Elementary particles and their main characteristics. Stable elementary particles Other existing and hypothetical particles
1. Elementary particles- these are microobjects, the dimensions of which do not exceed the size of atomic nuclei. Elementary particles include protons, neutrons, electrons, mesons, neutrinos, photons, etc.
The expression elementary particles should not be understood as structureless particles incapable of transformation. As science develops, the content of any scientific term gradually moves away from its etymology. Thus, the atom remained indivisible in people’s minds until its emergence at the beginning of the 19th century. chemical atomism. In modern scientific knowledge, an atom is a complex dynamic system capable of diverse rearrangements. Likewise, elementary particles, as their new properties are discovered, reveal an increasingly complex structure.
The most important property of elementary particles is their ability to be born and transform into each other during collisions. For such processes to occur, it is necessary that the colliding particles have high energy. Therefore, particle physics is also called high-energy physics.
According to their lifetime, all elementary particles are divided into three groups: stable, unstable and resonances.
Stable particles exist in a free state for an unlimited time. There are only 11 such particles: proton p, electron e, electron neutrino ν 0, muon neutrino νμ, taun neutrino ντ, their antiparticles p, e, ν e, νμ, ντ, and plus photon γ. Experimental evidence of spontaneous decay of these particles is still unknown.
Unstable particles have an average lifetime τ. which is very large compared to the characteristic nuclear flight time of 10 -23 s (the time it takes light to travel across the diameter of the nuclei). For example, for a neutron τ = 16 min, for a muon τ = 10 -6 s, for a charged pion τ = 10 -8 s, for hyperons and kaons τ = 10 -4 s.
Resonances have lifetimes comparable to the flight time of 10 -23 s. They are registered by resonances on the curves of reaction cross sections versus energy. Many resonances are interpreted as excited states of nucleons and other particles.
2. Fundamental interactions. The variety of interactions observed between elementary particles and in nature as a whole comes down to 4 main types: strong, electromagnetic, weak and gravitational. Strong interaction holds nucleons in atomic nuclei and is inherent in hadrons (protons, neutrons, mesons, hyperons, etc.). Electromagnetic interactions are those that manifest themselves at the macro level - elastic, viscous, molecular, chemical, etc. Weak interactions cause β-decay of nuclei and, along with electromagnetic forces, control the behavior of peptones - elementary particles with half-integer spin that do not participate in strong interactions. Gravitational interaction is inherent in all material objects.
Compare fundamental interactions with each other but their intensities. There is no unambiguous definition of this concept and no method for comparing intensities. Therefore, comparisons based on a set of phenomena are used.
For example, the ratio of the force of gravitational attraction between two protons to the force of Coulomb repulsion is G (m p m p /r 2) /(e 2 /4πε 0 r 2) = 4πε 0 G(m p 2 /e 2) =10 -36. This number is taken as a measure of the ratio of gravitational and electromagnetic interactions.
The ratio between strong and electromagnetic interactions, determined from the cross sections and energies of nuclear reactions, is estimated as 10 4: 1. The intensities of strong and weak interactions are compared in the same way.
Along with intensity, interaction time and distance are also used as a measure of interaction comparison. Usually, to compare times, we take the rates of processes at kinetic energies of colliding particles E = 1 GeV. At such energies, processes caused by strong interactions take place during a nuclear flight of 10 -23 s, processes caused by electromagnetic interactions take about 10 -19 s, weak interactions take about 10 -9 s, and gravitational interactions take about 10 +16 s. .
The mean free path of a particle in a substance is usually taken as distances for comparing interactions. Strongly interacting particles with E = 1 GeV are delayed by a layer of heavy metal up to 1 m thick. While a neutrino, capable of participating only in weak interaction, with an energy 100 times less (E = 10 MeV) can be retained by a layer of 10 9 km!
A. Strong interaction not only the most intense, but also the shortest-acting in nature. At distances exceeding 10 -15 m, its role becomes negligible. While ensuring the stability of nuclei, this interaction has virtually no effect on atomic phenomena. Strong interaction is not universal. It is not inherent in all particles, but only in hadrons - nucleons, mesons, hyperons, etc. There are particles - photons, electrons, muons, neutrinos - that are not subject to strong interaction and are not born due to it in collisions.
b. Electromagnetic interaction intensity is 4 orders of magnitude lower than strong. The main area of its manifestation is distances ranging from a core diameter of 10 -15 m and up to approximately 1 m. This includes the structure of atoms, molecules, crystals, chemical reactions, deformations, friction, light, radio waves and many other physical phenomena accessible to human perception .
The electromagnetic interaction is strongest for electrically charged particles. In neutral particles with non-zero spin it manifests itself weaker and only due to the fact that such particles have a magnetic moment of the order of М=eћ/2m. The electromagnetic interaction is even weaker in neutral pions π 0 and in neutrinos.
