagnetism. Composite particles like the proton and neutron have significant magnetic moments, just like the electron.

Quarks and leptons come in six flavors. Quarks - top, bottom, strange, charmed, charming, true (according to their letter designations in Latin u, d, s, c, t, b - up, down, strange, charm, top, bottom). Leptons - electron, electron-neutrino, muon, muon-neutrino, tau, tau-neutrino. Each of them has an electrical charge, but also a flavor. If we combine electromagnetism and the weak force to get the electroweak force, then each of the particles will have some kind of weak charge, or electroweak current, and a weak force constant. All this is described in the Standard Model, but it was quite difficult to verify this because electromagnetism is so strong.

In a new experiment, the results of which have recently been published, the contribution of the weak interaction has been measured for the first time. The experiment made it possible to determine the weak interaction of up and down quarks

And the weak charges of the proton and neutron. The predictions of the Standard Model for weak charges were:

Q W (p) = 0.0710 ± 0.0007,
Q W (n) = -0.9890 ± 0.0007.

And according to the scattering results, the experiment gave the following values:

Q W (p) = 0.063 ± 0.012,
Q W (n) = -0.975 ± 0.010.

Which agrees very well with the theory, taking into account the error. Experimenters say that by processing more data, they will further reduce the error. And if there are any surprises or discrepancies with standard model, it'll be cool! But nothing indicates this:

Therefore, particles have a weak charge, but we do not expand on it, since it is unrealistically difficult to measure. But we did it anyway, and apparently reaffirmed the Standard Model.

In 1896, the French scientist Henri Becquerel discovered the radioactivity of uranium. This was the first experimental signal about previously unknown forces of nature - the weak interaction. We now know that the weak force is behind many familiar phenomena - for example, it takes part in some thermonuclear reactions that support the radiation of the Sun and other stars.

The name "weak" went to this interaction due to a misunderstanding - for example, for a proton it is 1033 times stronger than the gravitational interaction (see Gravitation, Unity of the Forces of Nature). Rather, it is a destructive interaction, the only force of nature that does not hold matter together, but only destroys it. One could also call it "unprincipled", since in destruction it does not take into account the principles of spatial parity and temporal reversibility, which other forces observe.

The basic properties of the weak interaction became known as early as the 1930s, mainly due to the work of the Italian physicist E. Fermi. It turned out that, unlike gravitational and electrical forces, weak forces have a very small radius of action. In those years, it seemed that there was no radius of action at all - the interaction takes place at one point in space, and, moreover, instantly. This interaction is virtual a short time) turns each proton of the nucleus into a neutron, a positron into a positron and a neutrino, and each neutron into a proton, an electron and an antineutrino. In stable nuclei (see atomic nucleus) these transformations remain virtual, like the virtual production of electron-positron pairs or proton-antiproton pairs in vacuum. If the difference in the masses of nuclei that differ by one in charge is large enough, these virtual transformations become real, and the nucleus changes its charge by 1, throwing out an electron and an antineutrino (electron β-decay) or a positron and neutrino (positron β-decay). Neutrons have a mass that is approximately 1 MeV greater than the sum of the masses of a proton and an electric wave. Therefore, a free neutron decays into a proton, an electron, and an antineutrino with an energy release of approximately 1 MeV. The lifetime of a free neutron is about 10 minutes, although in a bound state, for example, in a deuteron, which consists of a neutron and a proton, these particles live indefinitely.

A similar event occurs with the muon (see Leptons) - it decays into an electron, a neutrino and an antineutrino. Before decaying, the muon lives for about 10 -6 s - much less than the neutron. Fermi's theory explained this by the difference in the masses of the particles involved. The more energy released during decay, the faster it goes. The release of energy during μ-decay is about 100 MeV, approximately 100 times greater than during neutron decay. The lifetime of a particle is inversely proportional to the fifth power of this energy.

