Post discovery of the main characteristics of elementary particles. Discovery of elementary particles

An elementary particle is considered a primary or indecomposable particle, of which all matter is composed. However, in modern physics, the term "elementary particle" is used to name a large group of the smallest particles of matter. This group includes protons, neutrons, electrons, photons, pi-mesons, muons, heavy leptons, neutrinos of three types, strange particles (K-mesons, hyperons), various resonances, "charmed" particles, upsilon particles, "beautiful" particles, intermediate bosons (W ±, Z 0). More than 500 particles in total. Particles claiming to be the primary elements of matter are called " truly elementary particles» .

In the history of science, the first particle to be discovered was electron- a carrier of a negative electric charge. The electron was first discovered by the English physicist Joseph Thomson, in 1897 In 1919 the English physicist Ernest Rutherford discovered proton- a particle that is part of atomic nuclei with a positive charge and mass 1840 times the mass of an electron. Another particle that makes up the nucleus is neutron, was discovered in 1932 by the English physicist James Chadwick... The concept of a photon as a particle originates from the work of the German physicist Max Plank, who put forward in 1900 the assumption about the quantization of the energy of electromagnetic radiation. In the development of Planck's idea, Einstein in 1905 established that electromagnetic radiation is a stream of individual quanta ( photons) and on this basis explained the laws of the photoelectric effect. Direct experimental evidence of the existence of a photon was carried out by American physicists Robert Milliken (1912) and A. Compton(1922).

Existence neutrino first hypothesized by Wulfgang Pauli(1930), and the electron neutrino was discovered experimentally only in 1962 by the American physicists F. Reines and K. Coone. The first antiparticle to be discovered is positron with the mass of an electron, but with a positive electric charge. It was discovered in the composition of cosmic rays by the American physicist K. Anderson in 1953. In 1946, Anderson and Neddermeier (USA) found it in the composition of cosmic rays muons with both signs of electric charge (µ - and µ +). Muons have a mass of about 200 electron masses, and their other properties are close to the electron and positron. In 1947, as part of cosmic rays, American physicists under the leadership of S. Powell discovered π - and π + - mesons... The existence of such particles was suggested by a Japanese physicist H. Yukawa in 1935 In the early 50s. a large group of particles was discovered with unusual properties, named “ strange. " The first particles of this group - K - and K + - mesons, Λ - hyperons were found in the composition of cosmic rays. Subsequent discoveries of "strange" particles were made with particle accelerators. Since the beginning of the 50s. accelerators have become the main tool for the study of EF. In 1955 was opened antiproton, 1956 - antineutron, 1960 – antisigma hyperon, and in 1964 - the most severe hyperon -. In 1960, accelerators were discovered resonant particles... They are unstable and very numerous, therefore they constitute the main part of the ECH.

In 1962, scientists found out that there are two different types neutrino: electronic and muon. In 1974, massive, and at the same time relatively stable " enchanted»Particles (D 0, D +, F +, etc.). In 1975, the heavy analogue of the electron and muon (τ - lepton), in 1981 - “ beautiful»Particles, and in 1983 - intermediate bosons(W ± and Z 0).

Thus, it was established that the world of ECH is very complex and diverse. Among elementary particles the electron is best known and used. It all started with the fact that, passing a current through an electrolyte, Faraday measured the amount of substances released on the electrodes, and came to the conclusion that in nature there is the smallest electric charge equal to the charge of a hydrogen ion.

English physicist J. Stoney, came up with a special name for the smallest electric charge - " electron". From the middle of the 19th century, physicists began experimenting with electric discharges in special glass tubes with electrodes soldered into the walls. When the gas was evacuated, the current in the circuit did not stop when the cathodes were heated. This current was accompanied by a beautiful and mysterious glow. It is clear that the current cannot flow through the void. The transfer of electricity from the cathode to the anode, called cathode rays... English physicist Joseph Thomson established the nature of cathode rays, experimentally showed that cathode rays are a stream of the smallest negatively charged particles. He, placing a glass tube in a magnetic field, investigated the deviation of the cathode rays from a straight line and found that the ratio of charge to mass (e / me) for electrons is a thousand times greater than that of the same ratio for hydrogen ions (e / m n) established earlier by Faraday.

