An elementary particle is the smallest, indivisible, structureless particle. An elementary particle that has no charge
Can you briefly and succinctly answer the question: “What is an electric charge?” This may seem simple at first glance, but in reality it turns out to be much more complicated.
Do we know what electric charge is?
The fact is that at the current level of knowledge we cannot yet decompose the concept of “charge” into simpler components. This is a fundamental, so to speak, primary concept.
We know that this is a certain property of elementary particles, the mechanism of interaction of charges is known, we can measure the charge and use its properties.
However, all this is a consequence of data obtained experimentally. The nature of this phenomenon is still not clear to us. Therefore, we cannot unambiguously determine what an electric charge is.
To do this, it is necessary to unpack a whole range of concepts. Explain the mechanism of interaction of charges and describe their properties. Therefore, it is easier to understand what the statement means: “this particle has (carries) an electric charge.”
The presence of an electric charge on a particle
However, later it was possible to establish that the number of elementary particles is much greater, and that the proton, electron and neutron are not indivisible and fundamental building materials of the Universe. They themselves can decompose into components and turn into other types of particles.
Therefore, the name "elementary particle" currently includes a fairly large class of particles smaller in size than atoms and atomic nuclei. In this case, particles can have a variety of properties and qualities.
However, such a property as electric charge comes in only two types, which are conventionally called positive and negative. The presence of a charge on a particle is its ability to repel or be attracted to another particle, which also carries a charge. The direction of interaction depends on the type of charges.
Like charges repel, unlike charges attract. Moreover, the force of interaction between charges is very large in comparison with the gravitational forces inherent in all bodies in the Universe without exception.
In the hydrogen nucleus, for example, an electron carrying a negative charge is attracted to a nucleus consisting of a proton and carrying a positive charge with a force 1039 times greater than the force with which the same electron is attracted by a proton due to gravitational interaction.
Particles may or may not carry a charge, depending on the type of particle. However, it is impossible to “remove” the charge from the particle, just as the existence of a charge outside the particle is impossible.
In addition to the proton and neutron, some other types of elementary particles carry a charge, but only these two particles can exist indefinitely.
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It is impossible to give a brief 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 by speed, etc. But the most basic, fundamental concepts, indivisible into simpler ones, devoid, according to science today, of any internal mechanism, can no longer be briefly explained in a satisfactory manner. Especially if objects are not directly perceived by our senses. It is these fundamental concepts that electric charge refers to.
Let us first try to find out not what an electric charge is, but what is hidden behind the statement given body or particle have an electrical charge.
You know that all bodies are built from tiny particles, indivisible into simpler (as far as science now knows) 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 in inverse proportion to the square of the distance, but this force is a huge number of times greater than the force of gravity. Thus, in the hydrogen atom, schematically shown 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 increasing distance and are many times greater than the forces of gravity, then these particles are said to have an electric charge. The particles themselves are called charged. There are particles without an 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 lack of 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 (after mass) most important characteristic elementary particles, which determines their behavior in the surrounding world.
Thus
Electric charge is a physical scalar quantity that characterizes the property of particles or bodies to enter into electromagnetic force interactions.
Electric charge is symbolized 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 is effective. A point charge is a charged body whose dimensions are significantly less than the distance from this body to the point of observation and other charged bodies. In particular, if they talk about the interaction of two point charges, they thereby assume that the distance between the two charged bodies under consideration is significantly greater than their linear dimensions.
Electric charge of an elementary particle
The electric charge of an elementary particle is not a special “mechanism” in the particle that could be removed from it, decomposed into its component parts and reassembled. The presence of an electric charge on an electron and other particles only means 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 the charge of an electron is called negative. The positive sign of a charge on a particle does not mean, of course, that it has any special advantages. The introduction of charges of two signs simply expresses the fact that charged particles can both attract and repel. If the charge signs are the same, the particles repel, and if the charge signs are different, they attract.
There is currently no explanation for the reasons for the existence of two types of electric charges. In any case, no fundamental differences are found between positive and negative charges. If the signs of the electric charges of particles changed to the opposite, then the nature of electromagnetic interactions in nature would not change.
Positive and negative charges are very well balanced in the Universe. And if the Universe is finite, then its total electric charge is, in all likelihood, equal to zero.
The most remarkable thing is that the electric charge of all elementary particles is strictly the same in magnitude. There is a minimum charge, called elementary, that 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 surprising thing. None modern theory cannot explain why the charges of all particles are the same, and is not able to calculate the value of the minimum electric charge. It is determined experimentally using various experiments.
In the 1960s, after the number of newly discovered elementary particles began to grow alarmingly, it was hypothesized that all strongly interacting particles are composite. More fundamental particles were called quarks. What was striking was that quarks should have a fractional electric charge: 1/3 and 2/3 of the elementary charge. To build protons and neutrons, two types of quarks are enough. And their maximum number, apparently, does not exceed six.
