1. Elementary particles are micro-objects, the size of which does 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 transformations. The content of any scientific term, as science develops, is gradually moving away from its etymology. So, the atom remained indivisible in the ideas of people until the emergence at the beginning of the 19th century. chemical atomistics.In modern scientific knowledge, an atom is a complex dynamic system capable of multiple rearrangements. Likewise, elementary particles, as their new properties are discovered, reveal their increasingly complex structure.

The most important property of elementary particles is their ability to be born and mutually transform into each other in collisions. For such processes to occur, it is necessary that the colliding particles have high energy. Therefore, elementary 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 indefinitely There are only 11 such particles: proton p, electron e, electron neutrino ν 0, muon neutrino νμ, taon neutrino ντ, their antiparticles p, e, ν e, νμ, ντ, and plus photon γ. Experimental facts of the spontaneous decay of these particles are still unknown.

Unstable particles have an average lifetime τ. which is very long in comparison with the characteristic time of a nuclear flight of 10 -23 s (the time the light travels across the nuclei). For example, τ \u003d 16 min for a neutron, τ \u003d 10 -6 s for a muon, τ \u003d 10 -8 s for a tied pion, and τ \u003d 10 -4 s for hyperons and kaons.

Resonances have lifetimes commensurate with a flight time of 10 -23 s. They are recorded by resonances on the curves of the dependence of the reaction cross sections on energy. Many resonances are interpreted as excited states of nucleons and other particles.

2. Fundamental interactions... The weight of the variety of interactions observed between elementary particles and in nature as a whole is reduced to 4 main types: strong, electromagnetic, weak and gravitational. A strong interaction keeps nucleons in atomic nuclei and is inherent in hadrons (protons, neutrons, mesons, hyperons, etc.). The interactions that manifest themselves at the macrolevel — elastic, viscous, molecular, chemical, etc. — are reduced to the electromagnetic one. 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. The gravitational interaction is inherent in all material objects.

Compare fundamental interactions with each other but their intensity. There is no unambiguous definition of this concept and no method for comparing intensities. Therefore, comparisons based on the totality 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) \u003d 4πε 0 G (m p 2 / e 2) \u003d 10 -36. This number is taken as a measure of the ratio of gravitational and electromagnetic interactions.

The ratio between the strong and electromagnetic interactions, determined from the cross sections and energies of nuclear reactions, is estimated as 10 4: 1. The intensities of the strong and weak interactions are compared in the same way.

Along with the intensity, the interaction time and distance are also used as a measure for comparing interactions. Usually, for comparison of times, the rates of processes are taken at the kinetic energies of colliding particles E \u003d 1 GeV. At such energies, the processes caused by strong interactions take place during a nuclear flight of 10 -23 s, processes caused by electromagnetic interactions - in a time of the order of 10 -19 s, weak - in a time of the order of 10 -9 s, gravitational - 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 \u003d 1 GeV are trapped by a layer of heavy metal up to 1 m thick. Whereas a neutrino, which can participate only in weak interaction, at an energy 100 times less (E \u003d 10 MeV) can be delayed by a layer of 10 9 km!

a. Strong interaction not only the most intense, but also the shortest in nature. At distances exceeding 10 -15 m, its role becomes insignificant. Providing the stability of nuclei, this interaction does not practically affect atomic phenomena. Strong interactions are not universal. It is inherent not to all particles, but only to hadrons - nucleons, mesons, hyperons, etc. There are particles - photons, electrons, muons, neutrinos, which are not subject to strong interaction and are not generated due to it in collisions.

b. Electromagnetic interaction in intensity it is 4 orders of magnitude inferior to the strong one. Its main area of \u200b\u200bmanifestation is distances ranging from a core diameter of 10-15 m and up to about 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 strongest electromagnetic interaction is with electrically charged particles. For neutral particles with nonzero spin, it manifests itself weaker and only due to the fact that such particles have a magnetic moment of the order of M \u003d eћ / 2m. The electromagnetic interaction is even weaker manifested 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 oppositely charged particles. Due to this, the EM interaction between atoms and any other objects with zero total charge has a relatively short range, although the Coulomb forces between charged particles are long-range.

e. Weak interactionnegligible compared to strong and electromagnetic. But with decreasing distances, it grows rapidly. If we assume that the dynamics of growth remains deep enough, then at distances of the order of 10 -20 m the weak interaction becomes equal to the strong one. But such distances are not yet available for experimental research.

Weak interaction determines some processes of interconversion of particles. For example, the particle sigma - plus - hyperon only under the influence of weak interaction decays into a proton and a neutral pion, Σ + \u003d\u003e p + π 0. Due to the weak interaction, β - decay occurs. Particles such as hyperons, kaons, muons in the absence of weak interaction would be stable.

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 grow with increasing mass of bodies. Therefore, gravity, despite all the insignificant relative intensity, in the interactions of cosmic bodies - planets, stars, galaxies - acquires a decisive role

In the world of elementary particles, the role of gravity is negligible. Therefore, in the physics of the atom, nucleus and elementary particles, the gravitational interaction is not taken into account.

3... Characteristics of elementary particles... Until the beginning of the 50s of the XX century, while the number of open 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 mean lifetime τ was also added here.

But gradually, in the laws of the 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 these 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 regularity, the quantum number of the baryonic charge B was introduced. They began to assume that such heavy baryon particles have B \u003d 1, and their antiparticles have B \u003d -1. For light particles, B \u003d 0. As a result, the open law took the form of the law of conservation of baryon charge.

Similarly, for light particles, quantum numbers were empirically introduced - lepton charges L - signs that some transformations are forbidden. We agreed to Consider that lepton charges Le \u003d +1 for electrons e - and electron neutrinos ν e, L µ \u003d + 1 for negative muons µ - and muon neutrinos ν µ, L τ \u003d +1 for negative taons τ - and taon neutrinos v τ. For the corresponding antiparticles, L \u003d -1. Like baryonic, lepton charges are conserved in all interactions.

During the discovery of hyperons, which are born in strong interactions, it turned out that their lifetime is not equal to the time of flight of 10 -23 s, which is typical for strongly interacting particles, but is 10 13 times longer. This seemed unexpected and strange and could be explained only by the fact that particles born in strong interactions decay in weak interactions. To reflect this property of particles, the quantum number of strangeness S was introduced. Strange particles have S \u003d + 1, their antiparticles have S \u003d - 1, and other particles have S \u003d 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. The proton has Q \u003d + 1, the electron has Q \u003d -1, the neutron, neutrino and other neutral particles have Q \u003d 0.

In addition to the named parameters, elementary particles have other characteristics that are not considered here.

