In the Universe every body lives in its own time and the basic elementary particles also. The lifetime of most elementary particles is rather short.

Some decay immediately after they are born, which is why we call them unstable particles.

After a short time, they decay into stable ones: protons, electrons, neutrinos, photons, gravitons and their antiparticles.

The most important micro-objects in our close space - protons and electrons. Some of the distant parts of the Universe may consist of antimatter, the most important particles there will be an antiproton and an antielectron (positron).

In total, several hundred elementary particles have been discovered: proton (p), neutron (n), electron (e -), as well as photon (g), pi-mesons (p), muons (m), neutrinos of three types (electronic ve , muon v m , with lepton v t), etc. obviously they will bring more new microparticles.

Appearance of particles:

protons and electrons

The appearance of protons and electrons dates back to about ten billion years.

Another type of micro-objects that play a significant role in the structure of near space is neutrons, which have a common name with a proton: nucleons. Neutrons themselves are unstable, they decay about ten minutes after they are generated. They can only be stable in the nucleus of an atom. A huge number of neutrons constantly arise in the depths of stars, where the nuclei of atoms are born from protons.

Neutrino

In the Universe, the birth of neutrinos is also constantly taking place, which are similar to an electron, but without a charge and with a small mass. In 1936, a variety of neutrinos was discovered: muon neutrinos, which arise during the transformation of protons into neutrons, in the depths of supermassive stars and during the decay of many unstable micro-objects. They are born when cosmic rays collide in interstellar space.

The big bang resulted in the appearance of a huge number of neutrinos and muon neutrinos. Their number in space is constantly increasing, because they are not absorbed by almost any matter.

Photons

Like photons, neutrinos and muon neutrinos fill the entire space. This phenomenon is called the "neutrino sea".
Since the Big Bang, there are a great many photons left, which we call relict or fossil. They are filled with all outer space, and their frequency, and hence the energy is constantly decreasing, as the universe is expanding.

At present, all cosmic bodies, primarily stars and nebulae, are involved in the formation of the photon part of the Universe. Photons are born on the surface of stars from the energy of electrons.

Particle connection

At the initial stage of the formation of the Universe, all basic elementary particles were free. Then there were no nuclei of atoms, no planets, no stars.

Atoms, and from them planets, stars and all substances, were formed later, when 300,000 years had passed and the incandescent matter cooled sufficiently during expansion.

Only the neutrino, muon neutrino and photon did not enter into any system: their mutual attraction is too weak. They have remained free particles.

Even at the initial stage of the formation of the Universe (300,000 years after its birth), free protons and electrons combined into hydrogen atoms (one proton and one electron connected by an electric force).

The proton is considered the main elementary particle with a charge of +1 and a mass of 1.672 10 −27 kg (slightly less than 2000 times heavier than an electron). The protons that found themselves in a massive star gradually turned into the main building "iron" of the Universe. Each of them released one percent of their rest mass. In supermassive stars, which shrink into small volumes as a result of their own gravity at the end of their lives, a proton can lose almost a fifth of its rest energy (and hence a fifth of its rest mass).

It is known that the "building microblocks" of the Universe are protons and electrons.

Finally, when a proton and an antiproton meet, no system arises, but all their rest energy is released in the form of photons ().

Scientists claim that there is also a ghostly basic elementary particle graviton that carries gravitational interaction similar to electromagnetism. However, the existence of a graviton has been proven only theoretically.

Thus, the main elementary particles arose and now represent our Universe, including the Earth: protons, electrons, neutrinos, photons, gravitons and many more discovered and undiscovered micro-objects.

These three particles (as well as others described below) mutually attract and repel each other according to their charges, which are only four types according to the number of fundamental forces of nature. Charges can be arranged in order of decreasing corresponding forces as follows: color charge (forces of interaction between quarks); electric charge (electric and magnetic forces); weak charge (strength 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 the strongest charge and the greatest forces.