Unit of measurement of electric charge
In the Universe, each body lives in its own time, and so do the basic elementary particles. The lifetime of most elementary particles is quite short.
Some disintegrate immediately after their birth, which is why we call them unstable particles.
They're through a short time decay into stable ones: protons, electrons, neutrinos, photons, gravitons and their antiparticles.
The most important microobjects in our nearby space - protons and electrons. Some of the distant parts of the Universe may consist of antimatter; the most important particles there will be the antiproton and 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 three types(electronic v e, muonic v m, with lepton v t), etc. Obviously they will bring more new microparticles.
Particle appearance:
Protons and electrons
The appearance of protons and electrons dates back to time, and their age is approximately ten billion years.
Another type of micro-objects that play a significant role in the structure of nearby space is neutrons, which have a common name with the proton: nucleons. Neutrons themselves are unstable; they decay about ten minutes after they are produced. They can only be stable in the nucleus of an atom. A huge number of neutrons constantly appear in the depths of stars, where atomic nuclei are born from protons.
Neutrino
In the Universe, there is also a constant birth of neutrinos, which are similar to an electron, but without a charge and with low mass. In 1936, a type of neutrino 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 creation of a huge number of neutrinos and muon neutrinos. Their number in space is constantly increasing because they are not absorbed by practically any matter.
Photons
Like photons, neutrinos and muon neutrinos fill all space. This phenomenon is called the “neutrino sea.”
From the time of the Big Bang, a great many photons remained, which we call relict or fossil. All outer space is filled with them, and their frequency, and therefore energy, is constantly decreasing as the Universe expands.
Currently, all cosmic bodies, primarily stars and nebulae, participate 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
IN initial stage formation of the Universe, all basic elementary particles were free. Then there were no atomic nuclei, no planets, no stars.
Atoms, and from them planets, stars and all substances were formed later, when 300,000 years passed and hot matter expanded into sufficiently cooled down. 
Only the neutrino, muon neutrino and photon did not enter any system: their mutual attraction is too weak. They remained free particles.
More on initial stage During 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 electrical 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). Protons that ended up in a massive star gradually turned into the main building blocks of the Universe. Each of them released one percent of its rest mass. In supermassive stars, which at the end of their lives are compressed into small volumes as a result of their own gravity, the proton can lose almost a fifth of its rest energy (and therefore 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, the graviton, which carries a gravitational interaction similar to electromagnetism. However, the presence of graviton has been proven only theoretically.
Thus, the basic 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) are mutually attracted and repelled according to their charges, of which there are only four types according to the number of fundamental forces of nature. The charges can be arranged in decreasing order of the corresponding forces as follows: color charge (forces of interaction between quarks); electric charge (electric and magnetic forces); weak charge (forces in some radioactive processes); finally, mass (gravitational force, or gravitational interaction). The word "color" here has nothing to do with color visible light; it is simply a characteristic of a strong charge and the greatest forces.
Charges are saved, i.e. the charge entering the system is equal to the charge leaving it. If the total electric charge of a certain number of particles before their interaction is equal to, say, 342 units, then after the interaction, regardless of its result, it will be equal to 342 units. This also applies to other charges: color (strong interaction charge), weak and mass (mass). Particles differ in their charges: in essence, they “are” these charges. Charges are like a “certificate” of the right to respond to the appropriate force. Thus, only colored particles are affected by color forces, only electrically charged particles are affected by electric forces, etc. The properties of a particle are determined greatest strength, acting on it. Only quarks are carriers of all charges and, therefore, are subject to the action of all forces, among which the dominant one is color. Electrons have all charges except color, and the dominant force for them is the electromagnetic force.
The most stable in nature are, as a rule, neutral combinations of particles in which the charge of particles of one sign is compensated by the total charge of particles of the other sign. This corresponds to the minimum energy of the entire system. (In the same way, two bar magnets are arranged in a line, with the north pole of one facing the south pole of the other, which corresponds to the minimum energy of the magnetic field.) Gravity is an exception to this rule: negative mass does not exist. There are no bodies that fall upward.
TYPES OF MATTER
Ordinary matter is formed from electrons and quarks, grouped into objects that are neutral in color and then in electrical charge. The color power is neutralized, as will be discussed in more detail below, when the particles are combined into triplets. (Hence the term “color” itself, taken from optics: three primary colors when mixed produce white.) Thus, quarks for which the color strength is the main one form triplets. But quarks, and they are divided into u-quarks (from the English up - top) and d-quarks (from the English down - bottom), also have an electric charge equal to u-quark and for d-quark. Two u-quark and one d-quarks give an electric charge of +1 and form a proton, and one u-quark and two d-quarks give zero electric charge and form a neutron.
