MINISTRY OF EDUCATION AND SCIENCE OF RUSSIA
Federal state budget educational institution
higher vocational education
"St. Petersburg State Electrotechnical University"LETI" named after. V. I. Ulyanova (Lenin)"
(SPbGETU)
Faculty of Economics and Management
Department of Physics
In the discipline "Concepts" modern natural science"
on the topic " Weak interaction"
Checked:
Altmark Alexander Moiseevich
Completed:
student gr. 3603
Kolisetskaya Maria Vladimirovna
Saint Petersburg
1. The weak interaction is one of the four fundamental interactions
History of the study
Role in nature
The weak force is one of the four fundamental forces
The weak force, or weak nuclear force, is one of the four fundamental interactions
The weak interaction is short-range - it manifests itself at distances significantly smaller than atomic nucleus
Vector bosons are carriers of the weak interaction
For the first time, weak interactions were observed during the decay of atomic nuclei. And, as it turned out, these decays are associated with the transformation of a proton into a neutron in the nucleus and vice versa:
r? n + e+ + ?e, n ? p + e- + e,
where n is a neutron, p is a proton, e- is an electron, ??e is an electron antineutrino.
Elementary particles are usually divided into three groups:
) photons; this group consists of only one particle - a photon - a quantum electromagnetic radiation;
) leptons (from the Greek “leptos” - light), participating only in electromagnetic and weak interactions. Leptons include the electron and muon neutrino, the electron, the muon and the heavy lepton discovered in 1975 - the t-lepton, or taon, with a mass of approximately 3487me, as well as their corresponding antiparticles. The name leptons is due to the fact that the masses of the first known leptons were smaller than the masses of all other particles. Leptons also include the secret neutrino, whose existence in lately also installed;
) hadrons (from the Greek “adros” - large, strong). Hadrons have strong interactions along with electromagnetic and weak ones. Of the particles discussed above, these include the proton, neutron, pions and kaons.
Properties of the weak interaction
The weak interaction has distinctive properties:
All fundamental fermions take part in weak interaction
Operation P changes the sign of any polar vector
The operation of spatial inversion transforms the system into a mirror symmetric one. Mirror symmetry observed in processes under the influence of strong and electromagnetic interactions. Mirror symmetry in these processes means that in mirror-symmetric states transitions are realized with the same probability.
G. ? Yang Zhenning, Li Zongdao received the Nobel Prize in Physics. For his in-depth studies of the so-called parity laws, which led to important discoveries in the field of elementary particles.
In addition to spatial parity, the weak interaction also does not preserve combined space-charge parity, that is, the only known interaction violates the principle of CP invariance
Charge symmetry means that if there is any process involving particles, then when they are replaced by antiparticles (charge conjugation), the process also exists and occurs with the same probability. Charge symmetry is absent in processes involving neutrinos and antineutrinos. In nature, only left-handed neutrinos and right-handed antineutrinos exist. If each of these particles (for definiteness, we will consider the electron neutrino? e and antineutrino e) is subjected to the operation of charge conjugation, then they will turn into non-existent objects with lepton numbers and helicities.
Thus, in weak interactions, P- and C-invariance are violated simultaneously. However, what if two consecutive operations are performed on a neutrino (antineutrino)? P- and C-transformations (the order of operations is not important), then we again obtain neutrinos that exist in nature. Sequence of operations and (or in reverse order) is called the CP transformation. The result of the CP transformation (combined inversion) of ?e and e is as follows:
Thus, for neutrinos and antineutrinos, the operation that transforms a particle into an antiparticle is not a charge conjugation operation, but a CP transformation.
History of the study
The study of weak interactions continued long period.
In 1896, Becquerel discovered that uranium salts emit penetrating radiation (γ decay of thorium). This was the beginning of the study of weak interactions.
In 1930, Pauli put forward the hypothesis that during ? decay, along with electrons (e), light neutral particles are emitted? neutrino (?). In the same year, Fermi proposed a quantum field theory of β-decay. The decay of a neutron (n) is a consequence of the interaction of two currents: the hadronic current converts a neutron into a proton (p), the leptonic current produces an electron + neutrino pair. In 1956, Reines first observed the reaction of er? ne+ in experiments near a nuclear reactor.
Lee and Yang explained the paradox in the decays of K+ mesons (? ~ ? mystery)? decay into 2 and 3 pions. It is associated with non-conservation of spatial parity. Mirror asymmetry has been discovered in the β-decay of nuclei, the decays of muons, pions, K-mesons and hyperons.