Charges persist, 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, 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, as it were, a “certificate” of the right to respond to the corresponding force. Thus, only colored particles are affected by color forces, only electrically charged particles are affected by electric forces, and so on. 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 color is dominant. 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 another sign. This corresponds to the minimum energy of the entire system. (Similarly, two bar magnets are in a line, with the north pole of one facing the south pole of the other, which corresponds to the minimum magnetic field energy.) Gravity is an exception to this rule: negative mass does not exist. There are no bodies that would fall up.

TYPES OF MATTER

Ordinary matter is formed from electrons and quarks, grouped into objects that are neutral in color, and then in electric charge. The color force is neutralized, which will be discussed in more detail below, when the particles are combined into triplets. (Hence the term "color" itself, taken from optics: the three primary colors, when mixed, give white.) Thus, quarks, for which the color power is the main one, form triplets. But quarks, and they are subdivided into u-quarks (from English up - upper) and d-quarks (from the English down - lower), they also have an electric charge equal to u-quark and for d-quark. Two u-quark and one d-quark give an electric charge +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 the nuclei carry a positive electric charge and, by attracting negative electrons that revolve around the nucleus like planets revolving around 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. Due to the power of color interaction, 99.945% of the mass of an atom is enclosed 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 causes electrical phenomena.

There are several hundred natural varieties of atoms (including isotopes) that differ in the number of neutrons and protons in the nucleus and, accordingly, in the number of electrons in 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) a few electrons, have become charged particles. Matter, consisting almost of one ions, is called plasma. Stars that burn due to thermonuclear reactions going on in the centers are composed mainly of plasma, and since stars are the most common form of matter in the universe, it can be said that the entire universe consists mainly of plasma. More precisely, stars are predominantly fully ionized gaseous hydrogen, 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 still invisible matter in the Universe. And there are particles that act as carriers of forces. 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 real, true nature of elementary particles and the forces acting between them. According to the most recent theories, particles can basically be extended geometric objects - "strings" in ten-dimensional space.

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 a truly invisible matter that permeates all of us and the entire Universe every second. It is a fast-moving gas of one kind of particles - electron neutrinos.

The electron neutrino is the partner of the electron, but has no electric charge. Neutrinos carry only the 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 the Einstein formula E = mc 2 , where c is the speed of light.

The key role of the neutrino is that it contributes to the transformation and-quarks in d quarks, resulting in the transformation of a proton into a neutron. The neutrino plays the role of the "carburetor needle" for stellar thermonuclear reactions, in which four protons (hydrogen nuclei) combine to form a helium nucleus. But since the helium nucleus consists not 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 fast the stars will burn. And the transformation process is determined by weak charges and forces of weak interaction 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 simply 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, then the stars would not burn at all. If they were stronger, then the stars would have burned out long ago.

But what about neutrinos? Since 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 through the Universe, until they enter, perhaps, into a new interaction of the STAR) .

Interaction carriers.

What causes forces that act between particles at a distance? Modern physics answers: due to the exchange of other particles. Imagine two skaters tossing a ball around. Giving the ball momentum when throwing and receiving momentum with the received ball, both get a push in the direction 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, it would seem, to the impossible: one of the skaters throws the ball in the direction from the other, but the 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), there would be attraction between the skaters.

Particles, due to the exchange of which interaction forces arise 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 strong interaction carrier particles are gluons (there are only eight of them). A photon is a carrier of electromagnetic interaction (it is one, and we perceive photons as light). The particles-carriers of the weak interaction are intermediate vector bosons (in 1983 and 1984 were discovered W + -, W- -bosons and neutral Z-boson). The particle-carrier of the gravitational interaction is still a hypothetical graviton (it must be 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 - with gravitational waves (not yet detected with certainty).

A particle capable of emitting gauge particles is said to be surrounded by an appropriate force field. 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 field of strong interaction. 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 on this below.

Antimatter.

Each particle corresponds to an antiparticle, with which the particle can mutually annihilate, i.e. "annihilate", as a result of which energy is released. "Pure" energy by itself, however, does not exist; as a result of annihilation, new particles (for example, photons) appear, carrying away this energy.