Stable protons and neutrons, attracted to each other by the residual color forces of interaction between their constituent quarks, form a color-neutral atomic nucleus. But nuclei carry a positive electrical charge and, attracting negative electrons that orbit around the nucleus like planets orbiting the Sun, tend to form a neutral atom. Electrons in their orbits are removed from the nucleus at distances tens of thousands of times greater than the radius of the nucleus - evidence that the electrical forces holding them are much weaker than nuclear ones. Thanks to the power of color interaction, 99.945% of an atom's mass is contained in its nucleus. Weight u- And d-quarks are about 600 times the mass of an electron. Therefore, electrons are much lighter and more mobile than nuclei. Their movement in matter is caused by electrical phenomena.
There are several hundred natural varieties of atoms (including isotopes), differing in the number of neutrons and protons in the nucleus and, accordingly, in the number of electrons in their orbits. The simplest is the hydrogen atom, consisting of a nucleus in the form of a proton and a single electron revolving around it. All “visible” matter in nature consists of atoms and partially “disassembled” atoms, which are called ions. Ions are atoms that, having lost (or gained) several electrons, have become charged particles. Matter consisting almost entirely of ions is called plasma. Stars that burn due to thermonuclear reactions occurring in the centers consist mainly of plasma, and since stars are the most common form of matter in the Universe, we can say that the entire Universe consists mainly of plasma. More precisely, stars are predominantly fully ionized hydrogen gas, i.e. a mixture of individual protons and electrons, and therefore, almost the entire visible Universe consists of it.
This is visible matter. But there is also invisible matter in the Universe. And there are particles that act as force carriers. There are antiparticles and excited states of some particles. All this leads to a clearly excessive abundance of “elementary” particles. In this abundance one can find an indication of the actual, true nature of elementary particles and the forces acting between them. According to the most recent theories, particles may be essentially extended geometric objects - “strings” in ten-dimensional space.
The invisible world.
The Universe contains not only visible matter (but also black holes and “ dark matter", such as cold planets that become visible if illuminated). There is also truly invisible matter that permeates all of us and the entire Universe every second. It is a fast-moving gas of particles of one type - electron neutrinos.
An electron neutrino is a partner of an electron, but has no electrical charge. Neutrinos carry only a so-called weak charge. Their rest mass is, in all likelihood, zero. But they interact with the gravitational field because they have kinetic energy E, which corresponds to the effective mass m, according to Einstein's formula E = mc 2 where c– speed of light.
The key role of the neutrino is that it contributes to the transformation And-quarks in d-quarks, as a result of which a proton turns into a neutron. Neutrinos act as the "carburetor needle" for stellar fusion reactions, in which four protons (hydrogen nuclei) combine to form a helium nucleus. But since the helium nucleus does not consist of four protons, but of two protons and two neutrons, for such nuclear fusion it is necessary that two And-quarks turned into two d-quark. The intensity of the transformation determines how quickly the stars will burn. And the transformation process is determined by weak charges and weak interaction forces between particles. Wherein And-quark (electric charge +2/3, weak charge +1/2), interacting with an electron (electric charge - 1, weak charge –1/2), forms d-quark (electric charge –1/3, weak charge –1/2) and electron neutrino (electric charge 0, weak charge +1/2). The color charges (or just colors) of the two quarks cancel out in this process without the neutrino. The role of the neutrino is to carry away the uncompensated weak charge. Therefore, the rate of transformation depends on how weak the weak forces are. If they were weaker than they are, the stars would not burn at all. If they were stronger, the stars would have burned out long ago.
What about neutrinos? Because these particles interact extremely weakly with other matter, they almost immediately leave the stars in which they were born. All stars shine, emitting neutrinos, and neutrinos shine through our bodies and the entire Earth day and night. So they wander around the Universe until they enter, perhaps, into a new interaction STAR).
Carriers of interactions.
What causes forces acting between particles at a distance? Modern physics answers: due to the exchange of other particles. Imagine two speed skaters throwing a ball around. By imparting momentum to the ball when thrown and receiving momentum with the received ball, both receive a push in a direction away from each other. This can explain the emergence of repulsive forces. But in quantum mechanics, which considers phenomena in the microworld, unusual stretching and delocalization of events are allowed, which leads to the seemingly impossible: one of the skaters throws the ball in the direction from different, but that one nonetheless Maybe catch this ball. It is not difficult to imagine that if this were possible (and in the world of elementary particles it is possible), attraction would arise between the skaters.
The particles, due to the exchange of which the interaction forces between the four “particles of matter” discussed above, are called gauge particles. Each of the four interactions – strong, electromagnetic, weak and gravitational – has its own set of gauge particles. The carrier particles of the strong interaction are gluons (there are only eight of them). A photon is a carrier of electromagnetic interaction (there is only one, and we perceive photons as light). The carrier particles of the weak interaction are intermediate vector bosons (they were discovered in 1983 and 1984 W + -, W- - bosons and neutral Z-boson). The carrier particle of gravitational interaction is the still hypothetical graviton (there should be only one). All these particles, except for the photon and graviton, which can travel infinitely long distances, exist only in the process of exchange between material particles. Photons fill the Universe with light, and gravitons fill the Universe with gravitational waves (not yet reliably detected).