In 1957, Gell-Mann, Feynman, Marshak, Sudarshan proposed universal theory weak interaction, based on the quark structure of hadrons. This theory, called V-A theories, led to the description of the weak interaction using Feynman diagrams. At the same time, fundamentally new phenomena were discovered: violation of CP invariance and neutral currents.
In the 1960s by Sheldon Lee Glashow
In 1991-2001, a study of the decays of Z0 bosons was carried out at the LEP2 accelerator (CERN), which showed that in nature there are only three generations of leptons: ?e, ?? And??.
Role in nature
nuclear interaction is weak
The most common process caused by weak interaction is the b-decay of radioactive atomic nuclei. Radioactivity phenomenon<#"justify">List of used literature
1. Novozhilov Yu.V. Introduction to the theory of elementary particles. M.: Nauka, 1972
Okun B. Weak interaction of elementary particles. M.: Fizmatgiz, 1963
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 best known 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 holding together nuclear particles(nucleons and quarks) and 10 10 less than the Coulomb force 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.
By modern ideas, formulated in Standard model, the weak force 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.
Peaks of Feynman diagrams describing possible processes with the participation calibration W-i 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 values isospin for particles that can exchange W and Z bosons ± 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 Nobel Prize for 1980. Non-conservation of CP symmetry occurs much less frequently than parity violation. It also means, since the conservation of CPT symmetry rests on the fundamental physical principles– Lorentz and short-range transformations, the possibility of breaking T-symmetry, i.e. non-invariance physical processes by changing 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. So important role weak interaction is connected with the fact that it does not obey a number of prohibitions characteristic of strong and electromagnetic interactions. In particular, the weak interaction turns charged leptons into neutrinos, and quarks of one flavor into quarks of another.
In 1896, French scientist Henri Becquerel discovered radioactivity in uranium. This was the first experimental signal about previously unknown forces of nature - weak interaction. We now know that the weak force lies behind many familiar phenomena - for example, it takes part in some thermonuclear reactions, supporting the radiation of the Sun and other stars.
The name “weak” was given to this interaction due to a misunderstanding - for example, for a proton it is 1033 times stronger gravitational interaction(see Gravity, This Unity of Nature). This is, rather, a destructive interaction, the only force of nature that does not hold the substance 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 are observed by other forces.
The basic properties of the weak interaction became known back in the 1930s, mainly thanks to the work of the Italian physicist E. Fermi. It turned out that, unlike gravitational and electrical forces, weak forces have a very short range of action. In those years, it seemed that there was no radius of action at all - interaction took place at one point in space, and, moreover, instantly. This interaction is virtual (on short time) converts each proton of the nucleus into a neutron, a positron into a positron and a neutrino, and each neutron into a proton, electron and antineutrino. In stable nuclei (see Atomic nucleus), these transformations remain virtual, like the virtual creation of electron-positron pairs or proton-antiproton pairs in a 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, emitting an electron and an antineutrino (electron decay) or a positron and a neutrino (positron decay). Neutrons have a mass that exceeds by approximately 1 MeV the sum of the masses of a proton and an electron. Therefore, a free neutron decays into a proton, an electron and an antineutrino, releasing an energy of approximately 1 MeV. Life time free neutron approximately 10 minutes, although bound state, for example, in deuteron, which consists of a neutron and a proton, these particles live indefinitely.
A similar event occurs with the muon (see Peptons) - it decays into an electron, neutrino and antineutrino. Before decaying, a muon lives about c - much less than a 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 -decay is about 100 MeV, approximately 100 times greater than during the decay of a neutron. 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, that is, it does not occur instantly and not at one point. According to modern theory, the weak interaction is not transmitted instantly, but a virtual electron-antineutrino pair is born s after the muon turns into a neutrino, and this happens at a distance of cm. Not a single ruler, not a single microscope can, of course, measure such a small distance, just as no stopwatch can measure such a small interval of time. As is almost always the case, in modern physics we must be content with indirect data. Physicists build various hypotheses about the mechanism of the process and test all sorts of consequences of these hypotheses. Those hypotheses that contradict at least one reliable experiment are discarded, and new experiments are carried out to test the remaining ones. This process, in the case of the weak interaction, continued for about 40 years, until physicists became convinced that the weak interaction was 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 and one neutral one (the icon at the top, as usual, indicates the charge in proton units). A charged vector boson “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 for the formula for the probability of a reaction; it is used here for illustration purposes only.