An extremely important property of EM interaction is the presence of both repulsion between like-charged particles and attraction between unlike-charged particles. Due to this, EM interactions between atoms and any other objects with zero net charge have a relatively short range, although Coulomb forces between charged particles are long-range.
e. Weak interaction negligible compared to strong and electromagnetic ones. But as distances decrease, it increases rapidly. If we assume that the growth dynamics remain deep enough, then at distances of the order of 10 -20 m the weak interaction will become equal to the strong one. But such distances are not yet available for experimental research.
Weak interaction causes some processes of interconversion of particles. For example, a sigma-plus-hyperon particle, only under the influence of the weak interaction, decays into a proton and a neutral pion, Σ + => p + π 0. Due to the weak interaction, β decay occurs. Particles such as hyperons, kaons, muons would be stable in the absence of weak interaction.
d. Gravitational interaction the weakest. But it is characterized by long-range action, absolute universality (all bodies gravitate) and the same sign between any pair of particles. The latter property leads to the fact that gravitational forces always increase with increasing mass of bodies. Therefore, gravity, despite its insignificant relative intensity, acquires a decisive role in the interactions of cosmic bodies - planets, stars, galaxies
In the world of elementary particles, the role of gravity is negligible. Therefore, in the physics of the atom, nucleus and elementary particles, gravitational interaction is not taken into account.
3. Characteristics of elementary particles. Until the early 50s of the 20th century, while the number of discovered particles was relatively small, general physical quantities were used to describe particles - mass m, kinetic energy E, momentum p and one quantum number - spin s, which made it possible to judge the magnitude of the mechanical and magnetic moments particles. For unstable particles, the average lifetime τ was added here.
But gradually, in the patterns of birth and decay of certain particles, it was possible to identify some features specific to these particles. To designate these properties, new quantum numbers had to be introduced. Some of them were called charges.
For example, it turned out that during the decay of heavy particles, for example, a neutron, it never happens that only light ones are formed, for example, electrons e - , e + and neutrinos. Conversely, when electrons and positrons collide, a neutron cannot be obtained, although the laws of conservation of energy and momentum are satisfied. To reflect this pattern, the quantum number baryon charge B was introduced. They began to believe that such heavy particles - baryons have B = 1, and their antiparticles B = -1. For light particles B = 0. As a result, the discovered pattern took the form of the law of conservation of baryon charge.
Similarly, for light particles, quantum numbers were empirically introduced - lepton charges L - signs of the prohibition of some transformations. We agreed to assume that lepton charges L e = +1 for electrons e - and electron neutrinos ν e ,L µ = + 1 for negative muons µ - and muonic neutrinos ν µ ,L τ = +1 for negative taons τ - and taon neutrinos v τ . For the corresponding antiparticles L= -1. Like baryon charges, leptonic charges are conserved in all interactions.
With the discovery of hyperons born in strong interactions, it turned out that their lifetime is not equal to the flight time of 10 -23 s, which is typical for strongly interacting particles, but 10 13 times longer. This seemed unexpected and strange and could only be explained by the fact that particles born in strong interactions decay in weak interactions. To reflect this property of particles, a quantum number strangeness S was introduced. Strange particles have S = + 1, their antiparticles have S = - 1, and other particles have S = 0.
The electric charge Q of microparticles is expressed through its ratio to the positive elementary charge e +. Therefore, the electric charge Q of particles is also an integer quantum number. For a proton Q = + 1, for an electron Q = -1, for a neutron, neutrino and other neutral particles Q = 0.
In addition to the named parameters, elementary particles have other characteristics that are not considered here.
4. Conservation laws in particle physics can be divided into three groups: general conservation laws, exact conservation laws of charges and approximate conservation laws.
A . Universal laws of conservation are carried out accurately regardless of the scale of phenomena - in the micro-, macro- and mega-world. These laws follow from the geometry of space-time. Homogeneity of time leads to the law of conservation of energy, homogeneity of space - to the law of conservation of momentum, isotropy of space - to the law of conservation of angular momentum, equality of ISO - to the law of conservation of the center of inertia. In addition to these 4 laws, this includes two more related to the symmetry of space - time relative to mirror reflections of coordinate axes. From the mirror symmetry of the coordinate axes it follows that the right-left symmetries of space are identical (the law of conservation of parity). The law associated with the mirror symmetry of time speaks of the identity of phenomena in the microcosm with respect to the change in the sign of time.
b. Exact laws of conservation of charges. Any physical system is assigned an integer charge of each kind. Each charge is additive and conserved. There are 5 such charges: electric Q, baryon B, three leigonic charges - electron L e, muon L µ ton L τ. All charges are integer and can have both positive and negative values of zero.
Electric charge has a double meaning. It represents not only a quantum number, but is also the source of the force field. Baryon and leptonic charges are not sources of the force field. For a complex system, the total charge of any type is equal to the sum of the corresponding charges of the elementary particles included in the system.
V. Approximate conservation laws are fulfilled only in certain types of fundamental interactions. They relate to such characteristics as the strangeness of S, etc.
All listed conservation laws are summarized in Table 26.2.