An antiparticle in most cases has the opposite properties with respect to the corresponding particle: if a particle moves to the left under the action 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, like, for example, a neutron, then its antiparticle consists of components with opposite charge signs. Thus, an antielectron has an electric charge of +1, a weak charge of +1/2 and is called a positron. The antineutron is made up of and-antiquarks with electric charge –2/3 and d-antiquarks with electric charge +1/3. Truly neutral particles are their own antiparticles: the photon's antiparticle is the photon.

According to modern theoretical concepts, each particle that exists in nature must have its own antiparticle. And many antiparticles, including positrons and antineutrons, were indeed obtained in the laboratory. The consequences of this are exceptionally important and underlie the entire experimental physics of elementary particles. 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 pair of particles and antiparticles (with zero net charge) can emerge from 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 quadruple (quartet) of material particles is repeated at least twice at higher mass values. In the second generation, the place of the electron is occupied 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 the muon (which accompanies the muon in weak interactions in the same way as the electron accompanies 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-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 there can be no more than four types of light neutrinos.

In experiments with high-energy particles, the electron, muon, tau-lepton and the corresponding neutrinos act as separate particles. They do not carry a color charge and only enter into 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, on the other hand, under the influence of color forces, combine into strongly interacting particles that dominate most experiments in high-energy physics. Such particles are called hadrons. They include two subclasses: baryons(e.g. proton and 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. The omega-minus hadrons, discovered in 1964 at the Brookhaven National Laboratory (USA), and the j-psy particle ( J/y-meson), discovered simultaneously in Brookhaven and at the Stanford Center for Linear Accelerators (also in the USA) in 1974. The existence of the omega-minus particle was predicted by M. Gell-Mann in his so-called " SU 3-theory” (another name is the “eight-fold way”), 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 combined electromagnetic and weak forces ( see below).

Particles of the second and third generations are no less real than those of the first. True, having arisen, they decay in millionths or billionths of a second into ordinary particles of the first generation: an electron, an electron neutrino, and also and- and d-quarks. The question of why there are several generations of particles in nature is still a mystery.

Different generations of quarks and leptons are often spoken of (which is, of course, somewhat eccentric) as different "flavors" of particles. The need to explain them is called the "flavor" problem.

BOSONS AND FERMIONS, FIELD AND SUBSTANCE

One of the fundamental differences between particles is the difference between bosons and fermions. All particles are divided into these two main classes. Like bosons can overlap or overlap, but like fermions can't. Superposition occurs (or does not occur) in the discrete energy states into which quantum mechanics divides nature. These states are, as it were, separate cells into which particles can be placed. So, in one cell you can put any number of identical bosons, but only one fermion.

As an example, consider such cells, or "states", for an electron revolving around 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 number 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 momenta and, therefore, two allowed cells, and in higher orbitals, eight or more cells.

Since an electron is a fermion, each cell can contain only one electron. From this follow very important consequences - the whole 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 order of increasing the number of protons in the nucleus per unit (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 successive change in the electronic structure of atoms from element to element determines the regularities in their chemical properties.

If the electrons were bosons, then all the electrons of 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 in the form in which we know it, the Universe would be impossible.

All leptons - electron, muon, tau-lepton and their corresponding neutrino - 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 very significant: fermions cannot combine, so the same applies to objects in the material world.

At the same time, all "gauge particles" 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, a laser is also possible.

Spin.

The difference between bosons and fermions is connected with another characteristic of elementary particles - back. Surprising as it may seem, but all fundamental particles have their own angular momentum or, in other words, rotate around their own axis. The angular momentum is a characteristic of rotational motion, just like the total momentum is 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, leptons and quarks have a spin equal to 1/2, and gauge particles have a spin equal to 1 (except for the graviton, which has not yet been observed experimentally, but theoretically should have a spin equal to 2). Since leptons and quarks are fermions, and gauge particles are bosons, it can be assumed 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 is integer, 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. Such an exchange constantly takes place in protons, neutrons and atomic nuclei. In the same way, photons exchanged between electrons and quarks create electrical attractive forces that hold electrons in an atom, and intermediate vector bosons exchanged between leptons and quarks create weak interaction forces responsible for the conversion of protons into neutrons during thermonuclear reactions in stars.