A particle capable of emitting gauge particles is said to be surrounded by a corresponding field of forces. Thus, electrons capable of emitting photons are surrounded by electrical and magnetic fields, as well as weak and gravitational fields. Quarks are also surrounded by all these fields, but also by the strong interaction field. Particles with a color charge in the field of color forces are affected by the color force. The same applies to other forces of nature. Therefore, we can say that the world consists of matter (material particles) and field (gauge particles). More about this below.
Antimatter.
Each particle has an antiparticle, with which the particle can mutually annihilate, i.e. "annihilate", resulting in the release of energy. “Pure” energy in itself, however, does not exist; As a result of annihilation, new particles (for example, photons) appear that carry away this energy.
In most cases, an antiparticle has properties opposite to the corresponding particle: if a particle moves to the left under the influence of strong, weak or electromagnetic fields, then its antiparticle will move to the right. In short, the antiparticle has opposite signs of all charges (except the mass charge). If a particle is composite, such as a neutron, then its antiparticle consists of components with opposite signs of charges. Thus, an antielectron has an electric charge of +1, a weak charge of +1/2 and is called a positron. Antineutron consists of And-antiquarks with electric charge –2/3 and d-antiquarks with electric charge +1/3. True neutral particles are their own antiparticles: the antiparticle of a photon is a photon.
According to modern theoretical concepts, each particle existing in nature should have its own antiparticle. And many antiparticles, including positrons and antineutrons, were indeed obtained in the laboratory. The consequences of this are extremely important and underlie all experimental particle physics. According to the theory of relativity, mass and energy are equivalent, and under certain conditions energy can be converted into mass. Since charge is conserved, and the charge of vacuum (empty space) is zero, any pairs of particles and antiparticles (with zero net charge) can emerge from the vacuum, like rabbits from a magician's hat, as long as there is enough energy to create their mass.
Generations of particles.
Accelerator experiments have shown that the quartet of material particles is repeated at least twice at higher mass values. In the second generation, the place of the electron is taken by the muon (with a mass approximately 200 times greater than the mass of the electron, but with the same values of all other charges), the place of the electron neutrino is taken by the muon (which accompanies the muon in weak interactions in the same way as the electron is accompanied by the electron neutrino), place And-quark occupies With-quark ( charmed), A d-quark – s-quark ( strange). In the third generation, the quartet consists of a tau lepton, a tau neutrino, t-quark and b-quark.
Weight t-a quark is about 500 times the mass of the lightest – d-quark. It has been experimentally established that there are only three types of light neutrinos. Thus, the fourth generation of particles either does not exist at all, or the corresponding neutrinos are very heavy. This is consistent with cosmological data, according to which no more than four types of light neutrinos can exist.
In experiments with high-energy particles, the electron, muon, tau lepton and corresponding neutrinos act as isolated particles. They do not carry a color charge and enter into only weak and electromagnetic interactions. Collectively they are called leptons.
| Table 2. GENERATIONS OF FUNDAMENTAL PARTICLES | ||||
| Particle | Rest mass, MeV/ With 2 | Electric charge | Color charge | Weak charge |
| SECOND GENERATION | ||||
| With-quark | 1500 | +2/3 | Red, green or blue | +1/2 |
| s-quark | 500 | –1/3 | Same | –1/2 |
| Muon neutrino | 0 | 0 | +1/2 | |
| Muon | 106 | 0 | 0 | –1/2 |
| THIRD GENERATION | ||||
| t-quark | 30000–174000 | +2/3 | Red, green or blue | +1/2 |
| b-quark | 4700 | –1/3 | Same | –1/2 |
| Tau neutrino | 0 | 0 | +1/2 | |
| Tau | 1777 | –1 | 0 | –1/2 |
Quarks, under the influence of color forces, combine into strongly interacting particles that dominate most high-energy physics experiments. Such particles are called hadrons. They include two subclasses: baryons(such as a proton and a neutron), which are made up of three quarks, and mesons, consisting of a quark and an antiquark. In 1947, the first meson, called the pion (or pi-meson), was discovered in cosmic rays, and for some time it was believed that the exchange of these particles - main reason nuclear forces. Omega-minus hadrons, discovered in 1964 at Brookhaven National Laboratory (USA), and the JPS particle ( J/y-meson), discovered simultaneously at Brookhaven and at the Stanford Linear Accelerator Center (also in the USA) in 1974. The existence of the omega minus particle was predicted by M. Gell-Mann in his so-called “ S.U. 3 theory" (another name is the "eight-fold path"), in which the possibility of the existence of quarks was first suggested (and this name was given to them). A decade later, the discovery of the particle J/y confirmed the existence With-quark and finally made everyone believe in both the quark model and the theory that united electromagnetic and weak forces ( see below).