4. Conservation laws in elementary particle physics can be divided into three troupes: general conservation laws, exact conservation laws for charges and approximate conservation laws.

a ... General conservation laws are carried out exactly regardless of the scale of the phenomena - in the micro-, macro- and megaworld. These laws follow from the geometry of space - time. The uniformity of time leads to the law of conservation of energy, the uniformity of space - to the law of conservation of momentum, isotropy of space - to the law of conservation of angular momentum, the equality of IFR - to the law of conservation of the center of inertia. In addition to these 4 laws, this includes two more, associated with the symmetry of space - time relative to the mirror reflections of the coordinate axes. From the mirror symmetry of the coordinate axes it follows that the right-left symmetries of the space are identical (the parity conservation law). The law associated with the mirror symmetry of time speaks of the identity of the 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, baryonic B, three leigons - electronic L e, muonium L µ ton L τ. All charges are integer and can have both positive and negative values \u200b\u200bto zero.

Electric charge has a double meaning. It is not only a quantum number, but is also the source of the force field. Baryon and lepton charges are not sources of a force field. For a complex system, the total charge of any kind is equal to the sum of the corresponding charges of the elementary particles included in the system.

in. Approximate conservation lawsare performed only in some types of fundamental interactions. They refer to characteristics such as strangeness S, etc.

All the 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 the pair electron + positron, they agreed to consider the electron e - a particle, and the positron e + - an antiparticle. The electron charges Q \u003d -1, B \u003d 0, Le \u003d +1, Lµ \u003d 0, Lτ \u003d 0. Positron charges Q \u003d +1, B \u003d 0, Le \u003d -1, Lµ \u003d 0, Lτ \u003d 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 truly neutral 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. The photon has zero rest mass and all charges. Photon is not subject to strong interactions. Its spin is 1, that is, statistically, it is a boson.

The 2nd class is formed by leptons. These are light particles with zero baryon charge. Each particle - a laptop, has one of the lenton charges 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 participating in strong interactions. All mesons have integer spin, that is, statistically they are bosons.

The 4th class is baryons. These are heavy particles with a nonzero baryon charge B ≠ O and with zero lepton ones, Le, Lµ, Lτ \u003d 0. They have half-integer spin (fermions) and participate in strong interactions. According to the ability of particles of the 3rd and 4th classes to participate in strong interactions, they are also called hadrons.

Table 26.3 lists well-known particles - not resonances with their basic characteristics. Particles and antiparticles are shown. Truly neutral particles with no antiparticles are placed in the middle of the column. Names are for particles only. The corresponding antiparticle is obtained simply by adding the prefix "anti" to the name of the Particle. For example, a proton is an antiproton, a neutron is an antineutron.

Antielectron e + has a historically developed name of positron. In relation to charged pions and kaons, the term "antiparticle" is practically not used. They differ only in Electric charge. Therefore, they simply speak of positive or negative pions and kaons.

The upper charge sign refers to the particle, the lower one to the antiparticle. For example, for a pair electron - positron Le \u003d ± 1. This means that the electron has Le \u003d + 1, and the positron has Le \u003d -1.

The following designations are adopted in the table: Q - electric charge, B baryon charge Le, Lµ, Lτ, - respectively, electronic, muon, taon leptope charges, S - strangeness, s- spin, τ - average lifetime.

Rest mass m is indicated in megaelectronvolts. From the relativistic equation mc 2 \u003d eU it follows m \u003d eU / c 2. The particle energy of 1 MeV corresponds to the mass m \u003d eU / c 2 \u003d 1.6 * 10 -19 / 9 * 10 16 \u003d 17.71 * 10 -31 kg. This is about two electron masses. Dividing by the electron mass m e \u003d 9, 11 * 10 -31 kg, we get m \u003d 1.94 m e.

The mass of an electron, expressed in terms of energy, is m e \u003d 0.511 MeV.

7. Quark model of hadrons... Elementary particles participating in strong interactions are called hadrons. These are mesons and baryons. In 1964, the Americans Murray Gell-Man and George Zweig hypothesized that the structure and properties of hadrons could be better understood if one assumes that hadrons are composed of more fundamental particles, called quarks by Gell-Man. 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: the u quark with a mass mu \u003d 5 MeV, a d quark with a mass md \u003d 7 MeV, an s quark with ms \u003d 150 MeV, a c quark with mc \u003d 1300 MeV, and a b quark with mb \u003d 5000 MeV. Each quark has its own antiquark.

All the listed quarks have the same spin 1/2 and the same baryon charge B \u003d 1/3. The u, c quarks have a fractional positive charge Q \u003d + 2/3, the d, s, b quarks have

fractional negative charge Q \u003d - 1/3. The s quark is the bearer of strangeness, the quark is the bearer of charm, and the b quark is beauty (Table 26.4).

Each hadron can be thought of 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 the hadron, then 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 \u003d\u003e uud. Electric charge of a proton Q \u003d + 2/3 + 2/3 - 1/3 \u003d 1, baryon charge of a proton B \u003d 1/3 + 1/3 + 1/3 \u003d 1, strangeness S \u003d O, spin s \u003d 1/2 - 1/2 + 1/2 \u003d 1/2.

The neutron also consists of three quarks, n \u003d\u003e udd. Q \u003d 2 / 3-1 / 3- 1/3 \u003d O, B \u003d 1/3 + 1/3 + 1/3 \u003d 1, S \u003d 0, s \u003d 1/2 - 1/2 + 1/2 \u003d 1/2. A combination of three quarks can 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, the Ω - - hyperon has spin 3/2.

Baryon antiparticles are formed from the corresponding antiquarks.

Mesons consist of any two quarks and an antiquark. For example, a positive pion π + (ud). Its charge is Q \u003d + 2 / 3- (-1/3) \u003d 1, B \u003d 1 / 3-1 / 3 \u003d O, S \u003d 0, spin 1/2 - 1/2 \u003d 0.

The quark model assumes that quarks exist inside hadrons, and experience shows that they cannot fly out of hadrons. But at least at those energies that are achievable with modern accelerators. It is highly probable that quarks cannot exist in a free state at all.

Modern high-energy physics believes that the interaction between quarks is carried out by means of special particles - gluons. The rest mass of gluons is zero, spin is one. The existence of about a dozen different types of gluons is allowed.

At the modern level of knowledge, electrons and others (see below), as well as quarks, internal. the structure is not found, although there are theoretical. models, according to which both leptons and quarks are built from more fundamental building blocks of the universe - preons (this term, however, is not yet generally accepted).

Historically, the first experimentally discovered E.ch. there was an electron, a proton, and then a neutron. It seemed that the aggregate of these particles and a quantum of e-magn. the photon field is sufficient to construct known forms of matter (atoms and molecules). Substance with this approach was built from protons, neutrons and electrons, and e-magn. the field (photons) carried out the interaction between them. However, it soon became clear that the world is much more complicated. It was found that each particle has its own, which differs from it only in the sign of charges (see below); for particles with zero values \u200b\u200bof all charges, the antiparticle coincides with the particle (for example, a photon). Further, with the development of experimental nuclear physics, more than 300 particles were added to the above four (or taking into account antiparticles - seven) particles. It can be considered established that most of these particles are built of quarks, the number of which is 6 (or 12, taking into account antiquarks).