Thomson, boldly accepting the hypothesis that electrons and hydrogen ions carry the same elementary charge, came to the conclusion that electrons have a negligible mass in comparison with atoms. There was a doubt about the indivisibility of the atom. Discovered by Henri Becquerel radioactivity of atoms in 1896, she finally shaken the assertions about the indivisibility of the atom. At the beginning of the 20th century, Ernest Rutherford proved that of the three types of rays -, β and γ, emitted by radium, β - rays, these are the same electrons that Thomson saw.

Questions for self-control

1. What are elementary particles?

2. How many elementary particles have been found by science?

3. What particles are called "truly elementary particles"

4. Which particle is the first particle discovered in the history of science?

5. Who and when discovered the electron?

6. Who and when discovered the proton?

7. Who and when discovered the neutron?

8. Who and when discovered the photon?

9. Who and when suggested the existence of neutrinos?

10. In what year, experimentally discovered neutrinos?

11. Who and when discovered the first antiparticle of the positron?

12. Who and when suggested the existence of mesons?

13. In what years was a large group of so-called "strange" particles discovered?

14. In what year were the "charmed" particles discovered?

15. In what year were the "beautiful" particles discovered?

16. In what year were the so-called "intermediate bosons" discovered?

17. Who and when discovered the radioactivity of atoms?

Introduction

1. Discovery of elementary particles

2. Theories of elementary particles

2.1. Quantum electrodynamics (QED)

2.2. Quark theory

2.3. Electroweak theory

2.4. Quantum chromodynamics

Conclusion

Literature

Introduction.

In the middle and second half of the twentieth century, in those branches of physics that are engaged in the study of the fundamental structure of matter, truly amazing results were obtained. First of all, this manifested itself in the discovery of a whole host of new subatomic particles. They are usually called elementary particles, but not all of them are really elementary. Many of them, in turn, consist of even more elementary particles.

The world of subatomic particles is truly diverse. These include protons and neutrons that make up atomic nuclei, as well as electrons revolving around nuclei. But there are also such particles that are practically not found in the substance around us. Their lifetime is extremely short, it is the smallest fractions of a second. After this extremely short time, they disintegrate into ordinary particles. There are an astounding number of such unstable short-lived particles: there are already several hundred known of them.

In the 60s and 70s, physicists were completely confused by the multiplicity, variety and unusualness of the newly discovered subatomic particles. There seemed to be no end to them. It is completely incomprehensible what so many particles are for. Are these elementary particles chaotic and random fragments of matter? Or perhaps they hold the key to understanding the structure of the Universe? The development of physics in the following decades has shown that there is no doubt about the existence of such a structure. At the end of the twentieth century. physics begins to understand what the meaning of each of the elementary particles is.

A deep and rational order is inherent in the world of subatomic particles. This order is based on fundamental physical interactions.

1. Discovery of elementary particles.

The discovery of elementary particles was a natural result of the general advances in the study of the structure of matter, achieved by physics at the end of the 19th century. It was prepared by comprehensive studies of the optical spectra of atoms, the study of electrical phenomena in liquids and gases, the discovery of photoelectricity, X-rays, natural radioactivity, which testified to the existence of a complex structure of matter.

Historically, the first discovered elementary particle was an electron - a carrier of a negative elementary electric charge in atoms. In 1897 J.J. Thomson established that the so-called. cathode rays are formed by a stream of tiny particles called electrons. In 1911 E. Rutherford, passing alpha particles from a natural radioactive source through thin foils various substances, found out that the positive charge in atoms is concentrated in compact formations - nuclei, and in 1919, among the particles knocked out of atomic nuclei, he found protons - particles with a unit positive charge and mass 1840 times greater than the mass of an electron. Another particle that makes up the nucleus, the neutron, was discovered in 1932 by J. Chadwick while studying the interaction of a-particles with beryllium. A neutron has a mass close to that of a proton, but does not have an electric charge. The discovery of the neutron completed the identification of particles - structural elements atoms and their nuclei.