Particles of the second and third generation are no less real than the first. True, having arisen, in millionths or billionths of a second they decay into ordinary particles of the first generation: electron, electron neutrino, and also And- And d-quarks. The question of why there are several generations of particles in nature still remains a mystery.
Different generations of quarks and leptons are often spoken of (which, of course, is somewhat eccentric) as different “flavors” of particles. The need to explain them is called the “flavor” problem.
BOSONS AND FERMIONS, FIELD AND MATTER
One of the fundamental differences between particles is the difference between bosons and fermions. All particles are divided into these two main classes. Identical bosons can overlap or overlap, but identical fermions cannot. Superposition occurs (or does not occur) in the discrete energy states into which quantum mechanics divides nature. These states are like separate cells into which particles can be placed. So, you can put as many identical bosons as you like into one cell, but only one fermion.
As an example, consider such cells, or “states,” for an electron orbiting the nucleus of an atom. Unlike planets solar system, the electron, according to the laws of quantum mechanics, cannot circulate in any elliptical orbit; for it there is only a discrete series of allowed “states of motion.” Sets of such states, grouped according to the distance from the electron to the nucleus, are called orbitals. In the first orbital there are two states with different angular momentum and, therefore, two allowed cells, and in higher orbitals there are eight or more cells.
Since the electron is a fermion, each cell can only contain one electron. Very important consequences follow from this - all of chemistry, since the chemical properties of substances are determined by the interactions between the corresponding atoms. If you go along periodic table elements from one atom to another in the order of increasing by one the number of protons in the nucleus (the number of electrons will also increase accordingly), then the first two electrons will occupy the first orbital, the next eight will be located in the second, etc. This consistent change in the electronic structure of atoms from element to element determines the patterns in their chemical properties.
If electrons were bosons, then all the electrons in an atom could occupy the same orbital, corresponding to the minimum energy. In this case, the properties of all matter in the Universe would be completely different, and the Universe in the form in which we know it would be impossible.
All leptons - electron, muon, tau lepton and their corresponding neutrinos - are fermions. The same can be said about quarks. Thus, all particles that form “matter”, the main filler of the Universe, as well as invisible neutrinos, are fermions. This is quite significant: fermions cannot combine, so the same applies to objects in the material world.
At the same time, all the “gauge particles” that are exchanged between interacting material particles and which create a field of forces ( see above), are bosons, which is also very important. So, for example, many photons can be in the same state, forming a magnetic field around a magnet or an electric field around an electric charge. Thanks to this, laser is also possible.
Spin.
The difference between bosons and fermions is associated with another characteristic of elementary particles - spin. Surprisingly, all fundamental particles have their own angular momentum or, more simply put, rotate around their own axis. Angle of impulse is a characteristic of rotational motion, just like the total impulse of translational motion. In any interaction, angular momentum and momentum are conserved.
In the microcosm, the angular momentum is quantized, i.e. takes discrete values. In suitable units of measurement, leptons and quarks have a spin of 1/2, and gauge particles have a spin of 1 (except for the graviton, which has not yet been observed experimentally, but theoretically should have a spin of 2). Since leptons and quarks are fermions, and gauge particles are bosons, we can assume that “fermionicity” is associated with spin 1/2, and “bosonicity” is associated with spin 1 (or 2). Indeed, both experiment and theory confirm that if a particle has a half-integer spin, then it is a fermion, and if it has an integer spin, then it is a boson.
GAUGE THEORIES AND GEOMETRY
In all cases, the forces arise due to the exchange of bosons between fermions. Thus, the color force of interaction between two quarks (quarks - fermions) arises due to the exchange of gluons. A similar exchange occurs constantly in protons, neutrons and atomic nuclei. Similarly, the photons exchanged between electrons and quarks create the electrical attractive forces that hold electrons in the atom, and the intermediate vector bosons exchanged between leptons and quarks create the weak forces responsible for converting protons into neutrons in thermonuclear reactions in stars.
The theory behind this exchange is elegant, simple, and probably correct. It is called gauge theory. But at present there are only independent gauge theories of strong, weak and electromagnetic interactions and a similar, although somewhat different, gauge theory of gravity. One of the most important physical problems is the reduction of these individual theories into a single and at the same time simple theory, in which they would all become different aspects a single reality - like the edges of a crystal.
