Elementary particles in the exact meaning of this term, the primary, further indecomposable particles, of which, by assumption, all matter consists. In modern physics, this term is not used in its exact meaning, but less strictly - to name a large group of the smallest particles of matter subject to the condition that they are not atoms or atomic nuclei (the proton is an exception). Elementary particles are specific quanta of matter, more precisely - quanta of the corresponding physical fields.

The most important quantum property of all elementary particles is the ability to be born and destroyed (emitted and absorbed) when interacting with other particles. In this respect, they are completely analogous to photons. All processes with elementary particles proceed through a sequence of acts of their absorption and emission. Only on this basis can one understand, for example, the process of birth p+-meson in the collision of two protons ( p + p ® p + n + p +) or the process of annihilation of an electron and a positron, when two g-quanta appear instead of the disappeared particles: e + + e -® g + g. Processes of elastic scattering of particles, for example, e - + p® e - + p, are also associated with the absorption of initial particles and the production of final particles. The decay of unstable elementary particles into lighter particles, accompanied by the release of energy, corresponds to the same regularity and is a process in which decay products are born at the moment of the decay itself and do not exist until that moment. In this respect, the decay of elementary particles is similar to the decay of an excited atom into the ground state and a photon. Examples of decays of elementary particles can be: total number known elementary particles (together with antiparticles) approaches 400. To describe the properties of individual elementary particles, a whole series of physical quantities, whose values ​​differ. The most famous among them are mass, mean lifetime, spin, electric charge, magnetic moment.

Weight and dimensions. All elementary particles are objects of exceptionally small masses and sizes. For most of them, the masses are of the order of magnitude of the mass of the proton, equal to 1.6 10 -27 kg (only the mass of the electron is noticeably smaller: 9.1.10 -31 kg). The sizes of a proton, a neutron, a p-meson determined from experience in order of magnitude are 10 -15 m. The dimensions of the electron and muon could not be determined, it is only known that they are less than 10 -17 m. Microscopic masses and sizes of elementary particles determine the quantum specificity of their behavior. Characteristic wavelengths that should be attributed to elementary particles in quantum theory (), where - Planck constant, t - particle mass, With is the speed of light), are close in order of magnitude to the typical sizes on which their interaction takes place (for example, for the p-meson » 1.4 10 -15 m ). This leads to the fact that quantum regularities are decisive in the behavior of elementary particles. The mass of elementary particles is expressed in energy units (MeV or GeV) in accordance with the Einstein relation W \u003d tc 2. In other words, the tables do not actually show the mass t particles, and their rest energy W0. This is convenient when compiling the energy balance equations for the processes of elementary particle interconversions. Let us indicate the masses of some particles:

m g= 0, me= 0.51 MeV, m p= 938.3 MeV, m n= 939.6 MeV.

The heaviest of the currently known elementary particles (the intermediate boson) is almost 100 times more massive than the proton.

Average life time elementary particle t serves as a measure of particle stability and is expressed in seconds. The half-life T 1/2 in elementary particle physics is not used, and as a measure of the stability of resonances, the width Г~ is taken, expressed in energy units.

Depending on the lifetime, elementary particles are divided into stable, quasi-stable and unstable(resonances). The electron (t>5.1031 years), proton (t>1030 years), photon and neutrino are stable within the accuracy of modern measurements. Quasi-stable particles include particles that decay due to electromagnetic and weak interactions. Their lifetimes >10 -20 sec. The neutron is a quasi-stable particle, and the last experimental value of its average lifetime (1986) is (898 ± 16) s. There are groups of particles with an average lifetime of the order of 10 -6 , 10 -8 , 10 -10 , 10 -13 s. For the shortest-lived particles, called resonances, t ~ 10 -24 -10 -23 s. For unstable particles in the tables, along with the lifetime, the types of decays are also indicated.