The muon turns into a neutrino, emitting a -boson, which decays into an electron and an antineutrino. The released energy is not enough for the real birth of a -boson, so it is born virtually, i.e. for a very short time. IN in this case this is s. During this time, the field corresponding to the -boson does not have time to form a wave, or otherwise, a real particle (see Fields and particles). A field clot of cm in size is formed, and after c an electron and an antineutrino are born from it.
For the decay of a neutron it would be possible to draw the same diagram, but here it would already mislead us. The fact is that the size of a neutron is cm, which is 1000 times greater than radius actions of weak forces. Therefore, these forces act inside the neutron, where the quarks are located. One of the three neutron quarks emits a -boson, transforming into another quark. Charges of quarks in a neutron: -1/3, - 1/3 and so one of the two quarks with negative charge-1/3 goes into a quark with positive charge. The result will be quarks with charges - 1/3, 2/3, 2/3, which together make up a proton. The reaction products - electron and antineutrino - freely fly out of the proton. But it’s a quark that emitted a -boson. received the kickback and began to move in opposite direction. Why doesn't he fly out?
It is held together by a strong interaction. This interaction will carry the quark along with its two inseparable companions, resulting in a moving proton. By similar scheme weak decays (associated with weak interactions) of the remaining hadrons occur. They all boil down to the emission of a vector boson by one of the quarks, the transition of this vector boson into leptons (, and -particles) and the 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, large number of various reactions comes down 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 gravitational or electromagnetic interaction, has not yet received a comprehensive explanation. In modern theories, the weak interaction is combined with the electromagnetic interaction (see Unity of the forces of nature).
On symmetry breaking by the weak interaction, see Parity, Neutrinos. The article The Unity of the Forces of Nature talks about the place of weak forces in the picture of the microworld
WEAK INTERACTION- one of four known foundations. interactions between . S.v. much weaker than strong and el-magnetic. interactions, but much stronger than gravitational ones. In the 80s It has been established that weak and el-magn. interactions - diff. 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 energies of GeV, characteristic of elementary particle physics. At such energies, the process caused by the strong interaction occurs in time s, el-magn. process over time, the characteristic time of processes occurring due to solar energy. ( 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 detained by an iron plate several times thick. tens of centimeters, while a neutrino, which only possesses a strong velocity, would pass, without experiencing a single collision, through an iron plate with a thickness of about a billion km. Gravity is even weaker. interaction, the strength of which at an energy of ~1 GeV is 10 33 times less than that of solar energy. However, usually the role of gravity. interactions are much more noticeable than the role of S. century. This is due to the fact that gravitational interaction, like electromagnetic interaction, has an infinitely large range of action; therefore, for example, gravitational forces act 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 short range of action: approx. 2*10 -16 cm (which is three orders of magnitude less than radius strong interaction). As a result of this, for example, S. v. between the nuclei of two neighboring atoms located at a distance of 10 -8 cm is negligibly small, incomparably weaker not only than electromagnetic, but also gravitational. interactions between them.
However, despite the small size and short action, S. century. plays a very important role in nature. So, if it were possible to “turn off” the solar energy, then the Sun would go out, since the process of converting a proton into a neutron, positron and 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 main source of energy from the Sun and most stars (see Hydrogen cycle). Processes of S. century. with the emission of neutrinos are generally extremely important in evolution of stars, because they cause energy loss by very hot stars in explosions supernovae with the formation of pulsars, etc. If there were no solar energy, muons, mesons, and strange and charmed particles, which decay as a result of solar energy, would be stable and widespread in ordinary matter. Such a large role of SE is due to the fact that it is not subject to a number of prohibitions characteristic of strong and el-magnetic power. interactions. In particular, S. v. transforms charged leptons into neutrinos, and one type (flavor) into quarks of other types.
The intensity of weak processes increases rapidly with increasing energy. So, neutron beta decay,energy release in Krom is small (~1 MeV), lasts approx. 10 3 s, which is 10 13 times greater than the lifetime of a hyperon, the energy release during its decay is ~100 MeV. The interaction cross section with nucleons for neutrinos with an energy of ~100 GeV is approx. a million times more than for neutrinos with energy ~1 MeV. According to theoretical According to the ideas, 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 inertia of colliding particles). At these energies and at large momentum transfers, effects associated with the existence of intermediate vector bosons. At distances between colliding particles much less than 2*10 -16 cm (Compton wavelength intermediate bosons), S.v. and el-magn. interactions have almost the same intensity.