5. Particles and antiparticles have the same mass, but all their charges are opposite The choice of a pair of particles and antiparticles is arbitrary. For example, in a pair electron + positron, they agreed to consider the electron e as a particle, and the positron e + as an antiparticle. Electron charges Q =-1, B = 0, Le = +1, Lµ= 0, Lτ =0. Positron charges Q = +1, V = 0, Le=-1, Lµ= 0, Lτ =0
All charges of the particle + antiparticle system are equal to zero. Such systems, in which all charges are equal to zero, are called truly neutral. There are true neutrals and particles. There are two of them: γ - quantum (photon) and η - meson. Particles and antiparticles are identical here.
6. Classification of elementary particles not completed yet. One of the classifications is currently based on the average lifetime τ, mass m, spin s, five types of charges, strangeness S and other parameters of particles. All particles are divided into 4 classes.
The 1st class is formed by one particle - a photon. A photon has zero rest mass and all charges. Photon is not subject to strong interactions. Its spin is 1, which means statistically it is a boson.
Class 2 is formed by leptons. These are light particles with zero baryon charge. Each particle - laptop - has one of its lenton charges that is not equal to zero. Leptons are not subject to strong interactions. The spin of all leptons is 1/2, that is, according to statistics, they are fermions.
The 3rd class is formed by mesons. These are particles with zero baryon and lepton charges that participate in strong interactions. All mesons have an integer spin, that is, according to statistics, they are bosons.
The 4th class consists of baryons. These are heavy particles with a non-zero baryon charge B ≠ O and with zero lepton charges, Le,Lµ,Lτ = 0. They have a half-integer spin (fermions) and participate in strong interactions. Due to the ability of particles of the 3rd and 4th classes to participate in strong interactions, they are also called hadrons.
Table 26.3 shows well-known particles - not resonances with their main characteristics. Particles and antiparticles are given. True neutral particles, which have no antiparticles, are placed in the middle of the column. Names are given for particles only. The corresponding antiparticle is obtained simply by adding the prefix “anti” to the name of the Particle. For example, proton - antiproton, neutron - antineutron.
The antielectron e + has the historical name positron. In relation to charged pions and kaons, the term “antiparticle” is practically not used. They differ only in Electric charge. Therefore, they simply talk about positive or negative pions and kaons.
The upper sign of the charge refers to the particle, the lower sign to the antiparticle. For example, for an electron - positron pair Le = ± 1. This means that the electron has Le = + 1, and the positron has Le = -1.
The following notations are used in the table: Q - electric charge, B baryon charge Le, Lµ, Lτ - respectively, electron, muon, taonic leptopic charges, S - strangeness, s - spin, τ - average lifetime.
The rest mass m is given in megaelectronvolts. From the relativistic equation mc 2 =еU it follows m=eU/c 2 . A particle energy of 1 MeV corresponds to a mass m=eU/c 2 =1.6 *10 -19 /9*10 16 =17.71*10 -31 kg. This is about two electron masses. Dividing by the mass of the electron m e = 9.11*10 -31 kg, we obtain m = 1.94 m e.
The mass of the electron, expressed in terms of energy, is m e =0.511 MeV.
7. Quark model of hadrons. Hadrons are elementary particles that participate in strong interactions. These are mesons and baryons. In 1964, Americans Murray Gell-Mann and George Zweig hypothesized that the structure and properties of hadrons could be better understood by assuming that hadrons were composed of more fundamental particles, which Gell-Mann called quarks. The quark hypothesis turned out to be very fruitful and is now generally accepted.
The number of putative quarks is constantly increasing. To date, 5 varieties (flavors) of quarks have been most well studied: quark u with mass m u = 5 MeV, quark d with mass m d = 7 MeV, quark s with ms = 150 MeV, quark c with mc = 1300 MeV and quark b with mb=5000 MeV. Each quark has its own antiquark.
All of the listed quarks have the same spin 1/2 and the same baryon charge B = 1/3. Quarks u, c have a fractional positive charge Q = + 2/3, quarks d, s, b have
fractional negative charge Q = - 1/3. Quark s is a carrier of strangeness, quark c is a carrier of charm, and quark b is a carrier of beauty (Table 26.4).
Each hadron can be represented as a combination of several quarks. The quantum numbers Q, B, S of hadrons are obtained as the sum of the corresponding numbers of the quarks that make up the hadron. If two identical quarks enter a hadron, their spins are opposite.
Baryons have half-integer spin, so they can consist of an odd number of quarks. For example, a proton consists of three quarks, p => uud. Electric charge of a proton Q =+ 2/3+2/3 – 1/3 = 1, baryon charge of a proton B = 1/3+ 1/3 + 1/3 = 1, strangeness S = O, spin s = 1/2 – 1/2 +1/2=1/2.
The neutron also consists of three quarks, n => udd. Q =2/3-1/3- 1/3 =O, B = 1/3+1/3+1/3=1, S = 0, s = 1/2 – 1/2 + 1/2 = 1/2. A combination of three quarks can be used to represent the following baryons: Λ 0 (uds), Σ + (uus), Σ 0 (uds), Σ - (dds),Ξ 0 (uss), Ξ - (dss),Ω - (sss) a°(uss). In the latter case, the spins of all quarks are directed in the same direction. Therefore Ω - - hyperon has spin 3/2.