The theory of such an 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 gauge theory of gravity similar to them, although in some ways different. One of the most important physical problems is the reduction of these separate theories into a single and at the same time simple theory, in which all of them would become different aspects of a single reality - like the facets of a crystal.

Table 3. SOME HADRONS

Table 3. SOME HADRONS
Particle	Symbol	Quark composition *	rest mass, MeV/ With 2	Electric charge
BARYONS
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
JPS	J/y	cў	3100	0
Upsilon	Ў	b	9460	0
* Quark composition: u- upper; d- lower; s- strange; c- enchanted b- beautiful. The line above the letter denotes antiquarks.

The simplest and oldest of 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 charges be compared? 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 the near electron to the far one and see how it reacts. The signal is a gauge particle - a photon. In order to be able to check the charge on distant particles, a photon is needed.

Mathematically, this theory is distinguished by extreme precision and beauty. From the "gauge principle" described above, all quantum electrodynamics (the quantum theory of electromagnetism) follows, 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 of different parts of the Universe, allowing measurements in the Universe. In mathematical terms, the field is interpreted geometrically as the curvature of some conceivable "internal" space. The measurement of charge is the measurement of the total "internal curvature" around the particle. Gauge theories of strong and weak interactions differ from electromagnetic gauge theory only in the internal geometric "structure" of the corresponding charge. The question of where exactly this inner space is located is being answered by multidimensional unified field theories, which are not considered 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	Carrier spin
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
Gravity- rational	10 –38	Ґ	graviton	0	2

The physics of elementary particles is not completed yet. It is still far from clear whether the available data are sufficient to fully understand the nature of particles and forces, as well as the true nature and dimensions of space and time. Do we need experiments with energies of 10 15 GeV for this, or will the effort of thought be enough? There is 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 be not so many fundamental ideas: the gauge principle, spaces of higher dimensions, collapse and expansion, and, above all, geometry.

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 XIX century. the electron was discovered, and then in the first decades of the 20th 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 enormous speeds are created, the existence of a large number of elementary particles - over 300 was established. Among them are both experimentally discovered and theoretically calculated, including resonances, quarks and virtual particles.

Term elementary particle originally 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 doubt that the particles have one structure or another, 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 an electron. There are elementary particles that do not have a rest mass, - photons. The rest of the particles on this basis are divided into leptons– light particles (electron and neutrino); mesons– medium particles with a mass ranging from one to a thousand electron masses; baryons- heavy particles whose mass exceeds a thousand masses of an electron 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. Approximately in 1963-1964. hypothesized that there is quarks– particles with a fractional electric charge. This hypothesis has not yet been experimentally confirmed.

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 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 before they even leave the atom or atomic nucleus. The resonant states have been calculated theoretically; it is not possible to fix them in real experiments.

In addition to charge, mass and lifetime, elementary particles are also described by concepts that have no analogues in classical physics: the concept back . Spin is the intrinsic angular momentum of a particle, not related to its displacement. 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. The electron belongs to fermions. According to the Pauli principle, an atom cannot have more than one electron with the same set of quantum numbers. n,m,l,s. The 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 remaining quantum numbers determine its filling, as mentioned above.

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

All particles that have a rest mass ( m 0), participate in gravitational interaction, charged - and in electromagnetic. Leptons also participate in weak interactions. Hadrons participate in all four fundamental interactions.

According to quantum field theory, all interactions are carried out through the exchange virtual particles , that is, particles whose existence 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 known four types of interactions - gravitational, electromagnetic, strong and weak - have a gauge nature and are described by gauge symmetries. That is, all interactions are, as it were, made “from one blank”. This inspires 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 the 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) - field quanta.