Naib. a common process caused by S. century - beta decay radioactive atomic nuclei. In 1934, E. Fermi built a theory of decay involving certain creatures. modifications formed the basis of the subsequent theory of the so-called. universal local four-fermion system. (Fermi interactions). According to Fermi's theory, the electron and neutrino (more precisely,) escaping from the radioactive nucleus were not in it before, but arose at the moment of decay. This phenomenon is similar to the emission of low energy photons ( visible light) excited atoms or high-energy photons (quanta) excited nuclei. The reason for such processes is the interaction of electricity. particles with el-magn. field: a moving charged particle creates electromagnetic current, which disturbs the electric magnet. field; As a result of interaction, the particle transfers energy to the quanta of this field - photons. Interaction of photons with el-magn. current is described by the expression A. Here e- elementary electrical charge, which is a constant el-magn. interactions (see Interaction constant), A- photon field operator (i.e., photon creation and annihilation operator), j em - el-magn density operator. current (Often, the expression for electromagnetic current also includes the multiplier e.) All charges contribute to j em. particles. For example, the term corresponding to the electron has the form: where is the operator of the annihilation of an electron or the birth of a positron, and is the operator of the birth of an electron or the annihilation of a positron. [For simplicity, it is not shown above that j um, 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 creation of electron-positron pairs by photons (see. Birth of couples)or annihilation these pairs into photons. Photon exchange between two charges. particles leads to their interaction with each other. As a result, for example, scattering of an electron by a proton occurs, which is shown schematically Feynman diagram, presented in Fig. 1. When a proton in the nucleus passes from one level to another, the same interaction can lead to the birth of an electron-positron pair (Fig. 2).
Fermi's decay theory is essentially similar to the el-magnetic theory. processes. Fermi based the theory on the interaction of two “weak currents” (see. Current in quantum field theory), but interacting with each other not at a distance by exchanging a particle - a field quantum (photon in the case of electromagnetic interaction), but contactally. This is the interaction between four fermion fields (four fermions p, n, e and neutrino v) in modern times. notation has the form: 
. Here G F- Fermi constant, or constant of weak four-fermion interaction, experimental. meaning of cut 
erg*cm 3 (the value has the dimension of the square of the length, and in units it is a constant 
, Where M- proton mass), - proton birth operator (antiproton annihilation), - neutron annihilation operator (antineutron birth), - electron birth operator (positron annihilation), v
- operator of neutrino destruction (antineutrino birth). (Here and henceforth, the operators of creation and annihilation of particles are indicated by the symbols of the corresponding particles, typed in bold.) The current that converts a neutron into a proton was subsequently called nucleon, and the current - lepton. Fermi postulated that, like an el-magn. current, weak currents are also four-dimensional vectors: Therefore, the Fermi interaction is called. vector. 
Similar to the birth 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” symbol above the symbols of the corresponding particles]. The interaction of lepton and nucleon currents should lead to other processes, for example. to reaction 
(Fig. 4), to steam (Fig. 5) and 
etc.
Creatures The difference between weak currents and electromagnetic ones is that a weak current changes the charge of particles, while an electric current changes the charge of particles. the current does not change: a weak current turns a neutron into a proton, an electron into a neutrino, and an electromagnetic one leaves a proton as a proton, and an electron as an electron. Therefore, weak tokii ev are called. charged currents. According to this terminology, an ordinary electric magnet. her current is neutral current.
Fermi's theory was based on the results of three different studies. areas: 1) experimental. research of the S. century itself (-decay), which led to the hypothesis of the existence of neutrinos; 2) experiment. research on the strong force (), which led to the discovery of protons and neutrons and the understanding that nuclei are made of these particles; 3) experiment. and theoretical el-magnetic research 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 research into the strong, weak and el-magnetic fields. interactions.
The theory of universal four-fermion sv. differs from Fermi's theory in a number of ways and points. These differences, established over subsequent years as a result of the study of elementary particles, boiled down to the following.
The hypothesis that S. v. does not preserve parity, was put forward by Lee Tsung-Dao and Yang Chen Ning in 1956 with theoretical decay research K-mesons; soon failure R- and C-parities were discovered experimentally in the decay of nuclei [Bu Chien-Shiung and co-workers], in the decay of the muon [R. Garwin (R. Garwin), L. Lederman (L. Lederman), V. Telegdi (V. Telegdi), J. Friedman (J. Friedman), etc.] and in the decays of other particles.