Antiparticles of baryons are formed from the corresponding antiquarks.
Mesons consist of any two quarks and an antiquark. For example, the positive pion is π + (ud). Its charge is Q = +2/3- (-1/3) = 1, B = 1/3-1/3= O, S = 0, spin 1/2 – 1/2= 0.
The quark model assumes that quarks exist inside hadrons, but experience shows that they cannot escape from hadrons. But at least at those energies that are achievable with modern accelerators. There is a high probability that quarks cannot exist in a free state at all.
Modern high-energy physics believes that the interaction between quarks is carried out through special particles - gluons. The rest mass of gluons is zero, the spin is equal to unity. It is possible that there are about a dozen different types of gluons.
These three particles (as well as others described below) are mutually attracted and repelled according to their charges, of which there are only four types according to the number of fundamental forces of nature. The charges can be arranged in decreasing order of the corresponding forces as follows: color charge (forces of interaction between quarks); electric charge (electric and magnetic forces); weak charge (forces in some radioactive processes); finally, mass (gravitational force, or gravitational interaction). The word "color" here has nothing to do with the color of visible light; it is simply a characteristic of a strong charge and the greatest forces.
Charges are saved, i.e. the charge entering the system is equal to the charge leaving it. If the total electric charge of a certain number of particles before their interaction is equal to, say, 342 units, then after the interaction, regardless of its result, it will be equal to 342 units. This also applies to other charges: color (strong interaction charge), weak and mass (mass). Particles differ in their charges: in essence, they “are” these charges. Charges are like a “certificate” of the right to respond to the appropriate force. Thus, only colored particles are affected by color forces, only electrically charged particles are affected by electric forces, etc. The properties of a particle are determined by the greatest force acting on it. Only quarks are carriers of all charges and, therefore, are subject to the action of all forces, among which the dominant one is color. Electrons have all charges except color, and the dominant force for them is the electromagnetic force.
The most stable in nature are, as a rule, neutral combinations of particles in which the charge of particles of one sign is compensated by the total charge of particles of the other sign. This corresponds to the minimum energy of the entire system. (In the same way, two bar magnets are arranged in a line, with the north pole of one facing the south pole of the other, which corresponds to the minimum energy of the magnetic field.) Gravity is an exception to this rule: negative mass does not exist. There are no bodies that fall upward.
TYPES OF MATTER
Ordinary matter is formed from electrons and quarks, grouped into objects that are neutral in color and then in electrical charge. The color power is neutralized, as will be discussed in more detail below, when the particles are combined into triplets. (Hence the term “color” itself, taken from optics: three primary colors when mixed produce white.) Thus, quarks for which the color strength is the main one form triplets. But quarks, and they are divided into u-quarks (from the English up - top) and d-quarks (from the English down - bottom), also have an electric charge equal to u-quark and for d-quark. Two u-quark and one d-quarks give an electric charge of +1 and form a proton, and one u-quark and two d-quarks give zero electric charge and form a neutron.
Stable protons and neutrons, attracted to each other by the residual color forces of interaction between their constituent quarks, form a color-neutral atomic nucleus. But nuclei carry a positive electrical charge and, attracting negative electrons that orbit around the nucleus like planets orbiting the Sun, tend to form a neutral atom. Electrons in their orbits are removed from the nucleus at distances tens of thousands of times greater than the radius of the nucleus - evidence that the electrical forces holding them are much weaker than nuclear ones. Thanks to the power of color interaction, 99.945% of an atom's mass is contained in its nucleus. Weight u- And d-quarks are about 600 times the mass of an electron. Therefore, electrons are much lighter and more mobile than nuclei. Their movement in matter is caused by electrical phenomena.
There are several hundred natural varieties of atoms (including isotopes), differing in the number of neutrons and protons in the nucleus and, accordingly, in the number of electrons in their orbits. The simplest is the hydrogen atom, consisting of a nucleus in the form of a proton and a single electron revolving around it. All “visible” matter in nature consists of atoms and partially “disassembled” atoms, which are called ions. Ions are atoms that, having lost (or gained) several electrons, have become charged particles. Matter consisting almost entirely of ions is called plasma. Stars that burn due to thermonuclear reactions occurring in the centers consist mainly of plasma, and since stars are the most common form of matter in the Universe, we can say that the entire Universe consists mainly of plasma. More precisely, stars are predominantly fully ionized hydrogen gas, i.e. a mixture of individual protons and electrons, and therefore, almost the entire visible Universe consists of it.
This is visible matter. But there is also invisible matter in the Universe. And there are particles that act as force carriers. There are antiparticles and excited states of some particles. All this leads to a clearly excessive abundance of “elementary” particles. In this abundance one can find an indication of the actual, true nature of elementary particles and the forces acting between them. According to the most recent theories, particles may be essentially extended geometric objects - “strings” in ten-dimensional space.