Another important achievement of the physics of the microworld was the discovery that E.ch. is inherent not only e-magn. interaction. With the study of the structure of atomic nuclei, it became clear that the forces that hold protons and neutrons in the nucleus are not electromagnetic.

The interaction characteristic of nucleons (protons and neutrons in the nucleus) is called strong. It turned out to be short-acting - at distances rexceeding 10 -13 cm, the strong interaction is negligible. However, with r Nuclear forces). The discovery of the instability of the neutron and certain atomic nuclei indicated the existence of another type of interaction, called weak. The three types of interactions listed above, as well as the gravitational interaction (see), exhaust the known types of fundamental physical. interactions. There is a point of view that all 4 (or at least 3) types of interactions are phenomena of the same nature and should be described in a single way.

Unified theory of the weak and el.-magn. interactions have already been built and confirmed by experience; there are theoretical models that uniformly describe all types of interactions (see).

2. Classification of elementary particles

Tab. 1. Elementary particles ( Q - Electric. charge, L - Lepton charge, B - Baryon charge, S - Weirdness, C - Charm).

Particle type	Symbol	Weight m, MeV	Spin, in units	Time life, with	Q	L	B	S	C
Leptons	e -	0,511	1/2	align \u003d "absmiddle" width \u003d "65" height \u003d "15"\u003e	-1	1	0	0	0
				stable 3)	0
		105			-1
				stable 3)	0
		1784			-1
				stable 3)	0
Mesons- carriers interactions		0	1	stable	0	0	0	0	0
	W
	Z 0				0
	gluon 5)	0 6)		stable 6)	0
Mesons (hadrons)		135	0		0	0	0	0	0
		140			+1			0	0
	K 0	498			0			+1	0
	K +	494			+1			+1	0
	D 0	1864			0			0	+1
	D +	1869		~ 10 -12	+1			0	+1
	F +	2020			+1			-1	+1
Baryons 8) (hadrons)	p	938,3	1/2	>10 38	+1	0	1	0	0
	n	939,6		900	0			0	0
		1115			0			-1	0
		1189			+1			-1	0
		1192			0			-1	0
		1197			-1			-1	0
		1315			0			-2	0
		1321			-1			-2	0
		1672			-1			-3	0
		2280		~ 10 -13	+1			0	1

Notes to the table:
1) In addition to the particles shown in the table, there are a large number of short-lived particles, the so-called. resonances with a lifetime of ~ 10 -20 -10 -24 s. For the given particles in the table of particles, their antiparticles are not indicated, having the same values \u200b\u200bof mass, life time, but opposite signs of quantum numbers Q, L, B, S, C.
2) It is believed that, although special. there is no reason for this; perhaps, .
3) If, then it is natural to expect that neutrinos are unstable, although their lifetime can be very long.
4) Given theoretical. appraisal.
5) Gluon as a free particle does not exist.
6) Theoretical. appraisal.
7) K 0 - and -mesons do not have a definite lifetime.
8) Baryons with large values \u200b\u200bmust exist C (up to 3), as well as with non-zero values C and S at the same time; discovered a meson (GeV), in which the quantum number ("beauty") is not equal to zero, attributed to b-quark.

Depending on the nature of the interaction, E.ch. subdivided into several. large groups (Table 1). E. ch., To-eye strong interaction is inherent, called. ... Hadrons include protons, neutrons and heavier particles of hyperons (all of them are united by a common name), as well as a large family. Particles that do not participate in strong interactions are called. ... This includes, in addition to the electron, two other charged leptons: the muon and the tau-lepton ("heavy lepton"), which are respectively 210 and 3600 times more massive than the electron. Each charged lepton has a neutral particle - (electronic, muonic or tau). The neutrino mass is zero or very small. There are 6 (with 12 antiparticles) types of leptons. Neutral leptons participate only in weak interactions; thinned - with weak and electromagnetic. Neutral leptons, however, can have very small magnitudes. moments. Hadrons participate in the strong, the weak and the e-magn. interactions. And, of course, all particles interact gravitationally. In addition to the above, there are particles - carriers of interactions: photon (carrier of electromagnetic interaction), W- and Z 0 -bosons (carriers of weak interaction). It is believed that there is a gravity carrier. interactions - graviton.

E.ch. are characterized by their mass, electric charge, proper angular momentum -.

The lightest particles (such as photons) have zero masses, and the heaviest known particles are 100 times the mass of a proton. Electric. charge E.ch. is an integer multiple of the electron charge. The sreen of particles can be either whole (0, 1, 2, ...) - in this case they are called bosons, or half-integer (1/2, 3/2, ...) - in this case they are called fermions.

Leptons are credited with the so-called. lepton charge Ltaken equal to +1 for particles and -1 for their antiparticles. The introduction of this charge is justified by the fact that in all processes occurring in a closed system, the total number of leptons, minutes, the number of antileptons is conserved. In addition, each pair of leptons has its own special lptonic charge, respectively. The introduction of these charges reflects the fact that, for example, an electron neutrino, incident on a neutron, can give birth to an electron, but not a muon or a lepton. The values \u200b\u200bare +1 for the indicated pairs of leptons and -1 for their antiparticles. Now, however, the possibility is widely discussed that a free neutrino can change its lepton charge with time, turning into a different type of neutrino (neutrino oscillations). As a result, at different distances from the place of their birth, neutrinos are capable of producing charged leptons of various types.

Baryons, like leptons, are assigned their own conserved baryon charge B... The nature of the conservation of lepton and baryon charges is not fully understood. Moreover, the grand unification models predict that this conservation is. just approximate, although the detection of a possible violation of conservation is, apparently, on the verge or beyond the present. experimental possibilities. All known leptons and baryons are fermions. Mesons have no baryon or lepton charge and yavl. bosons. In addition, hadrons are assigned specific quantum numbers (charges), called strangeness ( S), charm ( C), etc., to-rye, in contrast to B and L, do not persist in weak interactions, persist in strong and electromagnetic ones. Due to this, the lightest particles with (or), being unstable, have a rather long lifetime on the scale of the world of E.ch. (see Table 1), since only a weak interaction can lead to their disintegration.

3. Quark model of hadron structure

All hadrons, according to modern. representations, are built from more fundamental particles - quarks ( q). Like leptons, quarks are. fermions, their spin is 1/2, electric. charge +2/3 and -1/3 (in units of electron charge), charge of antiquarks -2/3 and +1/3, all quarks have a baryon charge B\u003d 1/3, lepton charge L\u003d 0. Similar to the lepton, quarks are also grouped into pairs. And, apparently, there is a quark-lepton symmetry: each pair of leptons corresponds to a pair of quarks (see Table 2). The pair (e,) corresponds to quarks, denoted by (u, d). These are the lightest quarks, their mass is 5-10 MeV, their strangeness, charm, and other similar quantum numbers are equal to zero. Nucleons can be constructed from three such quarks, i.e. proton and neutron: p \u003d ( uud), n \u003d ( udd). Dr. possible triplets of these quarks are also realized in nature, forming heavier particles, for example. a particle with a spin of 3/2 and a mass of 1240 MeV. Mesons are constructed from a quark-antiquark pair, in particular, the lightest known meson - meson :),) and, which are a mixture of and.