Conclusion about the existence of a particle electromagnetic field- photon - originates from the work of M. Planck (1900). Assuming that the energy of electromagnetic radiation from a black body is quantized, Planck obtained the correct formula for the radiation spectrum. Developing Planck's idea, A. Einstein (1905) postulated that electromagnetic radiation (light) is in fact a stream of individual quanta (photons), and on this basis he explained the laws of the photoelectric effect. Direct experimental evidence for the existence of a photon was given by R. Milliken (1912-1915) and A. Compton (1922).

The discovery of neutrinos, a particle that almost does not interact with matter, originates from a theoretical guess by W. Pauli (1930), which, due to the assumption of the birth of such a particle, made it possible to eliminate difficulties with the law of conservation of energy in the processes of beta decay of radioactive nuclei. The existence of neutrinos was experimentally confirmed only in 1953 (F. Reines and K. Cowen, USA).

From the 30s to the early 50s. the study of elementary particles was closely related to the study of cosmic rays. In 1932, in the composition of cosmic rays, K. Anderson discovered a positron (e +) - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle to be discovered. The existence of e + directly followed from the relativistic theory of the electron, developed by P. Dirac (1928-31) shortly before the discovery of the positron. In 1936, American physicists K. Anderson and S. Neddermeier discovered muons (of both signs of electric charge) while studying osmic rays - particles with a mass of about 200 electron masses, but otherwise surprisingly similar in properties to e-, e +.

In 1947, also in cosmic rays, S. Powell's group discovered p + and p - mesons with a mass of 274 electron masses, which play an important role in the interaction of protons with neutrons in nuclei. The existence of such particles was suggested by H. Yukawa in 1935.

Late 40s - early 50s were marked by the discovery of a large group of particles with unusual properties, called "strange". The first particles of this group K + - and K - mesons, L-, S + -, S- -, X- hyperons were discovered in cosmic rays, subsequent discoveries of strange particles were made at accelerators - installations that create intense fluxes of fast protons and electrons. When colliding with matter, accelerated protons and electrons give rise to new elementary particles, which become the subject of study.

Since the beginning of the 50s. accelerators have become the main tool for the study of elementary particles. In the 70s. the energies of particles accelerated in accelerators amounted to tens and hundreds of billions of electron volts (GeV). The desire to increase the energies of particles is due to the fact that high energies open up the possibility of studying the structure of matter at the smaller distances, the higher the energy of the colliding particles. Accelerators have significantly increased the rate of obtaining new data and in a short time have expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study in more detail their properties, in particular the features of their decay, and soon led to an important discovery: the elucidation of the possibility of changing the characteristics of some microprocesses during the operation of mirror reflection - the so-called. violation of spaces, parity (1956). The commissioning of proton accelerators with energies of billions of electron volts made it possible to discover heavy antiparticles: antiproton (1955), antineutron (1956), antisigma-hyperons (1960). In 1964, the heaviest hyperon W- (with a mass of about two proton masses) was discovered. In the 1960s. on accelerators was opened big number extremely unstable (in comparison with other unstable elementary particles) particles, called "resonances". The masses of most resonances exceed the mass of the proton. The first of them, D1 (1232), has been known since 1953. It turned out that resonances make up the bulk of elementary particles.

In 1962 it was found that there are two different neutrinos: electron and muon. In 1964, nonconservation of the so-called. combined parity (introduced by Li Tsung-tao and Yang Zhen-ning and independently by L. D. Landau in 1956), which means the need to revise the usual views on the behavior of physical processes during the operation of time reflection.

In 1974, massive (3-4 proton masses) and at the same time relatively stable y-particles were discovered, with a lifetime unusually long for resonances. They turned out to be closely related to a new family of elementary particles - “charmed”, the first representatives of which (D0, D +, Lс) were discovered in 1976. In 1975, the first information about the existence of a heavy analogue of an electron and a muon (heavy lepton t) was obtained. In 1977, частицы-particles with a mass of about ten proton masses were discovered.