Summarizing a huge experiment. material, M. Gell-Mann, P. Feynman, P. Marshak, and E. Sudarshan in 1957 proposed the theory of universal S. v. - 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 the lepton and quark currents, with each of these elementary currents containing the same combination of Dirac matrices:
As it turned out later, the charger. The lepton current, represented in 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 your neutrino.
Charge the hadronic current, represented by the term in Fermi theory, is the sum of quark currents. By 1992, five types of quarks were known , from which all known hadrons are constructed, and the existence of a sixth quark is assumed ( t With Q=+ 2 / 3). Charged quark currents, as well as 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 vector and axial currents with coefficients equal to unity.
The coefficients of nine charged quark currents are usually represented as a 3x3 matrix, the edges of which are parameterized by three angles and a phase factor characterizing the disturbance CP-invariance in weak decays. This matrix is called Kobayashi - Maskawa matrices (M. Kobayashi, T. Maskawa).
Lagrangian S. v. charged currents has the form: 
Eater, conjugated, etc.). This interaction of charged currents quantitatively describes a huge number of weak processes: leptonic, semi-leptonic ( 
etc.) and non-leptonic ( 
,, etc.). Many of these processes were discovered after 1957. During this period, two fundamentally new phenomena were also discovered: violation of CP invariance and neutral currents. 
Violation of CP invariance was discovered in 1964 in an experiment by J. Christenson, J. Cronin, V. Fitch and R. Turley, who observed decay of long-lived K° mesons into two mesons. Later, violation of CP invariance was also observed in semileptonic decays. To clarify the nature of the CP-non-invariant interaction, it would be extremely important to find k-l. CP-non-invariant process in decays or interactions of other particles. In particular, the search for the neutron dipole moment is of great interest (the presence of which would mean a violation of invariance with respect to time reversals, and therefore, according to the theorem SRT, and CP-invariance).
The existence of neutral currents was predicted by the unified theory of weak and electric currents. interactions created in the 60s. Sh. Glashow, S. Weinberg, A. Salam and others and later received the name. standard theory of electroweak interaction. According to this theory, S. v. 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 charge interaction. currents (Fig. 6), and Z 0-bosons are neutral (Fig. 7). In the standard theory, three intermediate bosons and a photon are vector quanta, the so-called. gauge fields, acting at asymptotically large transfers of four-dimensional momentum ( , m z, Where m w , m z- masses W- and Z-bosons in energy. units) are 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 discovered, as well as the effects of parity nonconservation in the interaction of electrons with nucleons, caused by the electron neutral current (these effects were first observed in experiments on parity nonconservation in atomic transitions conducted in Novosibirsk by L. M. Barkov and M. S. Zolotorev, as well as in experiments on electron scattering on 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 standard theory (experimental value p coincides with 1 within one percent of experimental accuracy and calculation accuracy radiation 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 is the case with 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- third projection of the so-called. weak isotopic spin, Q- particle charge, and - Weinberg angle.
The necessity of the existence of four vector fields of intermediate bosons W + , W -, Z 0 and photon A can be explained next. way. As is known, in el-magn. interaction electrical charge plays double role: on the one hand, it is a conserved quantity, and on the other, it is a source of el-magn. field that interacts between charged particles (interaction constant e). This is the role of electricity. charge is provided by a 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 el-magn. the field, which is a 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 is electrical. charge served as the starting point for the construction of theories and other types of interactions. In these theories, conserved quantities (for example, isotopic spin) are simultaneously sources of certain gauge fields that transfer interactions between particles. In the case of several types of “charges” (for example, 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. gauge fields. (Multiplets of gauge fields corresponding to local non-Abelian symmetries are called Young-Mills fields.) In particular, so that isotopic. spin [to which the local group responds SU(2)] acted as an interaction constant, three gauge fields with charges 1 and 0 are needed. Since in the S. century. charged currents of particle pairs are involved 
etc., then it is believed that these pairs are doublets of the weak isospin group, i.e. the group SU(2). Invariance of the theory under local group transformations S.U.(2) requires, as noted, the existence of a triplet of massless gauge fields W+, W - , W 0, the source of which is weak isospin (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 saving electricity. charge and weak isospin corresponds to the conservation of weak hypercharge [group [ U(1)]. A weak hypercharge is a source of a neutral gauge field B 0(interaction constant g"). Two mutually orthogonal linear superpositions of fields B° 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 relationship between the constant g, which characterizes the interaction of bosons with a weak current, and the constant e, characterizing the interaction of a photon with electricity. electric shock: 
In order for S. to was of a short-range nature, intermediate bosons should be massive, while the quanta of the original gauge fields - 
- massless. According to the standard theory, the appearance of mass in intermediate bosons occurs when spontaneous symmetry breaking SU(2) X U(1)to U(1) em. Moreover, one of the superpositions of fields B 0 And W 0- photon ( A) remains massless, the a- and Z-bosons acquire masses:
Let's experiment. data on neutral currents were given 
. The expected masses corresponded to this W- and Z-bosons, respectively, and
For detection W- and Z-bosons were specially created. installations in which these bosons are born during collisions of colliding high-energy beams. The first installation came into operation in 1981 at CERN. In 1983, reports appeared about the detection at CERN of the first cases of the birth of intermediate vector bosons. Birth data was published in 1989 W- And Z-bosons at the American proton-antiproton collider - Tevatron, at the Fermi National Accelerator Laboratory (FNAL). K con. 1980s full number W- and Z-bosons observed at the proton-antiproton colliders at CERN and FNAL numbered in the hundreds.