Four particles ( u, d,, e) form the so-called. first quark-lepton generation. Two more generations are known ( c, s,) and ( t, b,) (see Table 2) containing more massive particles.

Tab. 2. Quarks and leptons.

1st generation

2nd generation

III generation

Designations

u

d

e

c

s

t

b

Electric charge per unit electron charge

+2/3

-1/3

0

-1

+2/3

-1/3

0

-1

+2/3

-1/3

0

-1

Mass, MeV

0,5

1200

150

105

1784

Apparently, the cosmological data indicate the absence of subsequent quark-lepton generations (see below). On the other hand, three generations of particles are enough for theoretical. explanation of the difference between St. in particles and antiparticles. Each of the heavy quarks ( c, s and t, b) has, respectively, its quasi-conserved quantum number C, S or T, B... Because the S called strangeness, and the s-quark is called. strange; C called charm, B - beauty, for T the term has not yet been established. Particles, which include s-quark, called. strange. Replacing theoretically one, two or three quarks in a nucleon, one can explain the existence of all open strange baryons - hyperons (see Table 1). Similarly, when replacing u- or d-quark in -meson on s-quark it is fashionable to obtain the strange K-mesons found in nature. In exactly the same way, the observed charmed particles (c) have in their composition from-quark, etc. In principle, bound states of all six types of quarks are possible with each other, but only some of them are observed experimentally. However, all open hadrons can be described as bound states of these six quarks.

Each quark has a quantum number called a color. Yavl color analogue of electric. charge, albeit more complex. The presence of color explains the strong interaction of quarks, which is absent in colorless leptons.

Just as electric charges interact by means of photons, so the interaction of color charges is carried out by carriers of strong interaction - gluons. However, unlike a single photon, there are eight different types of gluons. Dr. the essential difference is that the photon does not have electric. charge and therefore does not interact with itself, and gluons, having a color charge, interact with each other. Apparently, this is the reason for a fundamentally new phenomenon called confinement or confinement of quarks. The fact is that, despite the rather high energies of particles accelerated in the present. accelerators, quarks cannot be observed in a free state. They, apparently, exist in nature only in the form of quark-antiquark pairs (), triplets ( qqq) or more complex formations, but necessarily such that electric. the charge of these objects turned out to be integer. All such objects have zero color charge. To put it very simply, the phenomenon of confinement is as follows. When an attempt is made to obtain a quark in a free state (that is, to "pull" it out of the hadron at a sufficiently large distance, imparting a high energy to it), the field strength of the uncompensated color charge of the quark turns out to be so strong that, due to the transferred energy, a pair is generated from the vacuum and the antiquark moves together with a quark, to-ry are trying to tear off. As a result, it is not a quark that is ejected, but a composite particle that has no color. For the same reason, gluons cannot be observed in a free state either. The phenomenon of confinement determines a small radius of action of a strong interaction.

The field of elementary particle physics, which studies the interaction of quarks and gluons, is called quantum chromodynamics. Quantum chromodynamics yavl. theory of strong interaction E.ch.

Thus, at present. the level of understanding of elementarity by the fundamental constituents of matter yavl. 6 leptons (with 12 antiparticles), 6x3 \u003d 18 quarks (with 36 antiparticles), as well as carriers of interaction: strong - 8 gluons, electromagnetic - photon, weak - W- and Z 0 -bosons. Leptons and quarks have spin 1/2, and interaction carriers have spin equal to 1, they are called vector bosons. The existence of all the listed particles is confirmed by experiment. In addition, the theory requires the existence of a constant scalar field in the entire space, with which various leptons and quarks interact in different ways, which determines the difference in their masses. The scalar field quanta are new ones predicted by E.ch. zero spin. They are called Higgs bosons (after the English physicist P. Higgs, 1964, who suggested their existence). The number of Higgs bosons can reach several. dozens. The interaction of W- and Z 0 -bosons with a scalar field determines the meaning. the mass of these particles and the small radius of weak interaction. Higgs bosons have so far been discovered experimentally. Moreover, a number of physicists consider their existence optional, but a full-fledged theoretical scheme without Higgs bosons has not yet been found.

Grand unification models require the introduction of additional vector particles - carriers of the interaction of hadrons with leptons. In the simplest version, there should be 12 such particles with a mass m ~ 10 14 -10 15 GeV. It is still impossible to obtain and study such particles experimentally, because the mass is far beyond the energies attainable in accelerators of both existing designs and generally conceivable. In interactions with these vector bosons, neither baryon nor lepton charge is conserved. Again, the number of particles at a new level of elementarity approaches or even exceeds a hundred. However, a large number of new particles are required only by theory, but not by experiment, and, possibly, other, still unknown theoretical. schemes will make it possible to do without a special set of already known particles.

The growth in the number of fundamental E.ch. forced theorists to look for models in which all families of quarks and leptons, as well as particles - carriers of interactions and Higgs bosons, would be considered as being composed of some more fundamental objects; one of the names suggested for the latter is preons.

Main the difficulty facing preon theory is that the mass of objects mcomposed of preons should be small compared to the inverse size of these objects r -one . On the other hand, according to quantum mechanics, generally speaking, the condition must be satisfied. There is no satisfactory solution to this problem yet. At the same time, the structure of matter should not necessarily resemble a toy "matryoshka", it cannot be ruled out that leptons and quarks exist and will forever remain the last stage in the fragmentation of matter. The decisive word here must belong to experiment. Unfortunately, experiments on existing accelerators cannot answer the questions posed.

4. Elementary particles and cosmology

In the primary plasma were all E.ch., the birth of which could occur at a given temperature of the plasma. With the expansion of the universe temp-ra T plasma fell, the most massive particles ceased to be born, and they were led to the fact that the number of massive stable E.ch. and antiparticles in the so-called element. of the accompanying volume (i.e., expanding at the rate of expansion of the Universe) decreased in proportion to exp ( mc 2 kT). If such a law of decreasing the concentration of E.ch. lasted until the present time (up to K), then practically no traces of E.ch. born in the early stages of the evolution of the universe would now be gone. However, when the concentration of such particles becomes sufficiently small, their mutual annihilation ceases, and subsequently the concentration of E.ch. falls only due to the expansion of the Metagalaxy (i.e., remains constant in the accompanying volume). This phenomenon is called. quenching (sometimes freezing) concentration. For weakly interacting particles, their present concentration should be of the order of the present. concentration of relict photons ... This is exactly the situation for neutrinos. The calculation shows that the number of relict neutrinos should be very large: (for each type of neutrino). The latter circumstance allows one to obtain a very strong limitation on the neutrino mass: eV. If the mass of all types of neutrinos exceeded the specified limit, then neutrinos would have a significant effect on the rate of expansion of the Universe and its age, calculated by the present day. the value of the Hubble constant and the mass density of relict neutrinos would be less than given by astrophysics. estimates and methods. The proof that the lower bound on the age of the Universe leads to the upper bound on the neutrino masses was given by S.S. Gershtein and Ya.B. Zeldovich (1966) and initiated the use of cosmological. methods to physics E.ch.