Spin is the intrinsic angular momentum of the particle, i.e., its angular momentum in the rest frame. Spin has no classical analogue, since an elementary particle cannot be imagined as a rotating ball. Usually, the spin J is expressed in units and takes only integer and half-integer values. Particle with spin J has 2J + 1 spin states differing in projection values Jz , which can be equal to -J, ( -J+ 1),0, .., (J- 1), J. for an electron, proton, neutron and neutrino J = 1/2, for a photon J= 1. Particles with spins from 0 (many mesons) to 6 (meson resonance, discovered at the Serpukhov accelerator in 1983) are known. The spin of an elementary particle is one of its most important characteristics. The value of the spin is unambiguous

determines the type of statistic that the given particle obeys. All particles with integer spins are bosons (Bose-Einstein statistics), all particles with half-integer spins are fermions (Fermi-Dirac statistics), for which the Pauli principle is valid. For example, electrons are fermions and photons are bosons.

Electric charge elementary particle q - physical quantity characterizing the ability of a particle to participate in electromagnetic interaction, expressed in units of elementary charge e= 1.6. 10 -19 C.

For all particles that exist in a free state, it takes integer values ​​- usually 0 and ±1, for some resonances ±2. This rule of quantization of electric charge is carried out with great accuracy.

Elementary particles are the main structural elements of the microworld. Elementary particles can be constituent(proton, neutron) and non-composite(electron, neutrino, photon). To date, more than 400 particles and their antiparticles have been discovered. Some elementary particles have unusual properties. Thus, for a long time it was believed that the neutrino particle has no rest mass. In the 30s. 20th century when studying beta decay, it was found that the energy distribution of electrons emitted by radioactive nuclei occurs continuously. It followed from this that either the law of conservation of energy is not fulfilled, or, in addition to electrons, difficult-to-detect particles are emitted, similar to photons with zero rest mass, which carry away part of the energy. Scientists have suggested that this is a neutrino. However, experimental registration of neutrinos was possible only in 1956 at huge underground installations. The difficulty of registering these particles lies in the fact that the capture of neutrino particles is extremely rare due to their high penetrating power. During the experiments, it was found that the rest mass of the neutrino is not equal to zero, although it does not differ much from zero. Antiparticles also have interesting properties. They have many of the same characteristics as their twin particles (mass, spin, lifetime, etc.), but differ from them in terms of electric charge or other characteristics.

In 1928, P. Dirac predicted the existence of an antiparticle of the electron - the positron, which was discovered four years later by K. Anderson as part of cosmic rays. An electron and a positron are not the only pair of twin particles; all elementary particles, except for neutral ones, have their own antiparticles. When a particle and an antiparticle collide, they annihilate (from lat. annihilatio- transformation into nothing) - the transformation of elementary particles and antiparticles into other particles, the number and type of which are determined by conservation laws. For example, as a result of the annihilation of an electron-positron pair, photons are born. The number of detected elementary particles increases with time. At the same time, the search for fundamental particles continues, which could be composite "building blocks" for building known particles. The hypothesis about the existence of particles of this kind, called quarks, was put forward in 1964. American physicist M. Gell-Man (Nobel Prize 1969).

Elementary particles have a large number of characteristics. One of the distinguishing features of quarks is that they have fractional electric charges. Quarks can combine with each other in pairs and triplets. The union of three quarks forms baryons(protons and neutrons). Quarks were not observed in the free state. However, the quark model made it possible to determine the quantum numbers of many elementary particles.

Elementary particles are classified according to the following features: particle mass, electric charge, type of physical interaction in which elementary particles participate, particle lifetime, spin, etc.

Depending on the rest mass of the particle (its rest mass, which is determined in relation to the rest mass of the electron, which is considered the lightest of all particles having mass), they distinguish:

♦ photons (gr. photos- particles that have no rest mass and move at the speed of light);

♦ leptons (gr. leptos– light) – light particles (electron and neutrino);

♦ mesons (gr. mesos- medium) - medium particles with a mass from one to a thousand masses of an electron (pi-meson, ka-meson, etc.);

♦ baryons (gr. barys- heavy) - heavy particles with a mass of more than a thousand masses of an electron (protons, neutrons, etc.).

Depending on the electric charge, there are:

♦ particles with a negative charge (for example, electrons);

♦ particles with a positive charge (eg proton, positrons);

♦ particles with zero charge (for example, neutrinos).

There are particles with a fractional charge - quarks. Taking into account the type of fundamental interaction in which particles participate, among them are:

♦ hadrons (gr. adros- large, strong), participating in electromagnetic, strong and weak interaction;

♦ leptons participating only in electromagnetic and weak interactions;

♦ particles – carriers of interactions (photons – carriers of electromagnetic interaction; gravitons – carriers of gravitational interaction; gluons – carriers of strong interaction; intermediate vector bosons – carriers of weak interaction).