As it turned out in recent decades, the weak interaction is nonlocal, i.e., it does not occur instantly and not at one point. According to modern theory, weak interaction is not transmitted instantly, and a virtual electron-antineutrino pair is born 10 -26 s after the muon passes into a neutrino, and this happens at a distance of 10 -16 cm. Not a single ruler, not a single microscope can , of course, to measure such a small distance, just as no stopwatch can measure such a small interval of time. As is almost always the case, modern physics we must be content with indirect data. Physicists build various hypotheses about the mechanism of the process and test all possible consequences of these hypotheses. Those hypotheses that contradict at least one reliable experiment are swept aside, and new experiments are put to verify the remaining ones. This process in the case of the weak interaction continued for about 40 years, until physicists came to the conclusion that the weak interaction is carried by supermassive particles - 100 times heavier than the proton. These particles have spin 1 and are called vector bosons (discovered in 1983 at CERN, Switzerland - France).

There are two charged vector bosons W + , W - and one neutral Z 0 (the icon at the top, as usual, indicates the charge in proton units). The charged vector boson W - "works" in the decays of the neutron and muon. The course of muon decay is shown in Fig. (above, right). Such drawings are called Feynman diagrams, they not only illustrate the process, but also help to calculate it. This is a kind of shorthand formula for the probability of a reaction; it is used here for illustration only.

The muon turns into a neutrino, emitting a W-boson, which decays into an electron and an antineutrino. The released energy is not enough for the real birth of the W-boson, so it is born virtually, i.e., for a very short time. In this case, it is 10 -26 s. During this time, the field corresponding to the W-boson does not have time to form a wave, or otherwise, a real particle (see Fields and particles). A field bunch 10 -16 cm in size is formed, and after 10 -26 s an electron and an antineutrino are born from it.

For the decay of the neutron, one could draw the same diagram, but here it would already mislead us. The fact is that the size of a neutron is 10 -13 cm, which is 1000 times greater than the radius of action of weak forces. Therefore, these forces act inside the neutron, where the quarks are located. One of the three quarks of the neutron emits a W-boson, while passing into another quark. The quark charges in the neutron are -1/3, -1/3, and +2/3, so that one of the two quarks with a negative charge of -1/3 goes over to a quark with a positive charge of +2/3. The result will be quarks with charges -1/3, 2/3, 2/3, which together make up the proton. The reaction products - an electron and an antineutrino - freely fly out of the proton. But the quark that emitted the W-boson received a recoil and began to move in the opposite direction. Why doesn't he fly?

It is held by the strong force. This interaction will drag its two inseparable satellites behind the quark, resulting in a moving proton. The weak decays (associated with the weak interaction) of the remaining hadrons occur according to a similar scheme. All of them come down to the emission of a vector boson by one of the quarks, the transition of this vector boson into leptons (μ-, e-, τ- and ν-particles) and further expansion of the reaction products.

Sometimes, however, hadronic decays also occur: a vector boson can decay into a quark-antiquark pair, which will turn into mesons.

So, a large number of various reactions is reduced to the interaction of quarks and leptons with vector bosons. This interaction is universal, that is, it is the same for quarks and leptons. The universality of the weak interaction, in contrast to the universality of the gravitational or electromagnetic interaction, has not yet received an exhaustive explanation. IN modern theories the weak interaction is combined with the electromagnetic interaction (see Unity of the Forces of Nature).

For symmetry breaking by the weak interaction, see Parity, Neutrino. The article The unity of the forces of nature tells about the place of weak forces in the picture of the microworld.

The carriers of the weak interaction are the vector bosons W + , W− and Z 0 . In this case, the interaction of the so-called charged weak currents and neutral weak currents is distinguished. Interaction of charged currents (with the participation of charged bosons W± ) leads to a change in particle charges and the transformation of some leptons and quarks into other leptons and quarks. Interaction of neutral currents (with the participation of a neutral boson Z 0 ) does not change the particle charges and transforms leptons and quarks into the same particles.

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    Using the Pauli hypothesis, Enrico Fermi developed in 1933 the first theory of beta decay. Interestingly, his work was refused to be published in the journal Nature, referring to the excessive abstractness of the article. Fermi's theory is based on the use of the secondary quantization method, similar to that which had already been applied by that time for the processes of emission and absorption of photons. One of the ideas voiced in the work was also the assertion that the particles emitted from the atom were not initially contained in it, but were born in the process of interaction.