Cosmological data also allow us to conclude that the number of different neutrinos cannot be arbitrarily large (V.F.Shvartsman, 1969). light elements (such as 4 He and deuterium) in the Universe is such that, i.e. all neutrinos have already been discovered. True, a number of physicists, not trusting the reliability of the existing data, adhere to a different assessment:. It is possible that soon the number of types of neutrinos will become known precisely, since discovered in 1983 Z 0 -boson of weak interactions should, according to the theoretical. predictions, decay into all types of neutrinos and therefore measuring its total decay probability will determine. Let us explain how the abundance of 4 He and 2 H can be determined. These elements were formed at a very early stage in the development of the Universe, when the temperature of the primary plasma was 1 MeV-100 keV (in energy units or 10 10 -10 9 K. At this temperature, the plasma contained approximately equal numbers of photons, all types neutrinos, electron-positron pairs and a small number of nucleons (~ 10 -10 of the number of light particles). The relative content of neutrons and protons is initially determined by thermodynamic equilibrium and is, where \u003d 1.3 MeV is the difference between the masses of a neutron and a proton. Transitions np occur in account of processes caused by weak interaction, for example, n + p + e -. As the Universe expands, the particle concentrations fall and the reaction rate of np-transitions becomes less than the expansion rate, the ratio of the concentrations n and p is hardened, i.e. N n / N p becomes constant if the slow decay of neutrons is neglected. This value determines the relative content (abundance) of 4 He, since due to the hydrogen chain, practically all neutrons are bound into 4 He nuclei. Obviously, the higher the rate of expansion and cooling, the higher the quenching temperature and, accordingly, the higher the ratio N n / N p... It can be shown that the greater the number of different types of particles in the primary plasma, the higher the expansion rate at a given temperature, therefore the addition of new types of neutrinos to the primary plasma entails an increase in the quenching temperature and, accordingly, an increase in the concentration of primary 4 He. Modern The data indicate that the proportion of 4 He (by mass) in the substance of the Metagalaxy is 22-25%, which is in good agreement with theory at \u003d 3. If the number of neutrino types were 10-20, the amount of 4 He would reach 40-50%, which is completely inconsistent with the observational data. The calculation, however, contains some uncertainty associated with the fact that the relative concentration of nucleons is known with poor accuracy. According to the data on the amount of 2 H in the Universe, such a limitation on the value f, with which\u003e 3 is excluded. Unfortunately, the ratio between the current amount of deuterium and the primary is rather poorly defined and this leaves a certain loophole for increasing the number.

Cosmology also allows you to draw conclusions about particles and processes, to-rye are far beyond energetic. limits available to modern. and future accelerators. A striking example of yavl. estimation of the concentration of magnetic monopoles - particles with an elementary magnitude. charge. The existence of these particles is predicted by the grand unification models. Their mass should be ~ 10 16 GeV, so that neither now nor in the foreseeable future there is any hope of obtaining these particles in the laboratory, just as, for example, antiprotons, W and Z 0 bosons are obtained.

The only way to find these particles is to search for them among the relic particles. Theoretical the expectations for the concentration of relict monopoles obtained within the framework of the simplest model contradict the existing observational data. This contradiction served as one of the prerequisites for the formulation of the model of the inflationary model of the Universe.

The relationship of physics E.ch. and cosmology has especially strengthened in recent times. Now not a single theoretical. model of interactions E.ch. cannot be recognized if it does not agree with the data of cosmology. On the other hand, the methods of physics by E.ch. allowed to solve a number of well-known cosmological problems, such as the problem of homogeneity and isotropy, the horizon of the Universe, the proximity of the density of matter to critical. value.

Further penetration into the depths of the microworld is associated with the transition from the level of atoms to the level of elementary particles. As the first elementary particle at the end of the 19th century. the electron was discovered, and then in the first decades of the XX century. - photon, proton, positron and neutron.

After the Second World War, thanks to the use of modern experimental technology, and above all powerful accelerators, in which conditions of high energies and tremendous speeds are created, the existence of a large number of elementary particles was established - over 300. Among them there are both experimentally discovered and theoretically calculated, including resonances, quarks and virtual particles.

Term elementary particleoriginally meant the simplest, further indecomposable particles that underlie any material formations. Later physicists realized the whole conventionality of the term "elementary" in relation to micro-objects. Now there is no longer any doubt that the particles have this or that structure, but, nevertheless, the historically established name continues to exist.

The main characteristics of elementary particles are mass, charge, average lifetime, spin and quantum numbers.

Rest mass elementary particles are determined in relation to the rest mass of the electron. There are elementary particles that do not have a rest mass, - photons... The rest of the particles are divided by this feature into leptons- light particles (electron and neutrino); mesons- average particles with a mass ranging from one to a thousand electron masses; baryons- heavy particles, whose mass exceeds a thousand electron masses and which include protons, neutrons, hyperons and many resonances.

Electric charge is another important characteristic of elementary particles. All known particles have a positive, negative or zero charge. Each particle, except for a photon and two mesons, corresponds to antiparticles with the opposite charge. Around 1963-1964. hypothesized about the existence quarks- particles with a fractional electric charge. This hypothesis has not yet been confirmed experimentally.

By life time particles are divided into stable and unstable . There are five stable particles: a photon, two types of neutrinos, an electron and a proton. It is the stable particles that play the most important role in the structure of macrobodies. All other particles are unstable, they exist for about 10 –10 –10 –24 s, after which they decay. Elementary particles with an average lifetime of 10 –23 –10 –22 s are called resonances... Due to their short lifetime, they decay even before they have time to leave the atom or atomic nucleus. Resonant states are calculated theoretically, but it is not possible to fix them in real experiments.

In addition to charge, mass and life time, elementary particles are also described by concepts that have no analogues in classical physics: the concept back . Spin is the proper angular momentum of a particle, which is not associated with its movement. Spin is characterized spin quantum number s, which can take integer (± 1) or half-integer (± 1/2) values. Particles with integer spin - bosons, with a half-integer - fermions... An electron belongs to fermions. According to Pauli's principle, an atom cannot have more than one electron with the same set of quantum numbers n,m,l,s... Electrons, which correspond to wave functions with the same number n, are very close in energy and form an electron shell in the atom. Differences in the number l determine the "subshell", the rest of the quantum numbers determine its filling, which was mentioned above.

In the characterization of elementary particles, there is another important concept interactions... As noted earlier, there are four types of interactions between elementary particles: gravitational, weak, electromagneticand strong(nuclear).

All particles with rest mass ( m 0), participate in gravitational interaction, charged - and in the electromagnetic one. Leptons also participate in weak interactions. Hadrons are involved in all four fundamental interactions.

