Until relatively recently, several hundred particles and antiparticles were considered elementary. A detailed study of their properties and interactions with other particles and the development of the theory showed that most of them are not actually elementary, since they themselves consist of the simplest or, as they say now, fundamental particles. Fundamental particles themselves are no longer composed of anything. Numerous experiments have shown that all fundamental particles behave like dimensionless point objects that do not have an internal structure, at least up to the smallest distances studied now, ~ 10 -16 cm.

Among the countless and diverse processes of interaction between particles, there are four basic or fundamental interactions: strong (nuclear), electromagnetic, weak and gravitational. In the world of particles, the gravitational interaction is very weak, its role is still unclear, and we will not talk about it further.

There are two groups of particles in nature: hadrons, which participate in all fundamental interactions, and leptons, which do not participate only in strong interactions.

According to modern concepts, interactions between particles are carried out through the emission and subsequent absorption of quanta of the corresponding field (strong, weak, electromagnetic) surrounding the particle. Such quanta are gauge bosons which are also fundamental particles. Bosons have their own angular momentum called spin is equal to integer value Planck's constant... The quanta of the field and, accordingly, the carriers of the strong interaction are gluons, denoted by the symbol g (gi), the quanta of the electromagnetic field are the quanta of light well known to us - photons, denoted by (gamma), and the quanta of the weak field and, accordingly, the carriers of weak interactions are W± (double ve) - and Z 0 (zet zero) bosons.

Unlike bosons, all other fundamental particles are fermions, that is, particles with a half-integer spin value equal to h/2.

Table 1 shows the symbols of fundamental fermions - leptons and quarks.

Each particle shown in table. 1, corresponds to an antiparticle, which differs from a particle only in the signs of the electric charge and other quantum numbers (see Table 2) and the direction of the spin relative to the direction of the particle momentum. We will denote antiparticles by the same symbols as particles, but with a wavy line above the symbol.

Particles in the table. 1 are designated by Greek and Latin letters, namely: letter (nu) - three different neutrinos, letters e - electron, (mu) - muon, (tau) - taon, letters u, c, t, d, s, b denote quarks ; their names and characteristics are given in table. 2.

Particles in the table. 1 are grouped into three generations I, II and III in accordance with the structure of modern theory. Our Universe is built of first generation particles - leptons and quarks and gauge bosons, but, as the modern science of the development of the Universe shows, at the initial stage of its development, particles of all three generations played an important role.

Leptons

Let us first consider in more detail the properties of leptons. The top line of the table. 1 contains three different neutrinos: electron, muonic and tau neutrinos. Their mass has not yet been accurately measured, but its upper limit has been determined, for example, for ne equal to 10 -5 of the value of the electron mass (that is, g).

Looking at the table. 1 involuntarily the question arises as to why nature needed the creation of three different neutrinos. There is no answer to this question yet, because such a comprehensive theory of fundamental particles has not been created that would indicate the necessity and sufficiency of all such particles and would describe their basic properties. Perhaps this problem will be resolved in the 21st century (or later).

The bottom line of the table. 1 begins with the particle we have most studied, the electron. The electron was discovered at the end of the last century by the English physicist J. Thomson. The role of electrons in our world is enormous. They are those negatively charged particles that, together with atomic nuclei, form all the atoms of the elements we know. Periodic table of Mendeleev... In each atom, the number of electrons is exactly equal to the number of protons in the atomic nucleus, which makes the atom electrically neutral.

The electron is stable, the main possibility of annihilation of an electron is its death upon collision with an antiparticle - the positron e +. This process was named annihilation :

As a result of annihilation, two gamma quanta are formed (this is how high-energy photons are called), which carry away both the rest energies e + and e - and their kinetic energies. At high energies e + and e - hadrons and quark pairs are formed (see, for example, (5) and Fig. 4).

Reaction (1) clearly illustrates the validity of the famous formula of A. Einstein about the equivalence of mass and energy: E = mc 2 .