    For a long time it was believed that the laws of nature are symmetrical with respect to mirror reflection, that is, the result of any experiment should be the same as the result of an experiment carried out on a mirror-symmetric installation. This symmetry with respect to spatial inversion (which is usually denoted as P) is related to the law conservation parity . However, in 1956, while theoretically considering the process of K-meson decay, Yang Zhenning and Li Zongdao suggested that the weak interaction may not obey this law. As early as 1957, Wu Jiansong's group confirmed this prediction in a beta decay experiment, which earned Yang and Li the 1957 Nobel Prize in Physics. Later, the same fact was confirmed in the decay of the muon and other particles.

    To explain the new experimental facts, in 1957 Murray Gell-Mann, Richard Feynman, Robert Marshak and George Sudarshan developed universal theory four-fermion weak interaction, called V − A-theory.

    In an effort to preserve the maximum possible symmetry of interactions, L. D. Landau suggested in 1957 that although P-symmetry is broken in weak interactions, combined symmetry must be preserved in them CP- a combination of mirror reflection and replacement of particles by antiparticles. However, in 1964, James-Cronin and Wahl-Fitch found a weak violation in the decays of neutral kaons CP-parity. It was the weak interaction that also turned out to be responsible for this violation, moreover, the theory in this case predicted that in addition to the two generations of quarks and leptons known by that time, there should be at least one more generation. This prediction was confirmed first in 1975 with the discovery of the tau lepton, and then in 1977 with the discovery of the b quark. Cronin and Fitch received the 1980 Nobel Prize in Physics.

    Properties

    All fundamental fermions (leptons and quarks) take part in the weak interaction. This is the only interaction in which neutrinos participate (apart from gravity, which is negligible in the laboratory), which explains the colossal penetrating power of these particles. Weak interaction allows leptons, quarks and their antiparticles to exchange energy, mass, electric charge and quantum numbers - that is, to turn into each other.

    The weak force gets its name from the fact that its characteristic intensity is much lower than that of electromagnetism. In elementary particle physics, the intensity of an interaction is usually characterized by the rate of processes caused by this interaction. The faster the processes proceed, the higher the intensity of interaction. At energies of interacting particles of the order of 1 GeV, the characteristic rate of processes due to weak interaction is about 10 −10 s, which is approximately 11 orders of magnitude higher than for electromagnetic processes, that is, weak processes are extremely slow processes.

    Another characteristic of the intensity of interaction is the length free path of particles in a substance. So, in order to stop a flying hadron due to the strong interaction, a plate of iron several centimeters thick is required. And a neutrino, which only participates in the weak interaction, can fly through a plate billions of kilometers thick.

    Among other things, the weak interaction has a very small radius of action - about 2 10 -18 m (this is approximately 1000 times smaller size nuclei). It is for this reason that, despite the fact that the weak interaction is much more intense than the gravitational one, the range of which is unlimited, it plays a noticeably smaller role. For example, even for nuclei located at a distance of 10 −10 m, the weak interaction is weaker not only electromagnetic, but also gravitational.

    In this case, the intensity of weak processes strongly depends on the energy of the interacting particles. The higher the energy, the higher the intensity. For example, due to the weak interaction, the neutron, the energy release during beta decay of which is approximately 0.8 MeV, decays in about 10 3 s, and the Λ-hyperon, with an energy release of about a hundred times more, already in 10 −10 s. The same is true for energetic neutrinos: the cross section for interaction with a nucleon of a neutrino with an energy of 100 GeV is six orders of magnitude larger than that of a neutrino with an energy of about 1 MeV. However, at energies of the order of several hundred GeV (in the center-of-mass system of colliding particles), the intensity of the weak interaction becomes comparable to the energy of the electromagnetic interaction, as a result of which they can be described uniformly as an electroweak interaction.

    The weak interaction is the only one of the fundamental interactions for which the law conservation parity does not hold, which means that the laws that weak processes obey change when the system is mirrored. Violation of the law of conservation of parity leads to the fact that only the left particles (whose spin is directed opposite to the momentum) are subject to weak interaction, but not the right ones (whose spin is co-directed with the momentum), and vice versa: the right antiparticles interact in a weak way, but the left ones are inert.