According to quantum field theory, all interactions are due to the exchange virtual particles , that is, particles, the existence of which can only be judged indirectly, by some of their manifestations through some secondary effects ( real particles can be directly fixed with instruments).

It turns out that all four known types of interactions - gravitational, electromagnetic, strong and weak - have a gauge nature and are described by gauge symmetries. That is, all interactions seem to be made “from one blank”. This gives rise to hope that it will be possible to find “the only key to all known locks” and describe the evolution of the Universe from a state represented by a single supersymmetric superfield, from a state in which differences between the types of interactions, between all kinds of particles of matter and field quanta have not yet been manifested.

There are a huge number of ways to classify elementary particles. So, for example, particles are divided into fermions (Fermi particles) - particles of matter and bosons (Bose particles) - quanta of fields.

According to another approach, particles are divided into 4 classes: photons, leptons, mesons, baryons.

Photons (quanta of the electromagnetic field) participate in electromagnetic interactions, but do not have strong, weak, gravitational interactions.

Leptons got their name from the Greek word leptos - light. These include particles that do not have strong interactions, muons (μ -, μ +), electrons (e -, e +), electron neutrinos (v e -, v e +) and muon neutrinos (v - m, v + m). All leptons have a spin of ½ and are therefore fermions. All leptons have a weak interaction. Those of them that have an electric charge (that is, muons and electrons) also have an electromagnetic interaction.

Mesons - strongly interacting unstable particles that do not carry the so-called baryonic charge. These include r-mesons, or pions (π +, π -, π 0), TO-mesons, or kaons (K +, K -, K 0), and this-mesons (η) . Weight TO-mesons is ~ 970 me (494 MeV for charged and 498 MeV for neutral TO-mesons). Lifetime TO-mesons has a magnitude of the order of 10 –8 s. They decay to form i-mesons and leptons or only leptons. Weight this-mesons is 549 MeV (1074me), the lifetime is about 10 –19 s. This-mesons decay with the formation of π-mesons and γ-photons. Unlike leptons, mesons have not only weak (and, if they are charged, electromagnetic), but also a strong interaction, which manifests itself when they interact with each other, as well as in the interaction between mesons and baryons. All mesons have zero spin, so they are bosons.

Class baryons combines nucleons (p, n) and unstable particles with a mass greater than the mass of nucleons, which are called hyperons. All baryons have strong interactions and, therefore, actively interact with atomic nuclei. All baryons have spin ½, so baryons are fermions. With the exception of the proton, all baryons are unstable. When baryons decay, along with other particles, a baryon is necessarily formed. This pattern is one of the manifestations baryonic charge conservation law.

In addition to the particles listed above, a large number of strongly interacting short-lived particles have been discovered, which are called resonances ... These particles are resonant states formed by two or more elementary particles. The lifetime of resonances is only ~ 10 –23 –10 –22 s.

Elementary particles, as well as complex microparticles, can be observed due to the traces they leave during their passage through the substance. The nature of the traces makes it possible to judge the sign of the particle's charge, its energy, momentum, etc. Charged particles cause ionization of molecules on their way. Neutral particles do not leave traces, but they can reveal themselves at the moment of decay into charged particles or at the moment of collision with any nucleus. Therefore, ultimately neutral particles are also detected by ionization caused by the charged particles they generate.

Particles and antiparticles... In 1928 the English physicist P. Dirac succeeded in finding a relativistic quantum-mechanical equation for the electron, from which a number of remarkable consequences follow. First of all, from this equation in a natural way, without any additional assumptions, the spin and the numerical value of the intrinsic magnetic moment of the electron are obtained. Thus, it turned out that spin is a quantity simultaneously both quantum and relativistic. But this does not exhaust the significance of the Dirac equation. It also made it possible to predict the existence of an electron antiparticle - positron... From the Dirac equation, not only positive, but also negative values \u200b\u200bare obtained for the total energy of a free electron. Studies of the equation show that for a given momentum of a particle, there are solutions to the equation corresponding to the energies: .

Between the greatest negative energy (- m e from 2) and the least positive energy (+ m e c 2) there is an interval of energy values \u200b\u200bthat cannot be realized. The width of this interval is 2 m e from 2. Consequently, two regions of energy eigenvalues \u200b\u200bare obtained: one starts with + m e from 2 and extends to + ∞, the other starts at - m e from 2 and extends to –∞.

A particle with negative energy must have very strange properties. Passing into states with less and less energy (that is, with a negative energy increasing in absolute value), it could release energy, say, in the form of radiation, and since | E| is not limited by anything, a particle with negative energy could emit an infinitely large amount of energy. A similar conclusion can be reached in the following way: from the relation E=m e from 2 it follows that a particle with negative energy will also have negative mass. Under the action of the decelerating force, a particle with negative mass should not decelerate, but accelerate, performing an infinitely large amount of work on the source of the decelerating force. In view of these difficulties, it would seem that it should be recognized that the state with negative energy should be excluded from consideration as leading to absurd results. This, however, would be contrary to some general principles of quantum mechanics. Therefore, Dirac chose a different path. He suggested that transitions of electrons to states with negative energy are usually not observed, for the reason that all available levels with negative energy are already occupied by electrons.

According to Dirac, vacuum is a state in which all levels of negative energy are populated by electrons, and levels with positive energy are free. Since all the levels below the forbidden band are occupied, the electrons at these levels do not show themselves in any way. If one of the electrons at negative levels is given energy E≥ 2m e from 2, then this electron will go into a state with positive energy and will behave in the usual way, like a particle with positive mass and negative charge. This first of the theoretically predicted particles was called a positron. When a positron meets an electron, they annihilate (disappear) - the electron passes from a positive level to a vacant negative one. The energy corresponding to the difference between these levels is released in the form of radiation. In fig. 4 arrow 1 depicts the process of creation of an electron-positron pair, and arrow 2 - their annihilation. The term “annihilation” should not be taken literally. Essentially, there is not a disappearance, but a transformation of some particles (electron and positron) into others (γ-photons).

There are particles that are identical with their antiparticles (that is, they do not have antiparticles). Such particles are called absolutely neutral. These include the photon, π 0 meson and η meson. Particles that are identical with their antiparticles are not capable of annihilation. This, however, does not mean that they cannot transform into other particles at all.

If baryons (i.e. nucleons and hyperons) are assigned a baryon charge (or baryon number) IN \u003d +1, antibaryons - baryon charge IN \u003d –1, and all other particles have baryon charge IN\u003d 0, then all processes involving baryons and antibaryons will be characterized by the conservation of charge baryons, just as the processes are characterized by the conservation of electric charge. The law of conservation of baryon charge determines the stability of the softest of baryons - the proton. The transformation of all quantities describing a physical system, in which all particles are replaced by antiparticles (for example, electrons by protons, and protons by electrons, etc.) is called conjugation charge.