According to the lifetime of the particles are divided into stable, quasi-stable and unstable. Most elementary particles are unstable, their lifetime is 10 -10 -10 -24 s. Stable particles do not decay for a long time. They can exist from infinity to 10 -10 s. The photon, neutrino, proton and electron are considered stable particles. Quasi-stable particles decay as a result of electromagnetic and weak interaction, otherwise they are called resonances. Their lifetime is 10 -24 -10 -26 s.

Scientists discovered in the study of nuclear processes, therefore, until the middle of the 20th century, elementary particle physics was a section of nuclear physics. At present, these branches of physics are close, but independent, united by the commonality of many of the problems considered and the research methods used. The main task of elementary particle physics is the study of the nature, properties and mutual transformations of elementary particles.

Currently, about 400 subnuclear particles are known, which are commonly called elementary. The vast majority of these particles are unstable . The only exceptions are the photon, electron, proton and neutrino. All other particles experience at certain intervals spontaneous transformation into other particles. Unstable elementary particles strongly differ from each other in lifetimes. The longest-lived particle is the neutron. The neutron lifetime is about 15 min. Other particles "live" for a much shorter time. For example, the average lifetime of a μ meson is 2.2∙10 -6 s, and that of a neutral π meson is 0.87∙10 -16 s. Many massive particles—hyperons—have an average lifetime on the order of 10–10 s.

There are several tens of particles with a lifetime exceeding 10–17 s. In terms of the scale of the microcosm, this is a significant time. Such particles are called relatively stable . Majority short lived elementary particles have lifetimes of the order of 10–22–10–23 s.

The ability for mutual transformations is the most important property of all elementary particles. They are capable of being born and destroyed (emitted and absorbed). This also applies to stable particles, with the only difference that the transformations of stable particles do not occur spontaneously, but upon interaction with other particles. An example would be annihilation (i.e. disappearance ) of an electron and a positron, accompanied by the production of photons of high energy. The reverse process can also take place. birth electron-positron pair, for example, when a photon of sufficiently high energy collides with a nucleus. Such a dangerous twin, as the positron is for the electron, the proton also has. It is called antiproton . The electric charge of the antiproton is negative. Currently antiparticles found in all particles. Antiparticles are opposed to particles because when any particle meets its antiparticle, they annihilate, i.e., both particles disappear, turning into radiation quanta or other particles.

Even the neutron has an antiparticle. The neutron and antineutron differ only in the signs of the magnetic moment and the so-called baryon charge. Is it possible for atoms to exist? antimatter , whose nuclei are made up of antinucleons and whose shells are made up of positrons. During the annihilation of antimatter with matter, the rest energy is converted into the energy of radiation quanta. This is a huge energy, much greater than that released in nuclear and thermonuclear reactions.

In the variety of elementary particles known to date, a more or less harmonious classification system is found.

Elementary particles are combined into three groups: photons , leptons and hadrons.

To the group photons the only particle is the photon, which is the carrier of the electromagnetic interaction.

The next group consists of light particles − leptons . This group includes two types of neutrinos (electronic and muon), electron and μ-meson. Leptons also include a number of particles not listed in the table. All leptons have spin 1/2.

The third large group consists of heavy particles called hadrons . This group is divided into two parts. Lighter particles form a subgroup mesons . The lightest of them are positively and negatively charged, as well as neutral π-mesons with masses of the order of 250 electron masses. Pions are quanta of the nuclear field, just as photons are quanta electromagnetic field. This subgroup also includes four K mesons and one η 0 meson. All mesons have spin equal to zero.

The second sub-group baryons – includes heavier particles. It is the most extensive. The lightest of the baryons are nucleons - protons and neutrons. They are followed by the so-called hyperons. Closes the table omega-minus-hyperon, discovered in 1964. This is a heavy particle with a mass of 3273 electron masses. All baryons have spin 1/2.