Indeed, during the annihilation of a positron and an electron at rest in the substance, the entire mass of their rest (equal to 1.22 MeV) is converted into the energy of quanta that have no rest mass.

In the second generation, the bottom line of the table. 1 located muon- a particle that, in all its properties, is analogous to an electron, but with an abnormally large mass. The mass of a muon is 207 times the mass of an electron. Unlike an electron, a muon is unstable. The time of his life t= 2.2 · 10 -6 s. A muon predominantly decays into an electron and two neutrinos according to the scheme

An even heavier analogue of the electron is. Its mass is more than 3 thousand times the mass of an electron (MeV / s 2), that is, taon is heavier than a proton and a neutron. Its lifetime is 2.9 · 10 -13 s, and of more than one hundred different schemes (channels) of its decay, the following are possible.

Presented in Fig. 1 fundamental fermions with spin ½ represent the "first bricks" of matter. They are presented leptons(electrons e, neutrinos, etc.) - particles not participating in strong nuclear interactions, and quarks that are involved in strong interactions. Nuclear particles are made of quarks - hadrons(protons, neutrons and mesons). Each of these particles has its own antiparticle, which must be placed in the same cell. Antiparticle designation is distinguished by a tilde (~) symbol.

Of six varieties of quarks or six aromas electric charge 2/3 (in units of elementary charge e) possess the upper ( u), enchanted by ( c) and true ( t) quarks, and the charge –1/3 - the lower ( d), strange ( s) and beautiful ( b) quarks. Antiquarks with the same flavors will have electric charges of –2/3 and 1/3, respectively.

In quantum chromodynamics (the theory of strong interaction), strong interaction charges of three types are attributed to quarks and antiquarks: red R(anti-red); green G(anti-green); blue B(anti-blue). Color (strong) interaction binds quarks in hadrons. The latter are divided into baryons consisting of three quarks, and mesons consisting of two quarks. For example, baryon protons and neutrons have the following quark composition:

p = (uud) and , n = (ddu) and .

As an example, we present the composition of the triplet of pi-mesons:

, ,

It is easy to see from these formulas that the charge of a proton is +1, while for an antiproton it is –1. Neutron and antineutron have zero charge. The quark spins in these particles add up so that their total spins are equal to ½. Such combinations of the same quarks are also possible, for which the total spins are 3/2. Such elementary particles (D ++, D +, D 0, D -) have been found and belong to resonances, i.e. short-lived hadrons.

The well-known process of radioactive b-decay, which is represented by the diagram

n ® p + e + ,

from the point of view of quark theory looks like

(udd) ® ( uud) + e+ or d ® u + e + .

Despite repeated attempts to find free quarks in experiments, it was not possible. This suggests that quarks, most likely, manifest themselves only in the composition of more complex particles ( quark trapping). To date, a complete explanation of this phenomenon has not been given.

Figure 1 shows that there is a symmetry between leptons and quarks, called quark-lepton symmetry. The particles of the top line have a charge one more than the particles of the bottom line. The particles in the first column belong to the first generation, the second to the second generation, and the third column to the third generation. The quarks themselves c, b and t were predicted based on this symmetry. The matter around us is made up of first-generation particles. What is the role of second and third generation particles? There is no definitive answer to this question yet.

These three particles (as well as others described below) are mutually attracted and repelled, respectively. charges, of which there are only four types according to the number of fundamental forces of nature. The charges can be arranged in decreasing order of the corresponding forces as follows: color charge (forces of interaction between quarks); electric charge (electric and magnetic forces); weak charge (forces in some radioactive processes); finally, mass (gravitational forces, 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 equal to, say, 342 units, then after the interaction, regardless of its result, it will be equal to 342 units. This also applies to other charges: color (strong interaction charge), weak and mass (mass). Particles differ in their charges: in essence, they "are" these charges. The charges are like a "certificate" about the right to respond to the corresponding force. So, only colored particles are affected by color forces, only electrically charged particles are affected by electric forces, etc. The properties of a particle are determined by the greatest force acting on it. Only quarks are carriers of all charges and, therefore, are subject to the action of all forces, among which the dominant one is color. Electrons have all charges except color, and the dominant force for them is electromagnetic force.