The invisible world.
There is not only visible matter in the Universe (but also black holes and “dark matter,” such as cold planets that become visible when illuminated). There is also truly invisible matter that permeates all of us and the entire Universe every second. It is a fast-moving gas of particles of one type - electron neutrinos.
An electron neutrino is a partner of an electron, but has no electrical charge. Neutrinos carry only a so-called weak charge. Their rest mass is, in all likelihood, zero. But they interact with the gravitational field because they have kinetic energy E, which corresponds to the effective mass m, according to Einstein's formula E = mc 2 where c– speed of light.
The key role of the neutrino is that it contributes to the transformation And-quarks in d-quarks, as a result of which a proton turns into a neutron. Neutrinos act as the "carburetor needle" for stellar fusion reactions, in which four protons (hydrogen nuclei) combine to form a helium nucleus. But since the helium nucleus does not consist of four protons, but of two protons and two neutrons, for such nuclear fusion it is necessary that two And-quarks turned into two d-quark. The intensity of the transformation determines how quickly the stars will burn. And the transformation process is determined by weak charges and weak interaction forces between particles. Wherein And-quark (electric charge +2/3, weak charge +1/2), interacting with an electron (electric charge - 1, weak charge –1/2), forms d-quark (electric charge –1/3, weak charge –1/2) and electron neutrino (electric charge 0, weak charge +1/2). The color charges (or just colors) of the two quarks cancel out in this process without the neutrino. The role of the neutrino is to carry away the uncompensated weak charge. Therefore, the rate of transformation depends on how weak the weak forces are. If they were weaker than they are, the stars would not burn at all. If they were stronger, the stars would have burned out long ago.
What about neutrinos? Because these particles interact extremely weakly with other matter, they almost immediately leave the stars in which they were born. All stars shine, emitting neutrinos, and neutrinos shine through our bodies and the entire Earth day and night. So they wander around the Universe until they enter, perhaps, into a new interaction STAR).
Carriers of interactions.
What causes forces acting between particles at a distance? Modern physics answers: due to the exchange of other particles. Imagine two speed skaters throwing a ball around. By imparting momentum to the ball when thrown and receiving momentum with the received ball, both receive a push in a direction away from each other. This can explain the emergence of repulsive forces. But in quantum mechanics, which considers phenomena in the microworld, unusual stretching and delocalization of events are allowed, which leads to the seemingly impossible: one of the skaters throws the ball in the direction from different, but that one nonetheless Maybe catch this ball. It is not difficult to imagine that if this were possible (and in the world of elementary particles it is possible), attraction would arise between the skaters.
The particles, due to the exchange of which the interaction forces between the four “particles of matter” discussed above, are called gauge particles. Each of the four interactions – strong, electromagnetic, weak and gravitational – has its own set of gauge particles. The carrier particles of the strong interaction are gluons (there are only eight of them). A photon is a carrier of electromagnetic interaction (there is only one, and we perceive photons as light). The carrier particles of the weak interaction are intermediate vector bosons (they were discovered in 1983 and 1984 W + -, W- - bosons and neutral Z-boson). The carrier particle of gravitational interaction is the still hypothetical graviton (there should be only one). All these particles, except for the photon and graviton, which can travel infinitely long distances, exist only in the process of exchange between material particles. Photons fill the Universe with light, and gravitons fill the Universe with gravitational waves (not yet reliably detected).
A particle capable of emitting gauge particles is said to be surrounded by a corresponding field of forces. Thus, electrons capable of emitting photons are surrounded by electric and magnetic fields, as well as weak and gravitational fields. Quarks are also surrounded by all these fields, but also by the strong interaction field. Particles with a color charge in the field of color forces are affected by the color force. The same applies to other forces of nature. Therefore, we can say that the world consists of matter (material particles) and field (gauge particles). More about this below.
Antimatter.
Each particle has an antiparticle, with which the particle can mutually annihilate, i.e. "annihilate", resulting in the release of energy. “Pure” energy in itself, however, does not exist; As a result of annihilation, new particles (for example, photons) appear that carry away this energy.
In most cases, an antiparticle has properties opposite to the corresponding particle: if a particle moves to the left under the influence of strong, weak or electromagnetic fields, then its antiparticle will move to the right. In short, the antiparticle has opposite signs of all charges (except the mass charge). If a particle is composite, such as a neutron, then its antiparticle consists of components with opposite signs of charges. Thus, an antielectron has an electric charge of +1, a weak charge of +1/2 and is called a positron. Antineutron consists of And-antiquarks with electric charge –2/3 and d-antiquarks with electric charge +1/3. True neutral particles are their own antiparticles: the antiparticle of a photon is a photon.
According to modern theoretical concepts, each particle existing in nature should have its own antiparticle. And many antiparticles, including positrons and antineutrons, were indeed obtained in the laboratory. The consequences of this are extremely important and underlie all experimental particle physics. According to the theory of relativity, mass and energy are equivalent, and under certain conditions energy can be converted into mass. Since charge is conserved, and the charge of vacuum (empty space) is zero, any pairs of particles and antiparticles (with zero net charge) can emerge from the vacuum, like rabbits from a magician's hat, as long as the energy is sufficient to create their mass.