    In addition to spatial parity, the weak interaction also does not preserve the combined space-charge parity, that is, the only known interaction violates the principle CP-invariance .

    Theoretical description

    Fermi theory

    The first theory of the weak interaction was developed by Enrico Fermi in the 1930s. His theory is based on a formal analogy between the process of β-decay and the electromagnetic processes of photon emission. Fermi's theory is based on the interaction of the so-called hadron and lepton currents. In this case, unlike electromagnetism, it is assumed that their interaction is of a contact nature and does not imply the presence of a carrier similar to a photon. IN modern notation the interaction between the four main fermions (proton, neutron, electron and neutrino) is described by an operator of the form

    G F 2 p ¯ ^ n ^ ⋅ e ¯ ^ ν ^ (\displaystyle (\frac (G_(F))(\sqrt (2)))(\hat (\overline (p)))(\hat (n) )\cdot (\hat (\overline (e)))(\hat (\nu ))),

    Where G F (\displaystyle G_(F))- the so-called  Fermi constant, numerically equal to approximately 10 −48 J/m³ or 10 − 5 / m p 2 (\displaystyle 10^(-5)/m_(p)^(2)) (m p (\displaystyle m_(p))- proton mass) in units, where ℏ = c = 1 (\displaystyle \hbar =c=1); p ¯ ^ (\displaystyle (\hat (\overline (p))))- proton creation operator (or antiproton annihilation), n ^ (\displaystyle (\hat(n)))- neutron annihilation operator (antineutron creation), e ¯ ^ (\displaystyle (\hat (\overline (e))))- operator of electron creation (positron annihilation), ν ^ (\displaystyle (\hat (\nu )))- neutrino annihilation operator (antineutrino generation).

    Work p ¯ ^ n ^ (\displaystyle (\hat (\overline (p)))(\hat (n))), responsible for the conversion of a neutron into a proton, was called the nucleon current, and e ¯ ^ ν ^ , (\displaystyle (\hat (\overline (e)))(\hat (\nu )),) converting an electron into a neutrino - lepton. It is postulated that these currents, similarly to electromagnetic currents, are 4-vectors p ¯ ^ γ μ n ^ (\displaystyle (\hat (\overline (p)))\gamma _(\mu )(\hat (n))) And e ¯ ^ γ μ ν ^ (\displaystyle (\hat (\overline (e)))\gamma _(\mu )(\hat (\nu ))) (γ μ , μ = 0 … 3 (\displaystyle \gamma _(\mu ),~\mu =0\dots 3)- Dirac matrices). Therefore, their interaction is called vector.

    The essential difference between the weak currents introduced by Fermi and the electromagnetic ones is that they change the charge of the particles: a positively charged proton becomes a neutral neutron, and a negatively charged electron becomes a neutral neutrino. In this regard, these currents are called charged currents.

    Universal V-A Theory

    The universal theory of the weak interaction, also called V−A-theory, was proposed in 1957 by M. Gell-Mann, R. Feynman, R. Marshak and J. Sudarshan. This theory took into account the recently proved fact of parity violation ( P-symmetries) in the case of weak interaction. For this, weak currents were represented as the sum of the vector current V and axial A(hence the name of the theory).

    The vector and axial currents behave in exactly the same way under Lorentz transformations. However, during spatial inversion, their behavior is different: the vector current remains unchanged during such a transformation, while the axial current changes sign, which leads to parity violation. In addition, currents V And A differ in the so-called charge parity (violate C-symmetry).

    Similarly, the hadronic current is the sum of the quark currents of all generations ( u- top, d- bottom, c- enchanted s- strange, t- true, b- lovely quarks):

    u ¯ ^ d ′ ^ + c ¯ ^ s ′ ^ + t ¯ ^ b ′ ^ . (\displaystyle (\hat (\overline (u)))(\hat (d^(\prime )))+(\hat (\overline (c)))(\hat (s^(\prime ))) +(\hat (\overline (t)))(\hat (b^(\prime ))).)