| Table 3. SOME HADRONS | ||||
| Particle | Symbol | Quark composition * | Rest mass, MeV/ With 2 | Electric charge |
| BARIONS | ||||
| Proton | p | uud | 938 | +1 |
| Neutron | n | udd | 940 | 0 |
| Omega minus | W – | sss | 1672 | –1 |
| MESONS | ||||
| Pi-plus | p + | u | 140 | +1 |
| Pi minus | p – | du | 140 | –1 |
| Fi | f | sє | 1020 | 0 |
| JP | J/y | cў | 3100 | 0 |
| Upsilon | Ў | b | 9460 | 0 |
| * Quark composition: u– top; d– lower; s- strange; c– enchanted; b- Beautiful. Antiques are indicated by a line above the letter. | ||||
The simplest and oldest of the gauge theories is the gauge theory of electromagnetic interaction. In it, the charge of an electron is compared (calibrated) with the charge of another electron distant from it. How can you compare charges? You can, for example, bring the second electron closer to the first and compare their interaction forces. But doesn't the charge of an electron change when it moves to another point in space? The only way checks - send a signal from a near electron to a distant one and see how it reacts. The signal is a gauge particle – a photon. To be able to test the charge on distant particles, a photon is needed.
Mathematically, this theory is extremely accurate and beautiful. From the “gauge principle” described above flows all of quantum electrodynamics (quantum theory of electromagnetism), as well as Maxwell’s theory of the electromagnetic field - one of the greatest scientific achievements 19th century
Why is such a simple principle so fruitful? Apparently, it expresses some kind of correlation different parts Universe, allowing measurements to be made in the Universe. In mathematical terms, the field is interpreted geometrically as the curvature of some conceivable “internal” space. Measuring charge is measuring the total “internal curvature” around the particle. The gauge theories of the strong and weak interactions differ from the electromagnetic gauge theory only in the internal geometric “structure” of the corresponding charge. The question of where exactly this internal space is is sought to be answered by multidimensional unified field theories, which are not discussed here.
| Table 4. FUNDAMENTAL INTERACTIONS | |||||
| Interaction | Relative intensity at a distance of 10–13 cm | Radius of action | Interaction carrier | Carrier rest mass, MeV/ With 2 | Spin the carrier |
| Strong | 1 | Gluon | 0 | 1 | |
| Electro- magnetic |
0,01 | Ґ | Photon | 0 | 1 |
| Weak | 10 –13 | W + | 80400 | 1 | |
| W – | 80400 | 1 | |||
| Z 0 | 91190 | 1 | |||
| Gravita- tional |
10 –38 | Ґ | Graviton | 0 | 2 |
Particle physics is not yet complete. It is still far from clear whether the available data is sufficient to fully understand the nature of particles and forces, as well as the true nature and dimension of space and time. Do we need experiments with energies of 10 15 GeV for this, or will the effort of thought be sufficient? No answer yet. But we can say with confidence that the final picture will be simple, elegant and beautiful. It is possible that there will not be so many fundamental ideas: the gauge principle, spaces of higher dimensions, collapse and expansion, and, above all, geometry.
Further penetration into the depths of the microworld is associated with the transition from the level of atoms to the level of elementary particles. As the first elementary particle at the end of the 19th century. the electron was discovered, and then in the first decades of the 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 was established - over 300. Among them there 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 entire convention of the term “elementary” in relation to micro-objects. Now there is no doubt that 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.
Resting mass elementary particles are determined in relation to the rest mass of the electron. There are elementary particles that do not have a rest mass - photons. The remaining particles according to this criterion are divided into leptons– light particles (electron and neutrino); mesons– medium-sized particles with a mass ranging from one to a thousand electron masses; baryons– heavy particles whose mass exceeds a thousand electron masses and which includes 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 the photon and two mesons, corresponds to antiparticles with opposite charges. Around 1963–1964 a hypothesis was put forward about the existence quarks– particles with a fractional electric charge. This hypothesis has not yet been confirmed experimentally.
By lifetime particles are divided into stable And unstable . There are five stable particles: the photon, two types of neutrinos, the electron and the proton. It is the stable particles that play vital 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. Resonant states were calculated theoretically; they could not be detected 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 that is not associated with its movement. Spin is characterized by 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. Electrons are classified as 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. 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 characteristics of elementary particles there is another important idea interaction. As noted earlier, four types of interactions between elementary particles are known: gravitational,weak,electromagnetic And strong(nuclear).
All particles having a rest mass ( m 0), participate in gravitational interaction, and charged ones also participate in electromagnetic interaction. Leptons also participate in weak interactions. Hadrons participate in all four fundamental interactions.
According to quantum field theory, all interactions are carried out due to 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 recorded using instruments).
It turns out that all four known types of interactions - gravitational, electromagnetic, strong and weak - have a gauge nature and are described by gauge symmetries. That is, all interactions are, as it were, made “from the same blank.” This gives us 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 appeared.
There are a huge number of ways to classify elementary particles. 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 (electromagnetic field quanta) participate in electromagnetic interactions, but do not have strong, weak, or gravitational interactions.
Leptons got their name from the Greek word leptos- easy. These include particles that do not have strong interaction: muons (μ – , μ +), electrons (е – , у +), electron neutrinos (v e – ,v e +) and muon neutrinos (v – m, v + m). All leptons have a spin of ½ and are therefore fermions. All leptons have a weak interaction. Those that have an electrical charge (that is, muons and electrons) also have an electromagnetic force.