The abundance of discovered and newly discovered hadrons led scientists to the idea that they are all built from some other more fundamental particles. In 1964, the American physicist M. Gell-Man put forward a hypothesis, confirmed by subsequent studies, that all heavy particles - hadrons - are built from more fundamental particles called quarks . Based on the quark hypothesis, not only was the structure of already known hadrons understood, but the existence of new ones was also predicted. The Gell-Mann theory assumed the existence of three quarks and three antiquarks, which combine with each other in various combinations. Thus, each baryon consists of three quarks, and an antibaryon consists of three antiquarks. Mesons are made up of quark-antiquark pairs.

With the acceptance of the quark hypothesis, it was possible to create a coherent system of elementary particles. However, the predicted properties of these hypothetical particles turned out to be rather unexpected. The electric charge of quarks must be expressed in fractional numbers equal to 2/3 and 1/3 of the elementary charge.

The model is made in the form of an interactive table. The user can choose the group under consideration (particles or antiparticles) and the data displayed in the tables (charge, spin, year of discovery).

Until now, only such particles as electron e proton p neutron n and photon were considered that are stable or quasi-stable, that is, they exist either indefinitely or for a sufficiently long time. However, the vast majority of elementary particles obtained at accelerators are not stable, that is, they decay, eventually turning into stable particles. The mass of the particle in nuclear physics It is customary to express in energy units, which are based on the Einstein law of the relationship between mass and energy E = mc2.

If this work does not suit you, there is a list of similar works at the bottom of the page. You can also use the search button

LECTURE #14

ELEMENTARY PARTICLES AND THEIR PROPERTIES

Currently, about 400 elementary particles are known. So far, only such particles as the electron have been considered. e , proton p , neutron n and photon, which are stable or quasi-stable, that is, they exist either indefinitely or for a sufficiently long time. However, the vast majority of elementary particles produced in accelerators are not stable, that is, they decay, eventually turning into stable particles.

To describe particles, a number of physical quantities are introduced that distinguish them: mass, average lifetime, electric charge, spin, and a number of others.

Particle mass in nuclear physics, it is customary to express in energy units, which are based on the Einstein law of the relationship between mass and energy E \u003d mc 2 . Unit of measurement electron volt (1 eV = 1.6 10 19 J); in practice, millions of electronvolts MeV are used (1 MeV = 10 6 eV) and gigaelectronvolt GeV (1 GeV = 10 9 eV). So the mass of an electron me = 0.51 MeV, proton m p = 938.3 MeV, neutron 939.6 MeV, photon mass is zero.

Average life time is a measure of particle stability and is expressed in seconds.

Particles known to us: electron, proton and photon are absolutely stable ( =  ), the neutron in the free state is quasi-stable, its lifetime is ≈898 s.

Spin intrinsic angular momentum of the particle. Spin is expressed in units h / 2  and accepts only integer and half-integer values. So for an electron, a proton and a neutron, the spin is equal for a photon This is the most important characteristic of elementary particles, which has no analogues in classical physics.

Electric chargecharacterizes the ability of a particle to participate in electromagnetic interactions, and this value is well known to us from electrostatics.

Own magnetic momentparticle characterizes the interaction of the particle with an external magnetic field.

It turned out that these characteristics are not enough to describe the behavior of elementary particles and new properties were introduced:oddity, charm, charm, color, fragranceand others that are characterized by their quantum numbers. Of course, the above names have nothing to do with the usual meaning of these words, but reflect the special properties of the particles.

FUNDAMENTAL INTERACTIONS

At present, four types of fundamental interactions are distinguished in physics: strong, electromagnetic, weak, and gravitational.

To strong interactions ( Sstrong) are primarily nuclear forces, uniting nucleons into a nucleus.

In electromagnetic interaction(E electromagnetic ) only electrically charged particles and photons are involved. One of its manifestations is the Coulomb forces that determine the existence of atoms. It is the electromagnetic interaction that is responsible for the vast majority of the macroscopic properties of matter (friction forces, elastic forces, etc.)

Weak interactions ( W week) manifested in beta transformations atomic nuclei. It leads to instability of many elementary particles and is typical for all particles, except for photons.

Gravitational interaction ( G gravitational ) manifests itself in the form of forces gravity and common to all bodies. The gravitational interaction is very weak and does not play a significant role in the microcosm.