Thus, over the years that have passed since the discovery of the electron, a huge number of various microparticles of matter have been identified. The world of elementary particles turned out to be quite complex. The properties of the elementary particles discovered were unexpected in many respects. To describe them, in addition to characteristics borrowed from classical physics, such as electric charge, mass, angular momentum, it was necessary to introduce many new special characteristics, in particular, to describe strange elementary particles - strangeness (K. Nishijima, M. Gell-Man , 1953), “charmed” elementary particles - “charm” (American physicists J. Bjorken, S. Glashow, 1964); already the names of the given characteristics reflect the unusual properties of elementary particles described by them.

Study internal structure matter and properties of elementary particles from its first steps was accompanied by a radical revision of many well-established concepts and concepts. The laws governing the behavior of matter in the small turned out to be so different from the laws classical mechanics and electrodynamics, which required completely new theoretical constructions for their description.

And the required quantities. The sequence of actions that must be performed in order to move from the initial data to the desired values is called an algorithm. 2. Historical development of models of elementary particles 2.1 Three stages in the development of physics of elementary particles Stage one. From electron to positron: 1897-1932 (Elementary particles - "atoms of Democritus" on a deeper level) When the Greek ...

A limited number of phenomena: Newtonian mechanics, or a far from optimal or perfect creation of technology: the Titanic liner, Tu-144 aircraft, Concorde, Chernobyl nuclear power plant, spaceships series "Shuttle" and much, much more. 3. Development systems approach in science 3.1 Early attempts to systematize physical knowledge The first really successful attempt to systematize knowledge about ...

III Microcosm

Movement and physical interaction.

Fundamental principles of modern physics and quantum mechanics: the principle of symmetry, the principle of complementarity and uncertainty relations, the principle of superposition, the principle of correspondence. "Apophatism" in the description of the structure and mechanics of the microworld.

Theological understanding of the tendencies towards the construction of the "Theory of Everything".

Literature for study:

1. Barbour I. Religion and Science: History and Present. - M .: The Bible-Theological Institute of St. ap. Andrey, 2001. - S. 199-216; 230-238; 253-256. (Electronic resource: http://www.mpda.ru/publ/text/59427.html)

2. A.A. Gorelov Concepts modern natural science... - M .: Higher education, 2006. - C. 110-120.

3. Green B. An elegant universe. Superstrings, hidden dimensions and the search for a definitive theory: Per. from English - M .: KomKniga, 2007.

4. Green B. The fabric of the cosmos: Space, time and the texture of reality: Per. from English - M .: URSS, 2009.

5. A.I. Osipov The path of reason in search of truth. - SPb .: Satis, 2007 .-- S. 100-110.

6. Sadokhin A.P. Concepts of modern natural science: a course of lectures. - M .: Omega-L, 2006 .-- S. 64-78.

7. Feynman R., The nature of physical laws. - M .: Nauka, 1987. (Electronic resource: http://vivovoco.rsl.ru/VV/Q_PROJECT/FEYNMAN/CONT.HTM)

The history of the discovery of elementary particles: atoms, hadrons, quarks, strings.

According to the ancient Greek philosophers Leucippus (Λεύκιππος, V century BC) and Democritus (Δημόκριτος; c. 460 BC - c. 370 BC) - the founders of atomism, the world is based on atoms- the smallest indivisible particles that stick together and form all living and nonliving.

By the 18th century. it became clear that the atom is elementary chemically indivisible particle, while molecule, - an elementary particle of a substance that preserves its properties, - consists of certain "sorts" of atoms. Atoms of the same type were named elements. In 1869 Dmitry Ivanovich Mendeleev created his Periodic table, including 64 elements (as of October 2009, 117 chemical elements with serial numbers from 1 to 116 and 118, of which 94 are found in nature (some are only in trace amounts), the remaining 23 are obtained artificially as a result nuclear reactions).

However, already in the 1910s. physicists come to the conclusion that the atom is divisible (ἄτομος - indivisible!). A number of models of the atom are created, of which the "planetary" model of the atom with the amended postulates has won recognition (E. Rutherford, Ernest Rutherford; 1871 - 1937, N. Bohr, Niels Bohr; 1885 - 1962).