Mesons – strongly interacting unstable particles that do not carry the so-called baryon charge. Among them is 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 the order of 10 –8 s. They disintegrate to form I-mesons and leptons or only leptons. Weight this-mesons is 549 MeV (1074me), the lifetime is about 10–19 s. This-mesons decay to form π-mesons and γ-photons. Unlike leptons, mesons have not only a weak (and, if they are charged, electromagnetic) interaction, but also a strong interaction, which manifests itself when they interact with each other, as well as during the interaction between mesons and baryons. All mesons have zero spin, so they are bosons.
Class baryons combines nucleons (p,n) and unstable particles with a mass greater than the mass of nucleons, called hyperons. All baryons have a strong interaction and, therefore, actively interact with atomic nuclei. The spin of all baryons is ½, so the baryons are fermions. With the exception of the proton, all baryons are unstable. During 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, we found big number strongly interacting short-lived particles, which are called resonances . These particles are resonant states formed by two or more elementary particles. The resonance lifetime is only ~ 10 –23 –10 –22 s.
Elementary particles, as well as complex microparticles, can be observed thanks to the traces that they leave as they pass through matter. The nature of the traces allows us to judge the sign of the particle’s charge, its energy, momentum, etc. Charged particles cause ionization of molecules along their path. 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. Consequently, neutral particles are ultimately also detected by the ionization caused by the charged particles they generate.
Particles and antiparticles. In 1928, the English physicist P. Dirac managed to find a relativistic quantum mechanical equation for the electron, from which a number of remarkable consequences follow. First of all, from this equation the spin and numerical value of the electron’s own magnetic moment are obtained naturally, without any additional assumptions. Thus, it turned out that spin is both a quantum and a relativistic quantity. But this does not exhaust the significance of the Dirac equation. It also made it possible to predict the existence of the electron’s antiparticle – 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 least 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. Transitioning into states with less and less energy (that is, with negative energy increasing in magnitude), it could release energy, say, in the form of radiation, and, since | E| unconstrained, a particle with negative energy could emit an infinitely large amount of energy. A similar conclusion can be reached in the following way: from the relation E=m e With 2 it follows that a particle with negative energy will also have a negative mass. Under the influence of a braking force, a particle with a negative mass should not slow down, but accelerate, performing an infinitely large amount of work on the source of the braking force. In view of these difficulties, it would seem that it would be necessary to 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. Therefore, Dirac chose a different path. He proposed 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, a vacuum is a state in which all levels of negative energy are occupied by electrons, and levels with positive energy are free. Since all levels lying below the forbidden band are occupied without exception, 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 positive mass and negative charge. This first theoretically predicted particle was called the positron. When a positron meets an electron, they annihilate (disappear) - the electron moves from a positive level to a vacant negative one. The energy corresponding to the difference between these levels is released in the form of radiation. In Fig. 4, arrow 1 depicts the process of creation of an electron-positron pair, and arrow 2 – their annihilation. The term “annihilation” should not be taken literally. Essentially, what occurs is not a disappearance, but a transformation of some particles (electron and positron) into others (γ-photons).
There are particles that are identical with their antiparticles (that is, they do not have antiparticles). Such particles are called absolutely neutral. These include the photon, π 0 meson and η meson. Particles identical with their antiparticles are not capable of annihilation. This, however, does not mean that they cannot be transformed into other particles at all.
If baryons (that is, nucleons and hyperons) are assigned a baryon charge (or baryon number) IN= +1, antibaryons – baryon charge IN= –1, and all other particles have a baryon charge IN= 0, then all processes occurring with the participation of baryons and antibaryons will be characterized by conservation of charge baryons, just as processes are characterized by conservation of electric charge. The law of conservation of baryon charge determines the stability of the softest baryon, the proton. The transformation of all quantities that describe a physical system, in which all particles are replaced by antiparticles (for example, electrons with protons, and protons with electrons, etc.), is called the conjugation charge.
Strange particles.TO-mesons and hyperons were discovered as part of cosmic rays in the early 50s of the XX century. Since 1953, they have been produced at accelerators. The behavior of these particles turned out to be so unusual that they were called strange. The unusual behavior of the strange particles was that they were clearly born due to strong interactions with a characteristic time of the order of 10–23 s, and their lifetimes turned out to be of the order of 10–8–10–10 s. The latter circumstance indicated that the decay of particles occurs as a result of weak interactions. It was completely unclear why the strange particles lived for so long. Since the same particles (π-mesons and protons) are involved in both the creation and decay of the λ-hyperon, it was surprising that the rate (that is, the probability) of both processes was so different. Further research showed that strange particles are born in pairs. This led to the idea that strong interactions cannot play a role in particle decay due to the fact that the presence of two strange particles is necessary for their manifestation. For the same reason, the single creation of strange particles turns out to be impossible.