The most stable in nature are, as a rule, neutral combinations of particles, in which the charge of particles of one sign is compensated by the total charge of particles of the other sign. This corresponds to the minimum energy of the entire system. (Likewise, two bar magnets line up with the north pole of one facing the south pole of the other, which corresponds to the minimum energy of the magnetic field.) Gravity is an exception to this rule: there is no negative mass. There are no bodies falling upwards.

TYPES OF MATTER

Ordinary matter is formed from electrons and quarks, grouping 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 very term "color" taken from optics: the three primary colors give white when mixed.) Thus, quarks, for which the color force is the main one, form triplets. But quarks, and they are subdivided into u-quarks (from the English up - upper) and d-quarks (from the English down - lower), also have an electric charge equal to u-quark and for d-quark. Two u-quark and one d-quark give an electric charge of +1 and form a proton, and one u-quark and two d-quarks give zero electric charge and form a neutron.

Stable protons and neutrons, attracted to each other by the residual color forces of interaction between their constituent quarks, form a color-neutral atomic nucleus. But the nuclei carry a positive electric charge and, attracting negative electrons that revolve around the nucleus like planets revolving around the Sun, tend to form a neutral atom. The electrons in their orbits are removed from the nucleus at distances tens of thousands of times greater than the radius of the nucleus, which is evidence that the electrical forces holding them are much weaker than nuclear forces. Thanks to the power of color interaction, 99.945% of the mass of an atom is contained in its nucleus. Weight u- and d-quarks are about 600 times the mass of an electron. Therefore, electrons are much lighter and more mobile than nuclei. Electrical phenomena are caused by their movement in matter.

There are several hundred natural varieties of atoms (including isotopes), differing in the number of neutrons and protons in the nucleus and, accordingly, in the number of electrons in orbits. The simplest is the hydrogen atom, which consists of a nucleus in the form of a proton and a single electron revolving around it. All "visible" matter in nature consists of atoms and partially "disassembled" atoms, which are called ions. Ions are atoms that, having lost (or gained) several electrons, become charged particles. Matter consisting of almost all ions is called plasma. Stars burning due to thermonuclear reactions going on in the centers consist mainly of plasma, and since stars are the most widespread form of matter in the Universe, we can say that the entire Universe consists mainly of plasma. More precisely, stars are predominantly fully ionized hydrogen gas, i.e. a mixture of individual protons and electrons, and therefore, almost the entire visible Universe consists of it.

This is visible matter. But there is 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.

The invisible world.

The universe contains more than just visible matter (but also black holes and "dark matter" such as cold planets that become visible when illuminated). There is also truly invisible matter that permeates all of us and the entire Universe every second. It is a rapidly 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, since they have kinetic energy E, which corresponds to the effective mass m, according to Einstein's formula E = mc 2, where c Is the speed of light.

The key role of the neutrino is that it facilitates the transformation and-quarks in d-quarks, as a result of which the proton turns into a neutron. The neutrino acts as a "carburetor needle" for stellar thermonuclear reactions, in which four protons (hydrogen nuclei) combine to form a helium nucleus. But since the helium nucleus does not consist of four protons, but of two protons and two neutrons, for such a nuclear fusion it is necessary that two and-quark turned into two d-quark. The intensity of the transformation depends on 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 just colors) of two quarks in this process are compensated without neutrinos. The role of neutrinos is to carry away an 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.

And 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).

Carriers of interactions.

What causes the forces acting between particles at a distance? Modern physics is responsible: through the exchange of other particles. Imagine two skaters throwing a ball. By imparting momentum to the ball when thrown and receiving momentum with the received ball, both receive a push in a direction away from each other. This is how the emergence of repulsive forces can be explained. But in quantum mechanics, which considers phenomena in the microworld, unusual stretching and delocalization of events is allowed, which leads to the seemingly impossible: one of the skaters throws the ball in the direction from another, but one nevertheless maybe catch this ball. It is not hard to figure out that if this were possible (and in the world of elementary particles it is possible), an attraction would arise between the skaters.