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

Mesons are strongly interacting unstable particles that do not carry the so-called baryon charge. Among them belongs R-mesons, or pions (π +, π -, π 0), To-mesons, or kaons (K + , K - , K 0), and this-mesons (η) . Weight To-mesons is ~970me (494 MeV for charged and 498 MeV for neutral To-mesons). Lifetime To-mesons has a magnitude of about 10–8 s. They break up to form I-mesons and leptons or only leptons. Weight this-mesons is equal to 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 a weak (and, if they are charged, electromagnetic), but also a strong interaction, which manifests itself in their interaction with each other, as well as in the interaction between mesons and baryons. The spin of all mesons is zero, so they are bosons.

Class baryons combines nucleons (p, n) and unstable particles with a mass greater than the mass of nucleons, called hyperons. All baryons have a strong interaction and, therefore, actively interact with atomic nuclei. The spin of all baryons is ½, so baryons are fermions. With the exception of the proton, all baryons are unstable. In the decay of baryons, along with other particles, a baryon is necessarily formed. This pattern is one of the manifestations baryon 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 that they leave when they pass through matter. The nature of the traces makes it possible to judge the sign of the charge of the particle, 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, eventually neutral particles are also detected by the ionization caused by the charged particles generated by them.

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 the spin is a quantity 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 antiparticle of the electron - positron. From the Dirac equation, not only positive but also negative values ​​are obtained for the total energy of a free electron. Studies of the equation show that for a given particle momentum, there are solutions to the equation corresponding to the energies: .

Between the greatest negative energy (- m e With 2) and the smallest positive energy (+ m e c 2) there is an interval of energy values ​​that cannot be realized. The width of this interval is 2 m e With 2. Consequently, two regions of energy eigenvalues ​​are obtained: one begins with + m e With 2 and extends to +∞, the other starts from - m e With 2 and extends to –∞.

A particle with negative energy must have very strange properties. Passing into states with ever lower energy (that is, with negative energy increasing in absolute value), it could release energy, say, in the form of radiation, moreover, since | E| is not limited by anything, a particle with negative energy could radiate an infinitely large amount of energy. A similar conclusion can be reached in the following way: from the relation E=m e With 2 it follows that the mass of a particle with negative energy will also be negative. Under the action of a decelerating force, a particle with a negative mass should not slow down, but accelerate, doing an infinitely large amount of work on the source of the decelerating force. In view of these difficulties, it would seem that one should admit that the state with negative energy should be excluded from consideration as leading to absurd results. This, however, would contradict some general principles of quantum mechanics. So 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 without exception are occupied, the electrons at these levels do not reveal themselves in any way. If one of the electrons located at negative levels is given energy E≥ 2m e With 2 , then this electron will go into a state with positive energy and will behave in the usual way, like a particle with a positive mass and a negative charge. This first theoretically predicted particle was called the 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. On fig. 4, arrow 1 depicts the process of the creation of an electron-positron pair, and arrow 2 - their annihilation The term “annihilation” should not be taken literally. In essence, what is happening is not the disappearance, but the 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 (that is, nucleons and hyperons) are assigned a baryon charge (or baryon number) AT= +1, antibaryons – baryon charge AT= –1, and for all other particles – the baryon charge AT= 0, then for all processes occurring with the participation of baryons and antibaryons, the conservation of charge baryons will be characteristic, just as the conservation of electric charge is characteristic of processes. The law of conservation of baryon charge determines the stability of the softest baryon, 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 the conjugation charge.

Strange particles.To-mesons and hyperons were discovered in the composition of cosmic rays in the early 1950s. Since 1953, they have been produced on accelerators. The behavior of these particles turned out to be so unusual that they were called strange. The unusual behavior of 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 particles decay as a result of weak interactions. It was completely incomprehensible why strange particles live so long. Since the same particles (π-mesons and protons) are involved in both the creation and decay of a λ-hyperon, it seemed surprising that the rate (that is, the probability) of both processes is so different. Further research showed that strange particles are produced in pairs. This led to the idea that strong interactions cannot play a role in the decay of particles due to the fact that the presence of two strange particles is necessary for their manifestation. For the same reason, the single production of strange particles is impossible.