The planetary model of the atom was soon found unsuitable due to a fundamental contradiction with the fact of the linear nature of the radiation spectrum: an electron rotating around a positively charged nucleus continuously emits, that is, it loses energy and soon must inevitably "fall" onto the nucleus. The situation was corrected by Bohr's postulates, in which the electron could not continuously lose energy, the radiation occurs as a result of a jump-like transition to the lower orbit. Creature quantum theory atom in the 1920s showed that Bohr's postulates must be abandoned. At the same time, the concept of the atomic nucleus remained the same as after Rutherford's experiments on scattering alpha particles at the beginning of the 20th century: the nucleus consists of protons and some, fewer electrons. The neutron was discovered by the English physicist J. Chadwick (1891 - 1974) in 1932. Then came the next act of drama. It was believed that the electron that escapes from the nucleus during beta decay is one of the electrons that were in the nucleus. But now it was already known that the nucleus consists of protons and neutrons. Where does the electron come from? The outstanding Italian physicist E. Fermi (Enrico Fermi; 1901 - 1954) put forward a paradoxical hypothesis. There are no electrons in the nucleus, during decay, an electron is born, and the neutron turns into a proton. Such a solution to the question seemed so unacceptable that the reputable journal Nature refused to publish Fermi's article on this topic. This is the first precedent for the birth of particles from energy. The chain of strange ideas did not end there. The Japanese theoretical physicist Hideki Yukawa (1907 - 1981) built a simple physical model in which, as a result of the exchange of nucleons by a particle with a nonzero mass, a force arises that keeps the nucleons in the nucleus. Yukawa also calculated the mass of this "virtual" particle. However, according to the concepts of physicists of that time, a particle can be recognized as existing if it is also found in a free state. Searches were undertaken for a Yukawa particle in cosmic rays, and seemingly the particle was found. However, the found particle had a lower mass than the Yukawa particle. In addition, data appeared that the found particle is similar to an electron, but heavier. Later, the particle was named mu-meson (Greek μέσος - middle). The search continued, and in the forties another completely suitable particle was found (it was called the pi-meson). In 1948, Yukawa received the Nobel Prize.

Thus, physicists realized the possibility of the existence of particles in a virtual state, i.e., when the nucleus splits, the particle is not detected, but actually provides the mutual attraction of nucleons in the nucleus. It turned out that not only atoms are indivisible, but also the "bricks" that make up their nuclei - protons and neutrons.

In the 1960s. it was proved that these particles also consist of even smaller particles with a fractional positive or negative charge ( 1 /3rd e or 2/3 e) - quarks... The hypothesis that "elementary" particles are built from specific subunits was first put forward by the American physicists M. Gell-Mann (born in 1929) and J. Zweig (born in 1937) in 1964 year. In the period from 1969 to 1994. managed to substantiate experimentally, at least indirectly, the possibility of the existence of quarks.

The word "quark" was borrowed by Gell-Mann from fiction novel J. Joyce's "Finnegans Wake", where one of the episodes contains the phrase "Three quarks for Muster Mark!" (usually translated as "Three quarks for M. Mark!"). The very word "quark" in this phrase is supposedly an onomatopoeia for the cry of seabirds or means something like "nonsense" in German slang.

Quarks do not exist autonomously, "by themselves", but only in a system - an "elementary" particle (proton, neutron, etc.), and are described by such specific parameters as "flavor" (6 types, see the diagram) and " color "(" red, "blue", "green", "anti-red", "anti-blue", "anti-green"). The total charge of 2 or 3 quarks combined into a system must be integer (0 or 1). The sum of the colors is also zero (white).

Quarks "stick" to each other due to strong physical interaction. It has been suggested that quarks are also involved in electromagnetic and weak interactions. Moreover, in the first case, the quarks do not change their color and aroma, and in the second, they change the aroma, preserving the color.

All in all, during the twentieth century, about 400 elementary particles were discovered. Some of them, as mentioned above, have a certain structure (proton, neutron), others are structureless (electron, neutrino, photon, quark).

Elementary particles have a fairly large number of parameters; therefore, there are several standard types of their classifications, given below.