In 1989, the electron-positroin colliders LEP at CERN and SLC at the Stanford Linear Accelerator Center (SLAC) began operating. 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 recorded. The study of Z-boson decays has shown that no other neutrinos, except those previously known, exist in nature. The Z-boson mass was measured with high accuracy: t z = 91.173 0.020 GeV (the mass of the W boson is known with significantly worse accuracy: m w= 80.220.26 GeV). Studying properties W- and Z-bosons confirmed the correctness of the basic (gauge) idea of the standard theory of electroweak interaction. However, to test the theory in full, it is also necessary to experimentally study the mechanism of spontaneous symmetry breaking. Within the standard theory, the source of spontaneous symmetry breaking 
is a special isodoublet scalar field that has a specific self-action 
, Where - dimensionless constant, and the constant h has the dimension of mass 
. The minimum interaction energy is achieved at, and, so, the lowest energy. state - vacuum - contains a non-zero vacuum field value. If this mechanism of symmetry breaking really occurs in nature, then there should be elementary scalar bosons - the so-called. Higgs boson(Higgs field quanta). 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 (this is possible). Unlike intermediate bosons, the masses of 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 el-magnetic interactions. processes can be calculated using perturbation theory, and the higher corrections are small, as in ordinary quantum (see. Renormalizability(In contrast, the four-fermion theory of variable speed is non-renormalizable and is not an internally consistent theory.)
There are theoretical models Great Unification, in which as a group 
electroweak interaction, and the group SU(3)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 ones (the so-called superunification).
Lit.: In 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, vol. 118, v. 3, p. 505; Taylor J., Gauge Theories of Weak Interactions, trans. from English, M., 1978; On the way to a unified field theory. Sat. art., translations, M., 1980; Okun L. B., Leptons and quarks, 2nd ed., M., 1990. L. B. Okun.
Weak interaction
Strong interaction is short-acting. Its range of action is about 10-13 cm.
Particles participating in strong interactions are called hadrons. In an ordinary stable substance, not too much high temperature strong interaction does not cause any processes. Its role is to create a strong bond between nucleons (protons and neutrons) in nuclei. The binding energy averages about 8 MeV per nucleon. Moreover, in collisions of nuclei or nucleons with sufficient high energy(on the order of hundreds of MeV), strong interaction leads to numerous nuclear reactions: fission of nuclei, transformation of some nuclei into others, etc.
Starting from energies of colliding nucleons of the order of several hundred MeV, strong interaction leads to the production of P-mesons. At even higher energies, K-mesons and hyperons, and many meson and baryon resonances are born (resonances are short-lived excited states of hadrons).
At the same time, 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.
The theoretical explanation of the nature of the strong interaction has been difficult to develop. A breakthrough emerged only in the early 1960s, when the quark model was proposed. In this theory, neutrons and protons are not considered as elementary particles, but as composite systems built from quarks
The strong interaction quanta are eight gluons. Gluons get their name from English word glue (glue), because they are responsible for the confinement of quarks. The rest masses of gluons are zero. At the same time, gluons have a colored charge, due to which they are capable of interacting with each other, as they say, of self-interaction, which leads to difficulties in describing the strong interaction mathematically due to its nonlinearity.
Its range of action is less than 10-15 cm. The weak interaction is several orders of magnitude weaker not only than the strong one, but also the electromagnetic one. Moreover, it is much stronger than gravitational force in the microcosm.
The first discovered and most common process caused by weak interactions is radioactive b-decay cores.