The particles, due to the exchange of which the forces of interaction 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 particles-carriers of strong interaction are gluons (there are eight of them). 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 should be one). All these particles, except for the photon and graviton, which can cover infinitely large 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 a corresponding 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 a 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.

An antiparticle corresponds to each particle, with which the particle can mutually annihilate, i.e. "Annihilate", resulting in the release of energy. "Pure" energy by itself, however, does not exist; as a result of annihilation, new particles (for example, photons) appear that carry this energy away.

An antiparticle in most cases has properties opposite to the corresponding particle: if a particle moves to the left under the influence of a strong, weak or electromagnetic field, then its antiparticle will move to the right. In short, an antiparticle has opposite signs of all charges (except for the mass charge). If a particle is composite, such as a neutron, then its antiparticle consists of components with opposite charge signs. So, an antielectron has an electric charge of +1, a weak charge of +1/2 and is called a positron. Antineutron consists of and-antiquarks with an electric charge of –2/3 and d- antiquarks with an electric charge of +1/3. Truly neutral particles are their own antiparticles: the antiparticle of a photon is a photon.

According to modern theoretical concepts, each particle existing in nature must have its own antiparticle. And many antiparticles, including positrons and antineutrons, were actually produced in the laboratory. The consequences of this are extremely important and underlie all 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 the charge is conserved, and the charge of the vacuum (empty space) is zero, from the vacuum, like rabbits from a magician's hat, any pair of particles and antiparticles (with zero total charge) can arise, as long as the energy is sufficient to create their mass.

Generations of particles.

Experiments at accelerators have shown that the four (quartet) of material particles is repeated at least twice at higher values ​​of mass. In the second generation, the place of the electron is taken by the muon (with a mass about 200 times the mass of the electron, but with the same values ​​of all other charges), the place of the electron neutrino is muon (which accompanies the muon in weak interactions in the same way as the electron is accompanied by the electron neutrino), a place and-quark takes with-quark ( charmed), a d-quark - s-quark ( strange). In the third generation, the quartet consists of tau lepton, tau neutrino, t-quark and b-quark.

Weight t- the quark is about 500 times the mass of the lightest - d-quark. It has been experimentally established that there are only three types of light neutrinos. Thus, the fourth generation of particles either does not exist at all, or the corresponding neutrinos are very heavy. This is consistent with cosmological data, according to which no more than four types of light neutrinos can exist.

In experiments with high-energy particles, the electron, muon, tau lepton and corresponding neutrinos act as separate particles. They do not carry a color charge and only enter into weak and electromagnetic interactions. Collectively, they are called leptons.

On the other hand, quarks, under the influence of color forces, combine into strongly interacting particles, which prevail in most experiments in high-energy physics. Such particles are called hadrons... They include two subclasses: baryons(for example, a proton and a neutron), which are composed 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-psi particle ( J/y-meson), discovered simultaneously in Brookhaven and at the Stanford Linear Accelerator Center (also in the USA) in 1974. The existence of the omega-minus particle was predicted by M. Gell-Mann in his so-called “ SU 3 -theory "(another name -" eightfold path "), in which the first assumption was made about the possibility of the existence of quarks (and this name was given to them). A Decade After Particle Discovery 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 generation are no less real than the first. True, having arisen, in a millionth or billionth of a second, they decay into ordinary particles of the first generation: an electron, an electron neutrino, and 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, of course, is somewhat eccentric) as different "flavors" of particles. The need to explain them is called the "flavor" problem.