    Unlike the lepton current, however, here the operators d ′ ^ , (\displaystyle (\hat (d^(\prime ))),) s ′ ^ (\displaystyle (\hat (s^(\prime )))) And b ′ ^ (\displaystyle (\hat (b^(\prime )))) are a linear combination of operators d ^ , (\displaystyle (\hat (d)),) s ^ (\displaystyle (\hat(s))) And b ^ , (\displaystyle (\hat (b)),) that is, the hadron current contains a total of not three, but nine terms. These terms can be combined into a single 3×3 matrix called the  Cabibbo -  Kobayashi -  Maskawa matrix. This matrix can be parameterized with three angles and a phase factor. The latter characterizes the degree of violation CP-invariance in the weak interaction.

    All terms in the charged current are the sum of the vector and axial operators with multipliers equal to one.

    L = G F 2 j w ^ j w † ^ , (\displaystyle (\mathcal (L))=(\frac (G_(F))(\sqrt (2)))(\hat (j_(w)))(\ hat (j_(w)^(\dagger ))),)

    Where j w ^ (\displaystyle (\hat (j_(w)))) is the charged current operator, and j w † ^ (\displaystyle (\hat (j_(w)^(\dagger ))))- conjugate to it (obtained by replacing e ¯ ^ ν e ^ → ν e ¯ ^ e ^ , (\displaystyle (\hat (\overline (e)))(\hat (\nu _(e)))\rightarrow (\hat (\overline (\ nu _(e))))(\hat (e)),) u ¯ ^ d ^ → d ¯ ^ u ^ (\displaystyle (\hat (\overline (u)))(\hat (d))\rightarrow (\hat (\overline (d)))(\hat (u ))) etc.)

    Theory of Weinberg - Salam

    In the modern form, the weak interaction is described as part of a single electroweak interaction in the framework of the Weinberg-Salam theory. This is a quantum field theory with a gauge group SU(2)× U(1) and the spontaneously broken symmetry of the vacuum state caused by the action of the Higgs boson field. The proof of the renormalizability of such a model by Martinus Veltman and Gerard "t Hooft was awarded the 1999 Nobel Prize in Physics.

    In this form, the theory of the weak interaction is included in the modern Standard Model, and it is the only interaction that breaks symmetries P And CP .

    According to the theory of the electroweak interaction, the weak interaction is not a contact, but has its own carriers - vector bosons W + , W− and Z 0 with non-zero mass and spin equal to 1. The mass of these bosons is about 90 GeV / s², which causes a small range of weak forces.

    In this case, charged bosons W± are responsible for the interaction of charged currents, and the existence of a neutral boson Z 0 means the existence of neutral currents as well. Such currents, indeed, were discovered experimentally. An example of interaction with their participation is, in particular, the elastic scattering of a neutrino by a proton. In such interactions, both the type of particles and their charges are preserved.

    To describe the interaction of neutral currents, the Lagrangian must be supplemented with a term of the form

    L = G F ρ 2 2 f 0 ^ f 0 ^ , (\displaystyle (\mathcal (L))=(\frac (G_(F)\rho )(2(\sqrt (2))))(\hat ( f_(0)))(\hat (f_(0))),)

    where ρ is a dimensionless parameter, equal to unity in the standard theory (experimentally it differs from unity by no more than 1%), f 0 ^ = ν e ¯ ^ ν e ^ + ⋯ + e ¯ ^ e ^ + ⋯ + u ¯ ^ u ^ + … (\displaystyle (\hat (f_(0)))=(\hat (\overline ( \nu _(e))))(\hat (\nu _(e)))+\dots +(\hat (\overline (e)))(\hat (e))+\dots +(\hat (\overline (u)))(\hat (u))+\dots )- self-adjoint neutral current operator.

    Unlike charged currents, the neutral current operator is diagonal, that is, it translates particles into themselves, and not into other leptons or quarks. Each of the terms of the neutral current operator is the sum of a vector operator with a multiplier and an axial operator with a multiplier I 3 − 2 Q sin 2 ⁡ θ w (\displaystyle I_(3)-2Q\sin ^(2)\theta _(w)), Where I 3 (\displaystyle I_(3))- the third projection of the so-called weak