Strange particles.TO-mesons and hyperons were discovered in the composition of cosmic rays in the early 50s of the XX century. Since 1953 they have been produced at accelerators. The behavior of these particles turned out to be so unusual that they were called strange. The unusual behavior of the strange particles was that they were obviously born due to strong interactions with a characteristic time of the order of 10 –23 s, and their lifetimes turned out to be of the order of 10 –8 –10 –10 s. The latter circumstance indicated that the decay of particles occurs as a result of weak interactions. It was completely incomprehensible why the strange particles last so long. Since the same particles (π-mesons and proton) are involved in both the production and decay of the λ-hyperon, it seemed surprising that the rates (that is, the probability) of both processes are so different. Further research showed that strange particles are born in pairs. This suggested that strong interactions cannot play a role in particle decay due to the fact that two strange particles must be present for their manifestation. For the same reason, the single production of strange particles is impossible.

To explain the prohibition of a single production of strange particles, M. Gell-Mann and K. Nishijima introduced a new quantum number into consideration, the total value of which should, according to their assumption, be conserved under strong interactions. This is a quantum number Swas named particle strangeness... In weak interactions, strangeness may not persist. Therefore, it is assigned only to strongly interacting particles - mesons and baryons.

Neutrino.Neutrino is the only particle that does not participate in either strong or electromagnetic interactions. Excluding the gravitational interaction, in which all particles participate, neutrinos can take part only in weak interactions.

For a long time it remained unclear how neutrinos differ from antineutrinos. The discovery of the conservation law for combined parity made it possible to answer this question: they differ in helicity. Under helicitymeans a certain relationship between the directions of the impulse Rand back Sparticles. Spirality is considered positive if the spin and momentum are in the same direction. In this case, the direction of motion of the particle ( R) and the direction of “rotation” corresponding to the spin form a right-hand screw. With oppositely directed back and momentum, helicity will be negative (translational motion and “rotation” form a left screw). According to the theory of longitudinal neutrinos developed by Yang, Lee, Landau and Salam, all neutrinos existing in nature, regardless of the way of their origin, are always completely longitudinally polarized (that is, their spin is directed parallel or antiparallel to the momentum R). Neutrino has negative(left) helicity (it corresponds to the ratio of directions S and Rshown in Fig. 5 (b), antineutrino - positive (right) helicity (a). Thus, helicity is what distinguishes neutrinos from antineutrinos.

Figure: 5.Diagram of the helicity of elementary particles

Systematics of elementary particles.The regularities observed in the world of elementary particles can be formulated in the form of conservation laws. Quite a lot of such laws have already accumulated. Some of them turn out to be not exact, but only approximate. Each conservation law expresses a certain symmetry of the system. Conservation of momentum R, angular momentum Land energy Ereflect the properties of symmetry of space and time: conservation Eis a consequence of the homogeneity of time, the preservation Rdue to the homogeneity of space, and the preservation L- its isotropy. The parity conservation law is related to the symmetry between right and left ( R-invariance). Symmetry with respect to charge conjugation (symmetry of particles and antiparticles) leads to conservation of charge parity ( FROM-invariance). Conservation laws of electric, baryon and lepton charges express a special symmetry FROM-function. Finally, the conservation law for the isotopic spin reflects the isotropy of the isotopic space. Failure to comply with one of the conservation laws means violation of the corresponding type of symmetry in this interaction.

In the world of elementary particles, the rule applies: everything is allowed that is not prohibited by conservation laws... The latter play the role of prohibition rules governing the interconversion of particles. First of all, we note the laws of conservation of energy, momentum and electric charge. These three laws explain the stability of the electron. It follows from the conservation of energy and momentum that the total rest mass of the decay products should be less than the rest mass of the decaying particle. This means that an electron could decay only into neutrinos and photons. But these particles are electrically neutral. So it turns out that the electron simply has no one to transfer its electric charge to, so it is stable.

Quarks.There were so many particles called elementary that serious doubts arose about their elementary character. Each of the strongly interacting particles is characterized by three independent additive quantum numbers: charge Q, hypercharge Haveand baryon charge IN... In this regard, a hypothesis appeared that all particles are built of three fundamental particles - carriers of these charges. In 1964, Gell-Mann and, independently of him, the Swiss physicist Zweig put forward a hypothesis according to which all elementary particles are built from three particles called quarks. Fractional quantum numbers are attributed to these particles, in particular, an electric charge equal to +; –⅓; + ⅓ respectively for each of the three quarks. These quarks are usually denoted by the letters U,D,S... In addition to quarks, antiquarks ( u,d, s). To date, 12 quarks are known - 6 quarks and 6 antiquarks. Mesons are formed from a quark-antiquark pair, and baryons are formed from three quarks. So, for example, a proton and a neutron are composed of three quarks, which makes the proton or neutron colorless. Accordingly, three charges of strong interactions are distinguished - red ( R), yellow ( Y) and green ( G).

Each quark is assigned the same magnetic moment (μV), the value of which cannot be determined from theory. Calculations made on the basis of this assumption give the value of the magnetic moment μ p = μ sq, and for a neutron μ n = – ⅔μ sq.

Thus, for the ratio of the magnetic moments, the value μ p / μ n = –⅔, which is in excellent agreement with the experimental value.

Basically, the color of a quark (like the sign of an electric charge) began to express a difference in the property that determines the mutual attraction and repulsion of quarks. By analogy with the quanta of the fields of various interactions (photons in electromagnetic interactions, r-mesons in strong interactions, etc.), particles-carriers of interactions between quarks were introduced. These particles were named gluons... They transfer color from one quark to another, causing the quarks to be held together. In quark physics, the confinement hypothesis is formulated (from the English. confinements- trapping) quarks, according to which it is impossible to subtract a quark from the whole. It can exist only as an element of the whole. The existence of quarks as real particles in physics is reliably substantiated.

The idea of \u200b\u200bquarks turned out to be very fruitful. It allowed not only to systematize the already known particles, but also to predict a number of new ones. The situation in the physics of elementary particles is reminiscent of the situation in the physics of the atom after the discovery of the periodic law in 1869 by D. I. Mendelev. Although the essence of this law was clarified only about 60 years after the creation of quantum mechanics, it made it possible to systematize the chemical elements known by that time and, in addition, led to the prediction of the existence of new elements and their properties. In exactly the same way, physicists have learned to systematize elementary particles, and the developed systematics in a number of cases made it possible to predict the existence of new particles and to anticipate their properties.

So, at the present time, quarks and leptons can be considered truly elementary; there are 12 of them, or together with anti-particles - 24. In addition, there are particles that provide four fundamental interactions (quanta of interaction). There are 13 of these particles: graviton, photon, W ± - and Z-particles and 8 gluons.