To explain the ban on the single production of strange particles, M. Gell-Mann and K. Nishijima introduced a new quantum number, the total value of which, according to their assumption, should be preserved under strong interactions. It's a quantum number S was named particle strangeness. In weak interactions, strangeness may not be conserved. Therefore, it is attributed only to strongly interacting particles - mesons and baryons.

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

For a long time it remained unclear how neutrinos differ from antineutrinos. The discovery of the law of conservation of combined parity made it possible to answer this question: they differ in helicity. Under helicity a certain relationship between the directions of momentum is understood R and back S particles. Helicity is considered positive if the spin and momentum are in the same direction. In this case, the direction of particle motion ( R) and the direction of “rotation” corresponding to the spin form a right screw. With oppositely directed spin 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 that exist in nature, regardless of the way they arise, 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 R shown in fig. 5 (b), antineutrino - positive (right) helicity (a). Thus, helicity is what distinguishes neutrinos from antineutrinos.

Rice. 5. Scheme of helicity of elementary particles

Systematics of elementary particles. The patterns observed in the world of elementary particles can be formulated as conservation laws. There are already quite a few such laws. Some of them are not exact, but only approximate. Each conservation law expresses a certain symmetry of the system. Laws of conservation of momentum R, angular momentum L and energy E reflect the symmetry properties of space and time: conservation E is a consequence of the homogeneity of time, the conservation R due to the homogeneity of space, and the conservation L- its isotropy. The parity conservation law is related to the symmetry between right and left ( R-invariance). Symmetry under charge conjugation (symmetry of particles and antiparticles) leads to conservation of charge parity ( FROM-invariance). The laws of conservation of electric, baryon and lepton charges express a special symmetry FROM-functions. Finally, the isotopic spin conservation law reflects the isotropy of the isotopic space. Failure to comply with one of the conservation laws means a violation in this interaction of the corresponding type of symmetry.

In the world of elementary particles, the following rule applies: everything is allowed that is not prohibited by conservation laws. The latter play the role of prohibition rules regulating the interconversions 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 must be less than the rest mass of the decaying particle. This means that the electron could only decay 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 are so many particles called elementary that there are serious doubts about their elementary nature. Each of the strongly interacting particles is characterized by three independent additive quantum numbers: the charge Q, hypercharge At and baryon charge AT. In this regard, a hypothesis appeared that all particles are built from 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. These particles are assigned fractional quantum numbers, in particular, an electric charge equal to +⅔; –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 are considered ( 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 made up 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 is not determined from the theory. Calculations made on the basis of this assumption give the proton the value of the magnetic moment μ p = μ q, and for the neutron μ n = – ⅔μ sq.

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

Basically, the color of the quark (like the sign of the electric charge) began to express the 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 interaction between quarks were introduced. These particles were named gluons. They transfer color from one quark to another, resulting in the quarks being held together. In quark physics, the confinement hypothesis has been formulated (from the English. confinements- captivity) of quarks, according to which it is impossible to subtract a quark from a 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 ​​quarks turned out to be very fruitful. It made it possible not only to systematize already known particles, but also to predict a number of new ones. The situation that has developed in elementary particle physics is reminiscent of the situation that was created in atomic physics after the discovery in 1869 by D. I. Mendelev of the periodic law. 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 few cases made it possible to predict the existence of new particles and anticipate their properties.