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This type of radioactivity was discovered in 1896 by A.A. Becquerelem. During the process of radioactive electron /b - -/ decay, one of the neutrons / n/ the atomic nucleus turns into a proton / r/ with electron emission / e-/ and electron antineutrino //:
n ® p + e-+
During the process of positronic /b + -/ decay the following transition occurs:
p® n + e++
In the first theory of b-decay, created in 1934 by E. Fermi, to explain this phenomenon it was necessary to introduce the hypothesis of the existence special type short-range forces that cause the transition
n ® p + e-+
Further research showed that the interaction introduced by Fermi has a universal character.
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It causes the decay of all unstable particles, the masses of which and the selection rules according to quantum numbers do not allow them to decay due to strong or electromagnetic interaction. Weak interaction is inherent in all particles except photons. The characteristic time of weak interaction processes at energies of the order of 100 MeV is 13-14 orders of magnitude longer than the characteristic time for strong interaction.
The weak interaction quanta are three bosons - W + , W − , Z°- bosons. Superscripts indicate sign electric charge these quanta. The weak interaction quanta have a significant mass, which leads to the fact that the weak interaction manifests itself at very short distances.
It must be taken into account that today already in unified theory weak and electromagnetic interactions are combined. There are a number theoretical schemes, in which an attempt is made to create a unified theory of all types of interaction. However, these schemes have not yet been developed enough to be tested experimentally.
26. Structural physics. Corpuscular approach to the description and explanation of nature. Reductionism
The objects of structural physics are the elements of the structure of matter (for example, molecules, atoms, elementary particles) and a more complex formation of them. This:
1) plasma - it is a gas in which a significant portion of the molecules or atoms are ionized;
2) crystals- This solids, in which atoms or molecules are arranged in an orderly manner and form a periodically repeating internal structure;
3) liquids- This physical state substances, ĸᴏᴛᴏᴩᴏᴇ combines the features of a solid state (conservation of volume, a certain tensile strength) and a gaseous state (shape variability).
The liquid is characterized by:
a) short-range order in the arrangement of particles (molecules, atoms);
b) small difference in the kinetic energy of thermal motion and their potential energy interactions.
4) stars,ᴛ.ᴇ. glowing gas (plasma) balls.
When selecting structural equations substances use the following criteria:
Spatial dimensions: particles of the same level have spatial dimensions of the same order (for example, all atoms have dimensions of the order of 10 -8 cm);
Process time: at one level it is approximately the same order of magnitude;
Objects of the same level consist of the same elements (for example, all nuclei consist of protons and neutrons);
The laws that explain processes at one level are qualitatively different from the laws that explain processes at another level;
Objects at different levels differ in their basic properties (for example, all atoms are electrically neutral, and all nuclei are positively electrically charged).
As new levels of structure and states of matter are discovered, the object domain of structural physics is expanding.
It is necessary to take into account that when solving specific physical problems questions related to the elucidation of structure, interaction and movement are closely intertwined.
At the root of structural physics is a corpuscular approach to describing and explaining nature.
For the first time, the concept of the atom as the last and indivisible particle of the body arose in Ancient Greece within the framework of the natural philosophical teachings of the school of Leucippus-Democritus. According to this view, there are only atoms in the world that move in the void. The ancient atomists considered the continuity of matter to be apparent. Different combinations of atoms form different visible bodies. This hypothesis was not based on experimental data. She was just a brilliant guess. But it determined everything for many centuries to come. further development natural sciences.
The Atom Hypothesis indivisible particles substances was revived in natural science, in particular in physics and chemistry, to explain some laws that were established experimentally (for example, the Boyle-Mariotte and Gay-Lussac laws for ideal gases, thermal expansion tel, etc.). Indeed, the Boyle-Marriott law states that the volume of a gas is inversely proportional to its pressure, but it does not explain why this is so. Likewise, when a body is heated, its size increases. But what is the reason for this expansion? In the kinetic theory of matter, these and other experimentally established patterns are explained with the help of atoms and molecules.
Indeed, the directly observed and measurable decrease in gas pressure with an increase in its volume in the kinetic theory of matter is explained as an increase in the free path of its constituent atoms and molecules. It is as a result of this that the volume occupied by the gas increases. Similarly, the expansion of bodies when heated in the kinetic theory of matter is explained by an increase average speed moving molecules.
Explanations in which they try to reduce the properties of complex substances or bodies to the properties of their simpler elements or components, called reductionism. This method of analysis made it possible to solve a large class of problems in natural science.