Fundamental interactions differ in a number of properties, among which, first of all, the intensity (α) of interaction and the radius of their action should be noted R . Usually, in order to compare different interactions, the ratio of their intensities is considered, which, in a rough approximation, is defined as the ratio of interaction energies. Assuming conditionally the intensity of the strong interaction as unity (α S = 1), the approximate values ​​of the intensities for other interactions are: α E ≈ 10 2 , α W ≈ 10 10 , α G ≈ 10 38 . Thus, the most intense interaction in the microcosm is the strong interaction, the least intense is the weak one, while the gravitational interaction is negligible.

Interaction radius R is determined by the dependence of the energy of this interaction on the distance between the particles. According to the law of Coulomb and universal gravitation, electromagnetic and gravitational forces are inversely proportional to the square of the distance between particles, that is, these forces decrease slowly. Therefore, the radius of their action is assumed to be equal to infinity: R E = ∞ and R G = ∞. The energy of strong and weak interactions decreases with distance very quickly according to an exponential law, and they affect only at small distances. As determined experimentally, their ranges R S ≈ 10 15 m and R W ≈ 10 18 m, that is, they are commensurate with the size of the nuclei and operate within the atomic nucleus.

CLASSIFICATION OF ELEMENTARY PARTICLES

1. Particles and antiparticles. All elementary particles, first of all, can be divided into two classes: particles and antiparticles. Each particle has its own antiparticle, and they are characterized by the following properties: the mass, lifetime and spin of the particle and antiparticle are the same, but other properties, such as charge and magnetic moment, are opposite in sign. Antiparticles are denoted by the same symbols as particles. To them, only the symbol ~, called "tilde", is added to the top. Examples of particles and antiparticles are the electron and positron (positively charged electron), proton p and antiproton An important property of particles and antiparticles is that when they meet, annihilation (destruction) of particles occurs with the appearance of either photons or other particles. This does not mean that in the first case matter is annihilated; in fact, there is a transition of one type of matter (particle) to another ( electromagnetic radiation). Under laboratory conditions, an antiatom was also obtained from an antiproton and a positron. All of the above leads to the idea that somewhere in the Universe, far from our ordinary matter, “anti-worlds” can exist (the meeting of such a world and an anti-world would lead to an explosion of colossal power due to annihilation). All known elementary particles, including antiparticles, are divided into three classes (Fig. 1): hadrons, leptons, and particles responsible for the transfer of interactions.

Rice. one

2. Hadrons these are elementary particles that participate in strong (nuclear) interactions. They make up the largest class of elementary particles: there are over 300 of them. The Greek word "hadros" means massive, strong. Russian word"core" also comes from this word. Hadrons are heavy particles and can be called relatives of the proton. Hadrons are divided into two classes: baryons particles that have a spin equal to and mesons with spin Protons and neutrons are the lightest baryons, other baryons (hyperons) surpass them in mass. Mesons are particles whose mass is intermediate between the mass of an electron and the mass of a proton.

3. Quarks . In the mid-60s, a hypothesis was put forward that all hadrons are built from more fundamental particles, called quarks . At present, it is believed that there are six types of quarks, the characteristics of which are given in Table. 1. There are also six antiquarks. All quarks have a spin equal to An interesting feature quarks is that they have a fractional electric charge.

Using combinations of different quarks, you can get any known hadron. For example, a proton is made up of two u -quarks and one d -quark (schematically, using the notation for quarks from the table: uud ). Indeed, the proton charge is: (2/3 + 2/3 1/3) e = + e . (It should not be forgotten that all other characteristics of a particle are checked in a similar way: spin, magnetic moment, and others). The neutron is made up of two d -quarks and one u-quark (ddu ). To explain the structure of other baryons and mesons, heavier quarks are involved.

Table 41.1

With the advent of the theory of quarks, physicists tried to find them experimentally. However, all attempts to find particles with a fractional charge were unsuccessful. It is currently believed thatFree quarks simply don't exist.and therefore cannot be found experimentally.

4. Leptons. Leptons are particles that do not participate in strong (nuclear) interactions, but participate in electromagnetic and weak interactions. In Greek, "leptos" means small, and "mite" a small coin. If hadrons are "relatives" of the proton, then leptons are "relatives" of the electron, and the electron itself belongs to the class of leptons. Like an electron, othersleptons truly elementary particles,because no lepton has an internal structure. The leptons include the electron, muon, -lepton and neutrino.