Generations of particles.
Accelerator experiments have shown that the quartet of material particles is repeated at least twice at higher mass values. In the second generation, the place of the electron is taken by the muon (with a mass approximately 200 times greater than the mass of the electron, but with the same values of all other charges), the place of the electron neutrino is taken by the muon (which accompanies the muon in weak interactions in the same way as the electron is accompanied by the electron neutrino), place And-quark occupies With-quark ( charmed), A d-quark – s-quark ( strange). In the third generation, the quartet consists of a tau lepton, a tau neutrino, t-quark and b-quark.
Weight t-a quark is about 500 times the mass of the lightest one – d-quark. It has been experimentally established that there are only three types of light neutrinos. Thus, the fourth generation of particles either does not exist at all, or the corresponding neutrinos are very heavy. This is consistent with cosmological data, according to which no more than four types of light neutrinos can exist.
In experiments with high-energy particles, the electron, muon, tau lepton and corresponding neutrinos act as isolated particles. They do not carry a color charge and enter into only weak and electromagnetic interactions. Collectively they are called leptons.
| Table 2. GENERATIONS OF FUNDAMENTAL PARTICLES | ||||
| Particle | Rest mass, MeV/ With 2 | Electric charge | Color charge | Weak charge |
| SECOND GENERATION | ||||
| With-quark | 1500 | +2/3 | Red, green or blue | +1/2 |
| s-quark | 500 | –1/3 | Same | –1/2 |
| Muon neutrino | 0 | 0 | +1/2 | |
| Muon | 106 | 0 | 0 | –1/2 |
| THIRD GENERATION | ||||
| t-quark | 30000–174000 | +2/3 | Red, green or blue | +1/2 |
| b-quark | 4700 | –1/3 | Same | –1/2 |
| Tau neutrino | 0 | 0 | +1/2 | |
| Tau | 1777 | –1 | 0 | –1/2 |
Quarks, under the influence of color forces, combine into strongly interacting particles that dominate most high-energy physics experiments. Such particles are called hadrons. They include two subclasses: baryons(such as a proton and a neutron), which are made up of three quarks, and mesons, consisting of a quark and an antiquark. In 1947, the first meson, called the pion (or pi-meson), was discovered in cosmic rays, and for some time it was believed that the exchange of these particles was the main cause of nuclear forces. Omega-minus hadrons, discovered in 1964 at Brookhaven National Laboratory (USA), and the JPS particle ( J/y-meson), discovered simultaneously at Brookhaven and at the Stanford Linear Accelerator Center (also in the USA) in 1974. The existence of the omega minus particle was predicted by M. Gell-Mann in his so-called “ S.U. 3 theory" (another name is the "eight-fold path"), in which the possibility of the existence of quarks was first suggested (and this name was given to them). A decade later, the discovery of the particle J/y confirmed the existence With-quark and finally made everyone believe in both the quark model and the theory that united electromagnetic and weak forces ( see below).
Particles of the second and third generation are no less real than the first. True, having arisen, in millionths or billionths of a second they decay into ordinary particles of the first generation: electron, electron neutrino, and also And- And d-quarks. The question of why there are several generations of particles in nature still remains a mystery.
Different generations of quarks and leptons are often spoken of (which, of course, is somewhat eccentric) as different “flavors” of particles. The need to explain them is called the “flavor” problem.
BOSONS AND FERMIONS, FIELD AND MATTER
One of the fundamental differences between particles is the difference between bosons and fermions. All particles are divided into these two main classes. Identical bosons can overlap or overlap, but identical fermions cannot. Superposition occurs (or does not occur) in the discrete energy states into which quantum mechanics divides nature. These states are like separate cells into which particles can be placed. So, you can put as many identical bosons as you like into one cell, but only one fermion.
As an example, consider such cells, or “states,” for an electron orbiting the nucleus of an atom. Unlike the planets of the Solar System, according to the laws of quantum mechanics, an electron cannot circulate in any elliptical orbit; for it there is only a discrete series of allowed “states of motion.” Sets of such states, grouped according to the distance from the electron to the nucleus, are called orbitals. In the first orbital there are two states with different angular momentum and, therefore, two allowed cells, and in higher orbitals there are eight or more cells.
Since the electron is a fermion, each cell can only contain one electron. Very important consequences follow from this - all of chemistry, since the chemical properties of substances are determined by the interactions between the corresponding atoms. If you go through the periodic system of elements from one atom to another in the order of increasing by one the number of protons in the nucleus (the number of electrons will also increase accordingly), then the first two electrons will occupy the first orbital, the next eight will be located in the second, etc. This consistent change in the electronic structure of atoms from element to element determines the patterns in their chemical properties.