So, at the present time, quarks and leptons can be considered truly elementary; there are 12 of them, or together with antiparticles - 24. In addition, there are particles that provide four fundamental interactions (interaction quanta). 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 an integral part. Apparently, this cannot continue indefinitely. 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 point, but extended, albeit extremely small (~10 -33 cm) formations, called superstrings. The described idea is not realizable in our four-dimensional space. This area of ​​physics is generally extremely abstract, and it is very difficult to find visual models that help a simplified perception of the ideas embedded in the theories 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 four-dimensional space-time. The most promising is the so-called M-theory (M - from mystery- a riddle, a mystery). She operates twelve-dimensional space . Ultimately, during the transition to the four-dimensional world directly perceived by us, all the “extra” dimensions “collapse”. M-theory is so far the only theory that makes it possible to reduce the four fundamental interactions to one - the so-called Superpower. It is also important that M-theory allows for 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.

With the words "electricity", "electric charge", "electric current" you have met many times and managed to get used to them. But try to answer the question: “What is an electric charge?” - and you will see that it is not so easy. The fact is that the concept of charge is a basic, primary concept that cannot be reduced at the present level of development of our knowledge to any simpler, elementary concepts.

Let us first try to find out what is meant by the statement: a given body or particle has an electric charge.

You know that all bodies are built from the smallest, indivisible into simpler (as far as science is now known) particles, which are therefore called elementary. All elementary particles have mass and due to this are attracted to each other according to the law of universal gravitation with a force that decreases relatively slowly as the distance between them increases, inversely proportional to the square of the distance. Most elementary particles, although not all, also have the ability to interact with each other with a force that also decreases inversely with the square of the distance, but this force is a huge number of times greater than the force of gravity. So. in the hydrogen atom, shown schematically in Figure 91, the electron is attracted to the nucleus (proton) with a force 101" times greater than the force of gravitational attraction.

If particles interact with each other with forces that slowly decrease with distance and are many times greater than the forces of universal gravitation, then these particles are said to have an electric charge. The particles themselves are called charged. There are particles without electric charge, but there is no electric charge without a particle.

Interactions between charged particles are called electromagnetic. Electric charge is a physical quantity that determines the intensity of electromagnetic interactions, just as mass determines the intensity of gravitational interactions.

The electric charge of an elementary particle is not a special "mechanism" in the particle, which could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge on an electron and other particles means only the existence

certain force interactions between them. But we, in essence, do not know anything about the charge, if we do not know the laws of these interactions. Knowledge of the laws of interactions should be included in our understanding of the charge. These laws are not simple, it is impossible to state them in a few words. This is why it is impossible to give a sufficiently satisfactory concise definition of what an electric charge is.

Two signs of electric charges. All bodies have mass and therefore attract each other. Charged bodies can both attract and repel each other. This most important fact, familiar to you from the 7th grade physics course, means that in nature there are particles with electric charges of opposite signs. Particles with the same sign of charge repel each other, and with different signs they attract.

The charge of elementary particles - protons, which are part of all atomic nuclei, is called positive, and the charge of electrons is called negative. There are no intrinsic differences between positive and negative charges. If the signs of the particle charges were reversed, then the nature of electromagnetic interactions would not change at all.

elemental charge. In addition to electrons and protons, there are several other types of charged elementary particles. But only electrons and protons can exist indefinitely in a free state. The rest of the charged particles live less than millionths of a second. They are born during collisions of fast elementary particles and, having existed for a negligible time, decay, turning into other particles. You will get acquainted with these particles in the X class.

Neutrons are particles that do not have an electric charge. Its mass only slightly exceeds the mass of a proton. Neutrons, along with protons, are part of the atomic nucleus.

If an elementary particle has a charge, then its value, as shown by numerous experiments, is strictly defined (one of these experiments - the experience of Millikan and Ioffe - was described in a textbook for grade VII)

There is a minimum charge, called elementary, which all charged elementary particles possess. Charges of elementary particles differ only in signs. It is impossible to separate part of the charge, for example, from an electron.