Muon is a particle whose basic properties coincide with the properties of an electron. It can be called a heavy electron, since the mass is 106 MeV (an electron has 0.51 MeV). Unlike the electron, the muon is not stable, its lifetime is 10 6 s (quite large on an atomic scale). Physicists managed to artificially obtain an atom in which a muon revolves around the nucleus ( -mesoatom). Mesons in this case obey the same laws as electrons. -mesoatom can enter into chemical reactions and form mesomolecules. However, it should be noted that the role of the muon in the universe is incomprehensible: it is possible to explain the structure of matter without it.

Neutrino . Neutrino (small neutron) was discovered as a result of research -decay. It turned out that at -decay, in addition to the electron, some other particle flies out of the nucleus that does not have a charge, which they called the neutrino and denoted by So, the free neutron, being a quasi-stable particle, eventually decays into a proton, an electron and a neutrino, which is called an electron neutrino ( more precisely antineutrino) according to the scheme: Further studies have shown that there are also muon neutrinos  , formed during the decay of muons, as well as tau neutrinos. The main properties of neutrinos:

a) a neutrino has no charge, it is a neutral particle;

b) the rest mass of the neutrino is zero or negligible;

c) the neutrino participates only in weak interaction, which manifests itself in particular in - decay.

These properties make the neutrino "invisible", a particle that is difficult to register, since it practically does not interact with anything. Therefore, the neutrino freely passes through the equipment with which they try to see it. Interaction of neutrinos with protons and neutrons in 10 12 times weaker than the electromagnetic force. The entire thickness of the globe neutrinos can pass without causing interactions. Therefore, neutrinos could not be "caught" for a long time. However, neutrinos have been detected.

Interaction carriers. Let us turn to the third type of elementary particles, which are responsible for the interaction between the previously considered particles and from which any substance is formed. Let's consider such particles. The carriers of interactions are photons, gluons and gravitons (Fig. 1).

Photons (γ) are carriers of electromagnetic interactions, their rest mass is zero, and they have no charge. The interaction of two charged particles occurs due to the exchange of photons between them. Note that quarks, all hadrons, charged leptons, as well as particles responsible for the weak interaction participate in the electromagnetic interaction.

Gluons [glue glue] (g ) carriers of strong interactions. They have no mass and are electrically neutral. These are the particles by which the interaction of quarks is carried out.

Intermediate bosons(W , Z ) carriers of weak interactions. They have an electrical charge q = ± e ) and have large masses: m W ≈ 81 GeV, m Z ≈ 93 GeV. Intermediate bosons were predicted theoretically and soon discovered, and all the predicted properties coincided with the experimental ones. Intermediate bosons can be emitted and absorbed by both quarks and leptons, and therefore all particles except photons and gravitons participate in the weak interaction.

Gravitons (G ) carriers of gravitational interaction. Gravitons have not yet been experimentally detected in the same way as gravitational waves. The alleged properties of the graviton are neutral particles that do not have a rest mass, with a spin

PERIODIC SYSTEM OF ELEMENTARY PARTICLES

According to modern theory, there are 17 elementary particles that form all known types of matter and carriers of all forces acting between particles. Material elementary particles (those that make up matter) can be represented as a kind of "periodic system" a table of quarks and leptons (Table 2).

table 2

This model includes 6 varieties of quarks and 6 leptons. These 12 particles are divided into columns according to their elementary charges. The rows correspond to the three families of the main material particles.

The main one is the first row, which contains the particles necessary to create an atom: u- and d - quarks form nucleons. Nucleons, in turn, form the nucleus of an atom. Negatively charged electrons are attracted to the nucleus to form atoms. Finally, atoms form molecules. The remaining fourth particle electron neutrino is not associated with matter. The neutrino plays a major role in -disintegration of the nucleus, when protons and neutrons can turn into each other. Thus, the first family of quarks and leptons is necessary for the existence of the world as we know it.

The second and third rows of the table are necessary to explain the properties of particles coming from space and created at accelerators. The question of what role the particles in the second and third rows play in the structure of matter remains open. However, with the help of these elementary particles, all particles known to us that exist in the Universe are explained.

Thus, modern theory suggests that at this stage in the development of physics, there are 17 elementary particles that can be used to explain the existence of matter from which the Universe is created.