1. By the rest mass of a particle (rest mass, determined in relation to the rest mass of an electron, which is considered the lightest of all particles with a mass):

photons(φῶς, φωτός - light) - particles that have no rest mass and move at the speed of light;

leptons(λεπτός - light) - light particles (electron and different types neutrino);

mesons(μέσος - medium, intermediate) - medium particles with a mass from one to a thousand electron masses;

baryons(βαρύς - heavy) - heavy particles with a mass of more than a thousand electron masses (protons, neutrons, hyperons, many resonances).

2. By electric charge, always a multiple of the fundamental unit of charge - the charge of an electron (-1), which is considered as a unit of counting charges. The particle charge can be negative, positive, or zero. As mentioned above, quarks are characterized by a fractional electric charge.

3. By the type of physical interaction (see below), in which certain elementary particles take part. According to this indicator, they can be divided into three groups:

· hadrons(ἁδρός - heavy, large, strong), participating in electromagnetic, strong and weak interactions (mesons and baryons);

· leptons, participating only in electromagnetic and weak interactions;

· particles - carriers of interactions (photons- carriers of electromagnetic interaction, gluons - carriers of strong interaction, heavy vector bosons- carriers of weak interaction, hypothetical gravitons - particles providing gravitational interaction).

4. According to the particle lifetime:

· stable "long-livers"(photon, neutrino, neutron, proton, electron; lifetime - to infinity);

· quasi-stable (resonances); the lifetime is 10 -24 -10 -26 s; decay as a result of electromagnetic and weak interactions;

· unstable(most elementary particles; their lifetime is 10 -10 - 10 -24 s).

5. On the back (from the English. spin- spindle, twirl (Xia)) - own moment momentum (momentum) of a particle, its internal degree of freedom, providing an additional physical state. Unlike the classical angular momentum, which can take any values, spin takes only five possible values. It can be an integer (0, 1, 2) or half-integer (1/2 (electron, proton, neutron), 3/2 (omega-hyperon)) number. Particles with half-integer spin are called fermions, and with integer - bosons(photons with spin 1; mesons - 0; gravitons - 2).

Each particle has its own antiparticle (substance and antimatter). When they meet, mutual destruction (annihilation) occurs and a large amount of energy is released.

The found regularities in the properties of elementary particles and their division into "families" or "generations" made it possible to raise the question of the presence of internal deep regularities that determine their properties (see the diagram).

There are theories explaining the structure of the microworld (for example, the Standard Model). In the 1970s. a very original string theory(John Henry Schwartz, Schwartz, b. 1941; G. Veneziano, Gabriele Veneziano; b. 1942; M. Green, Michael Greene, and others). String theory- the direction of mathematical physics, which studies not point particles, like many branches of physics, but one-dimensional extended geometric objects- quantum strings... The theory is based on the hypothesis that all fundamental particles and their interactions arise as a result of oscillations (excited states) and interactions of ultramicroscopic energy quantum strings on scales of the order of the so-called. Planck length 10 −33 m, just as sounds of different frequencies are generated by the vibration of the string of a musical instrument. Moreover, space and time itself are considered as derivatives of certain modes of string vibration. The universe, made up of an innumerable number of these vibrating strings, is like a sounding "cosmic symphony." Despite the resolution of a number of existing problems, string theory remains at present mainly a mathematical abstraction that requires experimental confirmation.

The discovery of elementary particles was a natural result of the general successes in the study of the structure of matter, achieved by physics at the end of the 19th century.

It was prepared by comprehensive studies of the optical spectra of atoms, the study of electrical phenomena in liquids and gases, the discovery of photoelectricity, X-rays, natural radioactivity, which testified to the existence of a complex structure of matter.

Historically, the first discovered elementary particle was an electron-carrier of a negative elementary electric charge in atoms. In 1897, J.J. Thomson established that cathode rays are formed by a stream of tiny particles, which were called electrons.

In 1911, E. Rutherford, passing alpha particles from a natural radioactive source through thin foils of various substances, found out that a positive charge in atoms is concentrated in compact formations - nuclei, and in 1919 he found protons among particles knocked out of atomic nuclei - particles with a unit positive charge and a mass 1840 times the mass of an electron. Another particle that makes up the nucleus, the neutron, was discovered in 1932 by J. Chadwick while studying the interaction of a-particles with beryllium. A neutron has a mass close to that of a proton, but does not have an electric charge. The discovery of the neutron completed the identification of particles - the structural elements of atoms and their nuclei.