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It is impossible to give a short definition of charge that is satisfactory in all respects. We are accustomed to finding understandable explanations for very complex formations and processes, such as the atom, liquid crystals, the distribution of molecules over velocities, and so on. But the most basic, fundamental concepts, indivisible into simpler ones, devoid, according to science today, of any internal mechanism, cannot be briefly explained in a satisfactory way. Especially if the objects are not directly perceived by our senses. It is to such fundamental concepts that the electric charge belongs.

Let us first try to find out not what an electric charge is, but what is hidden behind the statement, a given body or particle has an electric charge.

You know that all bodies are built from the smallest, indivisible into simpler (as far as science is now known) particles, which are therefore called elementary. All elementary particles have mass and due to this they are attracted to each other. According to the law of universal gravitation, the force of attraction decreases relatively slowly as the distance between them increases: inversely proportional to the square of the distance. In addition, most elementary particles, although not all, have the ability to interact with each other with a force that also decreases inversely with the square of the distance, but this force is a huge number, times greater than the force of gravity. So, in the hydrogen atom, shown schematically in Figure 1, the electron is attracted to the nucleus (proton) with a force 1039 times greater than the force of gravitational attraction.

If particles interact with each other with forces that slowly decrease with distance and are many times greater than the forces of universal gravitation, then these particles are said to have an electric charge. The particles themselves are called charged. There are particles without electric charge, but there is no electric charge without a particle.

Interactions between charged particles are called electromagnetic. When we say that electrons and protons are electrically charged, this means that they are capable of interactions of a certain type (electromagnetic), and nothing more. The absence of a charge on the particles means that it does not detect such interactions. Electric charge determines the intensity of electromagnetic interactions, just as mass determines the intensity of gravitational interactions. Electric charge is the second most important characteristic of elementary particles (after mass), which determines their behavior in the surrounding world.

In this way

Electric charge is a physical scalar quantity that characterizes the property of particles or bodies to enter into electromagnetic force interactions.

Electric charge is denoted by the letters q or Q.

Just as in mechanics the concept of a material point is often used, which makes it possible to significantly simplify the solution of many problems, when studying the interaction of charges, the concept of a point charge turns out to be effective. A point charge is a charged body whose dimensions are much smaller than the distance from this body to the point of observation and other charged bodies. In particular, if we talk about the interaction of two point charges, then we thereby assume that the distance between the two charged bodies under consideration is much greater than their linear dimensions.

Electric charge of an elementary particle

The electric charge of an elementary particle is not a special “mechanism” in a particle that could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge in an electron and other particles means only the existence of certain interactions between them.

In nature, there are particles with charges of opposite signs. The charge of a proton is called positive, and that of an electron is called negative. The positive sign of the charge of a particle does not mean, of course, that it has special advantages. The introduction of charges of two signs simply expresses the fact that charged particles can both attract and repel. Particles with the same sign of charge repel each other, and with different signs they attract.

There is no explanation of the reasons for the existence of two types of electric charges now. In any case, no fundamental differences between positive and negative charges are found. If the signs of the electric charges of the particles were reversed, then the nature of electromagnetic interactions in nature would not change.

Positive and negative charges are very well compensated in the Universe. And if the Universe is finite, then its total electric charge is, in all probability, equal to zero.

The most remarkable thing is that the electric charge of all elementary particles is strictly the same in absolute value. There is a minimum charge, called elementary, which all charged elementary particles possess. The charge can be positive, like a proton, or negative, like an electron, but the charge modulus is the same in all cases.

It is impossible to separate part of the charge, for example, from an electron. This is perhaps the most amazing thing. No modern theory can explain why the charges of all particles are the same, and cannot calculate the value of the minimum electric charge. It is determined experimentally with the help of various experiments.

In the 1960s, after the number of newly discovered elementary particles began to grow menacingly, a hypothesis was put forward that all strongly interacting particles are composite. The more fundamental particles were called quarks. It turned out to be striking that quarks should have a fractional electric charge: 1/3 and 2/3 of the elementary charge. To construct protons and neutrons, two kinds of quarks are sufficient. And their maximum number, apparently, does not exceed six.

Unit of electric charge