The existing theories of elementary particles cannot indicate what is the beginning of the series: atoms, nuclei, hadrons, quarksIn this series, each more complex material structure includes a simpler one as a component. Apparently, this cannot go on forever. It was assumed that the described chain of material structures is based on objects of a fundamentally different nature. It is shown that such objects can be not pointlike, but extended, although extremely small (~ 10 ‑33 cm) formations, called superstrings.The described idea is not realizable in our four-dimensional space. This area of \u200b\u200bphysics is generally extremely abstract, and it is very difficult to find visual models that help simplify the perception of ideas inherent in the theory of elementary particles. Nevertheless, these theories allow physicists to express the interconversion and interdependence of the “most elementary” micro-objects, their connection with the properties of the four-dimensional space-time. The most promising is the so-called M-theory (M - from mystery- riddle, mystery). She is operating twelve-dimensional space ... Ultimately, during the transition to the four-dimensional world directly perceived by us, all “extra” dimensions “collapse”. M-theory is still the only theory that makes it possible to reduce four fundamental interactions to one - the so-called Superpower.It is also important that M-theory admits the existence of different worlds and establishes the conditions that ensure the emergence of our world. M-theory is not yet sufficiently developed. It is believed that the final "Theory of everything" on the basis of M-theory will be built in the XXI century.

Unstable elementary particles

All other elementary particles are unstable, that is, they spontaneously decay into other particles in a free state. It has been experimentally established that the probability of decay of an unstable elementary particle does not depend on the duration of its existence and the time of observation of it. It is impossible to predict the moment of decay of a given elementary particle. It is possible to predict only the average lifetime of a large number of particles of the same type. Probability P (\\ displaystyle P) the fact that the particle decays within the next short period of time δ t (\\ displaystyle \\ delta t) equals δ t τ (\\ displaystyle (\\ frac (\\ delta t) (\\ tau))) and depends only on the constant τ (\\ displaystyle \\ tau) and does not depend on the background. This fact is one of the confirmations of the principle of identity of elementary particles. We obtain an equation for the dependence of the number of particles on time: N P \u003d N δ t τ \u003d - δ t d N d t (\\ displaystyle NP \u003d (\\ frac (N \\ delta t) (\\ tau)) \u003d - \\ delta t (\\ frac (dN) (dt))), d N d t \u003d - N τ (\\ displaystyle (\\ frac (dN) (dt)) \u003d - (\\ frac (N) (\\ tau)))... The solution to this equation has the form:, where N 0 (\\ displaystyle N_ (0)) - the number of particles at the initial moment. Thus, the lifetime of an unstable elementary particle is a random variable with an exponential distribution law.

The instability of particles is one of the manifestations of the property of interconversion of particles, which is a consequence of their interactions: strong, electromagnetic, weak, gravitational. The decay of unstable elementary particles occurs due to their interaction with zero-point oscillations of the field that is responsible for their decay. Interactions of particles cause transformations of particles and their aggregates into other particles, if such transformations are not prohibited by the laws of conservation of energy, momentum, angular momentum, electric charge, baryon charge, etc.

Lifetime of elementary particles

An important characteristic of elementary particles, along with mass, spin, electric charge, is its lifetime. Lifetime is a constant τ (\\ displaystyle \\ tau) in the law of exponential decay: N (t) \u003d N 0 exp \u2061 (- t / τ) (\\ displaystyle N (t) \u003d N_ (0) \\ exp (-t / \\ tau)) ... For example, the neutron lifetime τ n \u003d 880 (\\ displaystyle \\ tau _ (n) \u003d 880) sec, the lifetime of a charged pi-meson τ π + \u003d 2.6033 (5) × 10 - 8 (\\ displaystyle \\ tau _ (\\ pi ^ (+)) \u003d 2.6033 (5) \\ times 10 ^ (- 8)) sec. Lifetime τ (\\ displaystyle \\ tau) unstable particles depends on the type of interaction causing their decay. The longest lifetimes have elementary particles, whose decay is caused by weak interaction (neutron - 880 (\\ displaystyle 880) sec, muon - 2, 2 × 10 - 6 (\\ displaystyle 2.2 \\ times 10 ^ (- 6)) sec, charged peony - 2.6 × 10 - 8 (\\ displaystyle 2.6 \\ times 10 ^ (- 8)) sec, hyperon - 10 - 10 - 10 - 8 (\\ displaystyle 10 ^ (- 10) -10 ^ (- 8)) sec, kaon - 1.2 × 10 - 8 (\\ displaystyle 1.2 \\ times 10 ^ (- 8)) sec). Elementary particles, whose decay is caused by electromagnetic interaction (neutral pion - 8.2 × 10 - 17 (\\ displaystyle 8.2 \\ times 10 ^ (- 17)) sec, eta-meson - 5.1 × 10 - 19 (\\ displaystyle 5.1 \\ times 10 ^ (- 19)) sec). The shortest lifetimes have resonances - 10 - 24 - 10 - 22 (\\ displaystyle 10 ^ (- 24) -10 ^ (- 22)) sec.

For short-lived particles (resonances), instead of the lifetime, the width is used, which has the dimension of energy: Γ \u003d ℏ τ (\\ displaystyle \\ Gamma \u003d (\\ frac (\\ hbar) (\\ tau)))... This follows from the uncertainty relation between energy and time Δ E Δ t ≈ ℏ (\\ displaystyle \\ Delta E \\ Delta t \\ approx \\ hbar)... For example, the mass of the nucleon isobar Δ (\\ displaystyle \\ Delta) is equal to 1236 MeV, and its width is 120 MeV ( τ ≈ 5 × 10 - 24 (\\ displaystyle \\ tau \\ approx 5 \\ times 10 ^ (- 24)) c), which is about 10% by weight.

Decay probability ω (\\ displaystyle \\ omega) characterizes the intensity of decay of unstable particles and is equal to the fraction of particles of a certain ensemble decaying per unit time: ω \u003d 1 τ (\\ displaystyle \\ omega \u003d (\\ frac (1) (\\ tau)))where τ (\\ displaystyle \\ tau) is the lifetime of an elementary particle.

Many elementary particles have several decay methods. In this case, the total probability of decay of a particle in a certain time is equal to the sum of the probabilities of decay in different ways: 1 τ \u003d 1 τ 1 + 1 τ 2 +. ... ... + 1 τ N (\\ displaystyle (\\ frac (1) (\\ tau)) \u003d (\\ frac (1) (\\ tau _ (1))) + (\\ frac (1) (\\ tau _ (2))) + ... + (\\ frac (1) (\\ tau _ (N))))where N (\\ displaystyle N) - the number of decay methods, τ (\\ displaystyle \\ tau) - lifetime. The relative probability of decay by i (\\ displaystyle i)-th way is equal to: P i \u003d 1 τ i 1 τ (\\ displaystyle P_ (i) \u003d (\\ frac (\\ frac (1) (\\ tau _ (i))) (\\ frac (1) (\\ tau))))... Regardless of the number of types of its decay, an elementary particle always has only one lifetime τ (\\ displaystyle \\ tau) .

Elementary particle lifetime τ (\\ displaystyle \\ tau) and its half-life T 1/2 (\\ displaystyle T_ (1/2)) related by the ratio: T 1/2 \u003d ln \u2061 2 τ \u003d 0.693 τ (\\ displaystyle T_ (1/2) \u003d \\ ln (2) \\ tau \u003d 0.693 \\ tau)