The conclusion about the existence of a particle of an electromagnetic field - a photon - originates from the work of M. Planck (1900). Assuming that the energy of electromagnetic radiation from a black body is quantized, Planck obtained the correct formula for the radiation spectrum. Developing Planck's idea, A. Einstein (1905) postulated that electromagnetic radiation (light) is in fact a stream of individual quanta (photons), and on this basis he explained the laws of the photoelectric effect. Direct experimental evidence for the existence of a photon was given by R. Millikan (1912-1915) and A. Compton (1922).

From the 30s to the early 50s. the study of elementary particles was closely related to the study of cosmic rays. In 1932, in the composition of cosmic rays, K. Anderson discovered a positron (e +) - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle to be discovered. The existence of e + directly followed from the relativistic theory of the electron, developed by P. Dirac (1928-31) shortly before the discovery of the positron. In 1936, American physicists K. Anderson and S. Neddermeier discovered muons (of both signs of electric charge) while studying cosmic rays - particles with a mass of about 200 electron masses, but otherwise surprisingly similar in properties to e-, e +.

Since the beginning of the 50s. accelerators have become the main tool for the study of elementary particles. In the 70s. the energies of particles accelerated in accelerators amounted to tens and hundreds of billions of electron volts (GeV). The desire to increase the energies of particles is due to the fact that high energies open up the possibility of studying the structure of matter at the smaller distances, the higher the energy of the colliding particles. Accelerators have significantly increased the rate of obtaining new data and in a short time have expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study in more detail their properties, in particular the features of their decay, and soon led to an important discovery: the elucidation of the possibility of changing the characteristics of some microprocesses during the operation of mirror reflection - violation of spaces, parity (1956). The commissioning of proton accelerators with energies of billions of electron volts made it possible to discover heavy antiparticles: antiproton (1955), antineutron (1956), antisigma-hyperons (1960). In 1964, the heaviest hyperon W- (with a mass of about two proton masses) was discovered. In the 1960s. at accelerators, a large number of extremely unstable (in comparison with other unstable elementary particles) particles were discovered, which were called "resonances". The masses of most resonances exceed the mass of the proton. The first of these, the D1, has been around since 1953. It turned out that resonances make up the bulk of elementary particles.

In 1962, it was found that there are two different neutrinos: electron and muon. In 1964, nonconservation of the so-called. combined parity (introduced by Li Tsung-dao and Yang Zhen-ning and independently by L.D. Landau in 1956), which means the need to revise the usual views on the behavior of physical processes during the operation of time reflection.

In 1974, massive (3-4 proton masses) and at the same time relatively stable y-particles were discovered, with a lifetime unusually long for resonances. They turned out to be closely related to a new family of elementary particles - "charmed", the first representatives of which (D0, D +, Lс) were discovered in 1976. In 1975, the first information about the existence of a heavy analogue of an electron and a muon (heavy lepton t) was obtained. In 1977, Ў-particles with a mass of about ten proton masses were discovered.

Existence elementary particles scientists discovered in the study of nuclear processes, therefore, until the middle of the 20th century, elementary particle physics was a section nuclear physics... At present, these branches of physics are close, but independent, united by the commonality of many of the problems under consideration 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.

The idea that the world consists of fundamental particles , It has long history... For the first time, the idea of the existence of the smallest invisible particles that make up all surrounding objects was expressed 400 years BC by the Greek philosopher Democritus. He called these particles atoms, that is, indivisible particles. Science began to use the concept of atoms only at the beginning of the 19th century, when on this basis it was possible to explain a number of chemical phenomena. In the 30s of the XIX century, in the theory of electrolysis, developed by M. Faraday, the concept of an ion appeared and the measurement of the elementary charge was carried out. End of XIX century was marked by the discovery of the phenomenon of radioactivity (1896, A. Becquerel), as well as the discoveries of electrons (1897, J. Thomson) and α-particles (1899, E. Rutherford). In 1905, physics developed the concept of quanta of the electromagnetic field - photons (A. Einstein).