Until the end of the 19th century. It was believed that an atom is the smallest, indivisible, structureless particle of matter. At the same time, the discoveries of the electron and radioactivity showed that this is not so. Arises planetary model Rutherford atom. Then she is replaced by the model N. Bora. But as before, the thoughts of physicists are aimed at reducing all the diversity complex properties bodies and natural phenomena to simple properties a small number of primary particles. Subsequently, these particles were called elementary. Now they total number exceeds 350. For this reason, it is unlikely that all such particles can be called truly elementary, not containing other elements. This belief is strengthened by the hypothesis of the existence of quarks. According to it, known elementary particles consist of particles with fractional electric charges. They are called quarks.
According to the type of interaction in which elementary particles participate, all of them, except the photon, are classified into two groups:
1) hadrons. It is worth saying that they are characterized by the presence of strong interaction. Moreover, they can also participate in weak and electromagnetic interactions;
2) leptons. Οʜᴎ participate only in electromagnetic and weak interactions;
According to their lifespan, they are distinguished:
a) stable elementary particles. These are the electron, photon, proton and neutrino;
b) quasi-stable. These are particles that decay due to electromagnetic and weak interactions. For example, to + ® m + +;
c) unstable. Οʜᴎ decay due to strong interaction, for example, neutron.
The electric charges of elementary particles are multiples of the smallest charge inherent in the electron. At the same time, elementary particles are divided into particle – antiparticle pairs, for example e - - e + (they have all the same characteristics, and the signs of the electric charge are opposite). Electrically neutral particles also have antiparticles, for example, p -,- .
So, atomistic concept is based on the idea of the discrete structure of matter. The atomic approach explains the properties of a physical object based on the properties of its smallest particles, which at a certain stage of cognition are considered indivisible. Historically, such particles were first recognized as atoms, then as elementary particles, and now as quarks. The difficulty of this approach is the complete reduction of the complex to the simple, which does not take into account the qualitative differences between them.
Until the end of the first quarter of the twentieth century, the idea of the unity of the structure of the macro- and microcosmos was understood mechanistically, as the complete identity of laws and as the complete similarity of the structure of both.
Microparticles were interpreted as miniature copies of macrobodies, ᴛ.ᴇ. like extremely small balls (corpuscles) moving in precise orbits that are completely similar to planetary orbits, with the only difference being that celestial bodies are bound by forces of gravitational interaction, and microparticles by forces of electrical interaction.
After the discovery of the electron (Thomson, 1897 ᴦ.), the creation of the theory of quantum (Planck, 1900 ᴦ.), the introduction of the concept of photon (Einstein, 1905 ᴦ.), atomic doctrine acquired new character.
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The idea of discreteness was extended to the field of electrical and light phenomena, to the concept of energy (in the 19th century, the doctrine of energy served as the sphere of ideas about continuous values and state functions). The most important feature of modern atomic doctrine constitutes the atomism of action. It is due to the fact that the movement, properties and states of various micro-objects are amenable to quantization, ᴛ.ᴇ. are expressed in the form of discrete quantities and ratios. New atomism recognizes relative stability each discrete type of matter, its qualitative certainty, its relative indivisibility and intransformability within the known boundaries of natural phenomena. For example, being divisible by some by physical means, the atom is indivisible chemically, ᴛ.ᴇ. in chemical processes it behaves as something whole, indivisible. The molecule, being divisible chemically into atoms, in thermal motion (up to known limits) behaves as a whole, indivisible, etc.
Particularly important in the concept of new atomism is the recognition of the interconvertibility of any discrete types of matter.
Different levels structural organization physical reality(quarks, microparticles, nuclei, atoms, molecules, macrobodies, megasystems) have their own specific physical laws. But no matter how different the phenomena being studied are from the phenomena being studied classical physics, all experimental data must be described using classical concepts. There is a fundamental difference between the description of the behavior of the microobject under study and the description of the action of measuring instruments. This is the result of the fact that the action of measuring instruments must, in principle, be described in language classical physics, and the object being studied may not be described by this language.
Corpuscular approach to explanation physical phenomena and processes has always been combined with the continuum approach since the emergence of interaction physics. It was expressed in the concept of the field and the disclosure of its role in physical interaction. Representation of the field as a flow of a certain kind of particles ( quantum theory fields) and attribution to any physical object wave properties(Louis de Broglie's hypothesis) brought together these two approaches to the analysis of physical phenomena.
Weak interaction - concept and types. Classification and features of the category "Weak interaction" 2017, 2018.