If electrons were bosons, then all the electrons in an atom could occupy the same orbital, corresponding to the minimum energy. In this case, the properties of all matter in the Universe would be completely different, and the Universe in the form in which we know it would be impossible.
All leptons - electron, muon, tau lepton and their corresponding neutrinos - are fermions. The same can be said about quarks. Thus, all particles that form “matter”, the main filler of the Universe, as well as invisible neutrinos, are fermions. This is quite significant: fermions cannot combine, so the same applies to objects in the material world.
At the same time, all the “gauge particles” that are exchanged between interacting material particles and which create a field of forces ( see above), are bosons, which is also very important. So, for example, many photons can be in the same state, forming a magnetic field around a magnet or an electric field around an electric charge. Thanks to this, laser is also possible.
Spin.
The difference between bosons and fermions is associated with another characteristic of elementary particles - spin. Surprisingly, all fundamental particles have their own angular momentum or, more simply put, rotate around their own axis. Angle of impulse is a characteristic of rotational motion, just like the total impulse of translational motion. In any interaction, angular momentum and momentum are conserved.
In the microcosm, the angular momentum is quantized, i.e. takes discrete values. In suitable units of measurement, leptons and quarks have a spin of 1/2, and gauge particles have a spin of 1 (except for the graviton, which has not yet been observed experimentally, but theoretically should have a spin of 2). Since leptons and quarks are fermions, and gauge particles are bosons, we can assume that “fermionicity” is associated with spin 1/2, and “bosonicity” is associated with spin 1 (or 2). Indeed, both experiment and theory confirm that if a particle has a half-integer spin, then it is a fermion, and if it has an integer spin, then it is a boson.
GAUGE THEORIES AND GEOMETRY
In all cases, the forces arise due to the exchange of bosons between fermions. Thus, the color force of interaction between two quarks (quarks - fermions) arises due to the exchange of gluons. A similar exchange occurs constantly in protons, neutrons and atomic nuclei. Similarly, the photons exchanged between electrons and quarks create the electrical attractive forces that hold electrons in the atom, and the intermediate vector bosons exchanged between leptons and quarks create the weak forces responsible for converting protons into neutrons in thermonuclear reactions in stars.
The theory behind this exchange is elegant, simple, and probably correct. It is called gauge theory. But at present there are only independent gauge theories of strong, weak and electromagnetic interactions and a similar, although somewhat different, gauge theory of gravity. One of the most important physical problems is the reduction of these individual theories into a single and at the same time simple theory, in which they would all become different aspects of a single reality - like the faces of a crystal.
| Table 3. SOME HADRONS | ||||
| Particle | Symbol | Quark composition * | Rest mass, MeV/ With 2 | Electric charge |
| BARIONS | ||||
| Proton | p | uud | 938 | +1 |
| Neutron | n | udd | 940 | 0 |
| Omega minus | W – | sss | 1672 | –1 |
| MESONS | ||||
| Pi-plus | p + | u | 140 | +1 |
| Pi minus | p – | du | 140 | –1 |
| Fi | f | sє | 1020 | 0 |
| JP | J/y | cў | 3100 | 0 |
| Upsilon | Ў | b | 9460 | 0 |
| * Quark composition: u– top; d– lower; s- strange; c– enchanted; b- Beautiful. Antiques are indicated by a line above the letter. | ||||
The simplest and oldest of the gauge theories is the gauge theory of electromagnetic interaction. In it, the charge of an electron is compared (calibrated) with the charge of another electron distant from it. How can you compare charges? You can, for example, bring the second electron closer to the first and compare their interaction forces. But doesn't the charge of an electron change when it moves to another point in space? The only way to check is to send a signal from a near electron to a far one and see how it reacts. The signal is a gauge particle – a photon. To be able to test the charge on distant particles, a photon is needed.
Mathematically, this theory is extremely accurate and beautiful. From the “gauge principle” described above flows all of quantum electrodynamics (quantum theory of electromagnetism), as well as Maxwell’s theory of the electromagnetic field - one of the greatest scientific achievements of the 19th century.
Why is such a simple principle so fruitful? Apparently, it expresses a certain correlation between different parts of the Universe, allowing measurements to be made in the Universe. In mathematical terms, the field is interpreted geometrically as the curvature of some conceivable “internal” space. Measuring charge is measuring the total “internal curvature” around the particle. The gauge theories of the strong and weak interactions differ from the electromagnetic gauge theory only in the internal geometric “structure” of the corresponding charge. The question of where exactly this internal space is is sought to be answered by multidimensional unified field theories, which are not discussed here.
| Table 4. FUNDAMENTAL INTERACTIONS | |||||
| Interaction | Relative intensity at a distance of 10–13 cm | Radius of action | Interaction carrier | Carrier rest mass, MeV/ With 2 | Spin the carrier |
| Strong | 1 | Gluon | 0 | 1 | |
| Electro- magnetic |
0,01 | Ґ | Photon | 0 | 1 |
| Weak | 10 –13 | W + | 80400 | 1 | |
| W – | 80400 | 1 | |||
| Z 0 | 91190 | 1 | |||
| Gravita- tional |
10 –38 | Ґ | Graviton | 0 | 2 |
Particle physics is not yet complete. It is still far from clear whether the available data is sufficient to fully understand the nature of particles and forces, as well as the true nature and dimension of space and time. Do we need experiments with energies of 10 15 GeV for this, or will the effort of thought be sufficient? No answer yet. But we can say with confidence that the final picture will be simple, elegant and beautiful. It is possible that there will not be so many fundamental ideas: the gauge principle, spaces of higher dimensions, collapse and expansion, and, above all, geometry.