BOSONS AND FERMIONES, 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. Identical bosons can superimpose or overlap, but identical fermions cannot. Superposition occurs (or does not occur) in discrete energy states into which quantum mechanics divides nature. These states are, as it were, separate cells in which particles can be placed. So, in one cell you can put as many identical bosons as you like, 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, an electron, according to the laws of quantum mechanics, cannot revolve in any elliptical orbit, for it there is only a discrete series of allowed "states of motion". Sets of such states, grouped according to the distance from the electron to the nucleus, are called orbitals... In the first orbital, there are two states with different angular momenta and, therefore, two allowed cells, and in higher orbitals, there are eight or more cells.

Since the electron is a fermion, there can be only one electron in each cell. Very important consequences follow from this - all chemistry, since the chemical properties of substances are determined by interactions between the corresponding atoms. If we go through the periodic table of elements from one atom to another in the order of increasing by one the number of protons in the nucleus (the number of electrons will also increase accordingly), then the first two electrons will occupy the first orbital, the next eight will be located on the second, etc. This successive change in the electronic structure of atoms from element to element is responsible for 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. Moreover, 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 neutrinos - are fermions. The same can be said about quarks. Thus, all particles that form "matter", the main filler of the Universe, as well as invisible neutrinos, are fermions. This is very significant: fermions cannot be combined, so the same applies to objects in the material world.

At the same time, all the "gauge particles" that are exchanged between interacting material particles and which create a field of forces ( see above) are bosons, which is also very important. So, for example, many photons can be in one 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 associated with another characteristic of elementary particles - spin... Surprisingly, but all fundamental particles have their own angular momentum, or, more simply, rotate around their axis. Moment of impulse is a characteristic of rotary motion, as well as total impulse - translational. In any interactions, 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 spin equal to 1/2, and gauge particles have spin equal to 1 (except for the graviton, which has not yet been observed experimentally, but should theoretically have 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 a whole, then a boson.

SIZING THEORY AND GEOMETRY

In all cases, the forces arise from the exchange of bosons between fermions. So, the color force of interaction between two quarks (quarks - fermions) arises due to the exchange of gluons. This exchange is constantly taking place in protons, neutrons and atomic nuclei. Likewise, the photons exchanged between electrons and quarks create electrical attractive forces that hold electrons in the atom, and the intermediate vector bosons that leptons and quarks exchange create weak forces responsible for the conversion of protons into neutrons in 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 similar, although somewhat different, gauge theory of gravity. 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 they would all become different aspects of a single reality - like the facet of a crystal.

The simplest and oldest gauge theory is the gauge theory of electromagnetic interaction. In it, the charge of an electron is compared (calibrated) with the charge of another electron, far 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 to the far electron and see how it reacts. The signal is a calibration particle - a photon. In order to be able to check the charge on distant particles, a photon is needed.

Mathematically, this theory is extremely accurate and beautiful. The whole of quantum electrodynamics (the quantum theory of electromagnetism), as well as Maxwell's theory of the electromagnetic field, one of the greatest scientific achievements of the 19th century, follows from the "gauge principle" described above.

Why is such a simple principle so fruitful? Apparently, it expresses a certain correlation between different parts of the Universe, allowing measurements to be taken in the Universe. Mathematically, the field is interpreted geometrically as the curvature of some conceivable "internal" space. Measurement of charge, however, is a measurement of the total "internal curvature" around a particle. Gauge theories of strong and weak interactions differ from electromagnetic gauge theory only by the internal geometrical "structure" of the corresponding charge. The question of where exactly this internal space is located is being answered by multidimensional unified field theories, which are not considered here.

Particle physics has not yet been completed. It is still far from clear whether the available data are sufficient for a complete understanding of the nature of particles and forces, as well as the true nature and dimension of space and time. Do we need experiments with energies of 10 15 GeV for this, or will the efforts of thought be enough? There is no answer yet. But we can say with confidence that the final picture will be simple, graceful and beautiful. It is possible that there will not be so many fundamental ideas: the gauge principle, spaces of higher dimensions, collapse and expansion, and above all, geometry.

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Microcosm structures

Previously, elementary particles were called particles that are part of an atom and cannot be decomposed into more elementary components, namely electrons and nuclei.