To explain the prohibition of 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 conserved under strong interactions. This is a quantum number S was named the strangeness of the particle. In weak interactions, the strangeness may not be preserved. Therefore, it is attributed only to strongly interacting particles - mesons and baryons.
Neutrino. Neutrino is the only particle that does not participate in either strong or electromagnetic interactions. Excluding the gravitational interaction, in which all particles participate, neutrinos can only take part in weak interactions.
For a long time, it remained unclear how a neutrino differs from an antineutrino. 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 the impulse is understood R and back S particles. Helicity is considered positive if 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-handed screw. When the spin and momentum are oppositely directed, the helicity will be negative (the translational movement and “rotation” form a left-handed screw). According to the theory of longitudinal neutrinos developed by Yang, Lee, Landau and Salam, all neutrinos existing in nature, regardless of the method of their origin, are always completely longitudinally polarized (that is, their spin is directed parallel or antiparallel to the momentum R). Neutrino has negative(left) helicity (corresponding to the ratio of directions S And R, shown in Fig. 5 (b), antineutrino – positive (right-handed) 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 in the form of conservation laws. Quite a lot of such laws have already accumulated. Some of them turn out to be not exact, but only approximate. Each conservation law expresses a certain symmetry of the system. Laws of conservation of momentum R, angular momentum L and energy E reflect the properties of symmetry of space and time: conservation E is a consequence of the homogeneity of time, the preservation R due to the homogeneity of space, and the preservation L– its isotropy. The law of conservation of parity is associated with the symmetry between right and left ( R-invariance). Symmetry with respect to charge conjugation (symmetry of particles and antiparticles) leads to the conservation of charge parity ( WITH-invariance). The laws of conservation of electric, baryon and lepton charges express a special symmetry WITH-functions. Finally, the law of conservation of isotopic spin reflects the isotropy of isotopic space. Failure to comply with one of the conservation laws means a violation of the corresponding type of symmetry in this interaction.
In the world of elementary particles the following rule applies: everything that is not prohibited by conservation laws is permitted. The latter play the role of exclusion rules governing the interconversion of particles. First of all, let us note the laws of conservation of energy, momentum and electric charge. These three laws explain the stability of the electron. From the conservation of energy and momentum it follows that the total rest mass of the decay products must be less than the rest mass of the decaying particle. This means that an 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 have become so many particles called elementary that serious doubts have arisen about their elementary nature. Each of the strongly interacting particles is characterized by three independent additive quantum numbers: charge Q, hypercharge U and baryon charge IN. In this regard, a hypothesis arose 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 designated 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. For example, a proton and a neutron are composed of three quarks, which makes the proton or neutron colorless. Accordingly, three charges of strong interactions are distinguished - red ( R), yellow ( Y) and green ( G).
Each quark is assigned the same magnetic moment (μV), the value of which is not determined from theory. Calculations made on the basis of this assumption give the value of the magnetic moment μ p for the proton = μ kv, and for a neutron μ n = – ⅔μ sq.
Thus, for the ratio of magnetic moments the value μ p is obtained / 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 quanta of fields of various interactions (photons in electromagnetic interactions, R-mesons in strong interactions, etc.) particles that carried the interaction between quarks were introduced. These particles were named gluons. They transfer color from one quark to another, causing the quarks to be held together. In quark physics, the confinement hypothesis was formulated (from the English. confinements– capture) of quarks, according to which it is impossible to subtract a quark from the whole. It can only exist 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 whole series of new ones. The situation that has developed in the physics of elementary particles is reminiscent of the situation created in atomic physics after the discovery of the periodic law in 1869 by D. I. Mendelev. Although the essence of this law was clarified only about 60 years after the creation of quantum mechanics, it made it possible to systematize the chemical elements known by that time and, in addition, led to the prediction of the existence of new elements and their properties. In the same way, physicists have learned to systematize elementary particles, and the developed taxonomy has, in rare cases, made it possible to predict the existence of new particles and anticipate their properties.
So, at present, quarks and leptons can be considered truly elementary; There are 12 of them, or together with anti-chatits - 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.
Existing theories of elementary particles cannot indicate what is the beginning of the series: atoms, nuclei, hadrons, quarksIn this series, each more complex material structure includes a simpler one as a component. 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 may not be pointlike, 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 simplify the perception of the ideas inherent in the theories of elementary particles. Nevertheless, these theories allow physicists to express the mutual transformation 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- riddle, secret). She's operating twelve-dimensional space . Ultimately, during the transition to the four-dimensional world that we directly perceive, all “extra” dimensions are “collapsed.” M-theory is so far the only theory that makes it possible to reduce four fundamental interactions to one - the so-called Superpower. It is also important that M-theory 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" based on M-theory will be built in the 21st century.