13.1. The concept of “elementary particles”
In the precise meaning of the term “elementary” are the primary indivisible simplest particles without internal structure that make up matter.
By 1932, four types of particles were known: electrons, protons, neutrons and photons. These particles (with the exception of the photon) are indeed constituents of observable matter.
By 1956, about 30 elementary particles had already been discovered. Thus, as part of cosmic radiation, positrons (1932), muons (1936), p(pi) - mesons (1947), strange particles K (ka) - mesons and hyperons were discovered. Subsequent discoveries in this area were made with the help of large accelerators that impart energies of the order of hundreds and thousands of MeV to particles. Thus, antiprotons (1955) and antineutrons (1956), heavy hyperons and resonances (60s), “charmed” and “lovely” particles (70s), t(tau) - lepton ( 1975), n(upsilon) - a particle with a mass of about ten (!) proton masses, “beautiful” particles (1981), intermediate vector bosons (1983). Several hundred particles are now known and their number continues to grow.
The common property of all these elementary particles is that they are specific forms of existence of matter that is not associated into nuclei and atoms. For this reason, the term “ subnuclear particles". Most of these particles do not satisfy the strict definition of elementarity, since (according to modern concepts) they are composite systems, that is, they have an internal structure. However, in accordance with established practice, the term “elementary particles” is retained. Particles that claim to be the primary elements of matter (for example, electrons) are called “ truly elementary".
13.1.1. Basic properties of elementary particles
All elementary particles have very small masses: from 10 -22 (for intermediate bosons) to ~ 10 -27 (for electrons). The lightest particles are neutrinos (its mass is assumed to be 10 thousand times less than the mass of an electron). The size of elementary particles is also extremely small: from 10 -13 cm (for hadrons) to< 10 -16 см у электронов и мюонов.
Microscopic masses and sizes determine quantum specificity behavior of elementary particles. The most important quantum property is the ability to be born and destroyed (emitted and absorbed) when interacting with other particles.
Most elementary particles unstable: born in cosmic rays or accelerators, they live for a fraction of a second and then undergo decay. A measure of particle stability is the average lifetime t. Electron, proton, photon and neutrino - absolutely stable particles(t®¥), in any case, their decay has not been experimentally detected. Neutron quasi-stable(t=(898±16)s. There are groups of unstable particles with average lifetimes of the order of 10 -6, 10 -8, 10 -10, 10 -13, 10 -16, 10 -20 s. The most meekly living particles are resonances : t~(10 -22 ¸10 -23)s.
Common characteristics of elementary particles are also spin, electric charge q and intrinsic magnetic moment. Spin is usually expressed in units and takes only integer or half-integer values. It determines the number of possible spin states of a particle, as well as the type of statistics to which these particles are subject. According to this criterion, all particles are divided into fermions(particles with half-integer spin) and bosons(particles with integer spin). The electric charge of a particle is an integer multiple of the elementary charge |e| = 1.6 × 10 -19 Cl. For known elementary particles, the electric charge in units of e takes on the following values: q = 0, ±1, ±2. Particles with fractional charge - quarks- do not occur in a free state (see clause 5.3.2).
The intrinsic magnetic moment characterizes the interaction of a particle at rest with an external magnetic field. Vectors and
parallel or antiparallel.
In addition to those listed, elementary particles are also characterized by a number of quantum characteristics, called “internal” (lepton charge, baryon charge, strangeness, etc.).
13.1.2 Particles and antiparticles
Almost every particle corresponds antiparticle- a particle with the same mass, lifetime, spin; their other characteristics are equal in magnitude, but opposite in sign (electric charge, magnetic moment, internal quantum characteristics). Some particles (for example, a photon) do not have any internal quantum numbers and, therefore, are identical to their antiparticles - this is true neutral particles.
The conclusion about the existence of antiparticles was first made by P. Dirac (1930). He derived a relativistic quantum equation that describes the state of a particle with half-integer spin. For a free particle, the Dirac equation leads to a relativistic relationship between the momentum (p), energy (E) and mass (m) of the particle:
For an electron at rest (p e =0), the following energy levels are possible: 
And 
, energy range 
"prohibited".
In quantum field theory, the state of a particle with negative energy is interpreted as the state of an antiparticle, which has positive energy but an opposite electric charge. All possible negative energy levels are filled but not observable. A photon with energy is capable of transferring an electron from a state with negative energy to a state with positive energy (see Fig. 5.1) - the electron becomes observable.