Later it was found that the nuclei are composed of simpler particles - nucleons(protons and neutrons), which in turn are composed of other particles. That's why elementary particles began to be considered the smallest particles of matter , excluding atoms and their nuclei .

To date, hundreds of elementary particles have been discovered, which requires their classification:

- by types of interactions

- by the times of life

- the largest back

Elementary particles are divided into the following groups:

Composite and fundamental (structureless) particles

Compound particles

Hadrons (heavy)- particles participating in all kinds of fundamental interactions. They consist of quarks and are subdivided, in turn, into: mesons- hadrons with integer spin, that is, they are bosons; baryons- hadrons with half-integer spin, that is, fermions. These include, in particular, the particles that make up the nucleus of the atom - the proton and the neutron, i.e. nucleons.

Fundamental (structureless) particles

Leptons (lungs)- fermions, which have the form of point particles (ie, not consisting of anything) up to scales of the order of 10 - 18 m. They do not participate in strong interactions. Participation in electromagnetic interactions was experimentally observed only for charged leptons (electrons, muons, tau leptons) and was not observed for neutrinos.

Quarks- fractionally charged particles that make up hadrons. Not observed in a free state.

Gauge bosons- particles through the exchange of which interactions are carried out:

- photon - a particle that carries electromagnetic interaction;

- eight gluons - particles that carry strong interactions;

- three intermediate vector bosons W + , W- and Z 0, carrying weak interaction;

- graviton is a hypothetical particle that carries gravitational interaction. The existence of gravitons, although not yet proven experimentally due to the weakness of the gravitational interaction, is considered quite probable; however, the graviton is not part of the Standard Model of elementary particles.

According to modern concepts, fundamental particles (or "truly" elementary particles) that do not have an internal structure and finite size include:

Quarks and leptons

Particles providing fundamental interactions: gravitons, photons, vector bosons, gluons.

Classification of elementary particles by lifetimes:

- stable: particles whose lifetime is very long (tends to infinity in the limit). These include electrons , protons , neutrino ... Neutrons are also stable inside nuclei, but they are unstable outside the nucleus.

- unstable (quasi-stable): elementary particles are those particles that decay due to electromagnetic and weak interactions, and the lifetime of which is more than 10–20 sec. These particles include free neutron (i.e. a neutron outside the nucleus of an atom)

- resonances (unstable, short-lived). Resonances include elementary particles that decay due to strong interaction. The life time for them is less than 10 -20 sec.

Particle classification by participation in interactions:

- leptons : these include neutrons. All of them do not participate in the vortex of intranuclear interactions, i.e. not subject to strong interaction. They participate in weak interaction, and having an electric charge, they also participate in electromagnetic interaction.

- hadrons : particles that exist inside an atomic nucleus and participate in strong interactions. The most famous of them are proton and neutron .

Known for today six leptons :

The same family with an electron includes muons and tau particles, which are similar to an electron, but more massive than it. Muons and tau particles are unstable and decay over time into several other particles, including an electron

Three electrically neutral particles with zero (or close to zero, scientists have not yet decided on this) mass, called neutrino ... Each of the three neutrinos (electron neutrino, muonic neutrino, tau neutrino) is paired with one of the three types of particles of the electronic family.

The most famous hadrons , protons and neutrinos, there are hundreds of relatives, which are born in great numbers and immediately decay in the process of various nuclear reactions. With the exception of the proton, they are all unstable, and they can be classified according to the composition of the particles into which they decay:

If there is a proton among the final products of particle decay, then it is called baryon

If there is no proton among the decay products, then the particle is called meson .

The confused picture of the subatomic world, which became more complicated with the discovery of each new hadron, gave way to a new picture, with the emergence of the concept of quarks. According to the quark model, all hadrons (but not leptons) are composed of even more elementary particles - quarks. So baryons (in particular the proton) consist of three quarks, and mesons - from a pair of quark - antiquark.