In 1911, the atomic nucleus was discovered (E. Rutherford) and it was finally proved that atoms have a complex structure. In 1919, Rutherford discovered protons in the fission products of atomic nuclei of a number of elements. In 1932, J. Chadwick discovered the neutron. It became clear that the nuclei of atoms, like the atoms themselves, have a complex structure. The proton-neutron theory of the structure of nuclei arose (D. D. Ivanenko and V. Heisenberg). In the same 1932, a positron was discovered in cosmic rays (K. Anderson). A positron is a positively charged particle with the same mass and the same (modulo) charge as an electron. The existence of the positron was predicted by P. Dirac in 1928. During these years, the mutual transformations of protons and neutrons were discovered and investigated, and it became clear that these particles are also not invariable elementary "bricks" of nature. In 1937, particles with a mass of 207 electron masses were discovered in cosmic rays, called muons (μ-mesons). Then in 1947-1950 were opened peonies (i.e. π-mesons), which, according to modern concepts, carry out the interaction between nucleons in the nucleus. In subsequent years, the number of newly discovered particles began to grow rapidly. This was facilitated by studies of cosmic rays, the development of accelerator technology, and the study of nuclear reactions.

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

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

The ability to mutually transform 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 when interacting with other particles. An example is annihilation (i.e. disappearance) electron and positron, accompanied by the production of high-energy photons. The reverse process can also take place - birth electron-positron pair, for example, when a photon of sufficiently high energy collides with a nucleus. The proton also has such a dangerous double as the positron for the electron. It is called antiproton ... The electric charge of the antiproton is negative. Currently antiparticles found on all particles. Antiparticles are opposed to particles because when any particle meets its antiparticle, they annihilate, that is, both particles disappear, turning into radiation quanta or other particles.

Even a neutron has been found to have an antiparticle. The neutron and antineutron differ only in the signs of the magnetic moment and the so-called baryon charge. The existence of atoms is possible antimatter, the nuclei of which are composed of antinucleons, and the shell, of positrons. During the annihilation of antimatter with matter, the rest energy is converted into the energy of radiation quanta. This is a tremendous energy, significantly superior to that which is released during nuclear and thermonuclear reactions.

In the variety of elementary particles known to date, a more or less harmonious classification system is found. Table 6.9.1 presents some information about the properties of elementary particles with a lifetime of more than 10 –20 s. Of the many properties characterizing an elementary particle, only the particle mass (in electron masses), electric charge (in units of elementary charge) and angular momentum (the so-called spin ) in units of Planck's constant ħ = h/ 2π. The table also shows the average lifetime of a particle.

Elementary particles are grouped 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 (electron and muon), electron and μ-meson. Leptons also include a number of particles not listed in the table. All leptons have a spin

The third large group is made up of heavy particles called hadrons... This group is divided into two parts. Lighter particles make up a subgroup mesons ... The lightest of them are positively and negatively charged, as well as neutral π-mesons with masses of about 250 electron masses (Table 6.9.1). Peonies are quanta of a nuclear field, just as photons are quanta of an electromagnetic field. This subgroup also includes four K mesons and one η 0 meson. All mesons have zero spin.

The second subgroup - baryons - includes heavier particles. It is the most extensive. The lightest of baryons are nucleons - protons and neutrons. They are followed by the so-called hyperons. Omega-minus-hyperon, discovered in 1964, closes the table. It is a heavy particle with a mass of 3273 electron masses. All baryons have a spin

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-Mann put forward a hypothesis, confirmed by subsequent studies, that all heavy particles - hadrons - are built from more fundamental particles called quarks ... On the basis of 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, connecting with each other in various combinations. Thus, each baryon consists of three quarks, and an antibaryon consists of three antiquarks. Mesons are composed of quark – antiquark pairs.

With the adoption of the quark hypothesis, it was possible to create a harmonious system of elementary particles. However, the predicted properties of these hypothetical particles turned out to be rather unexpected. The electric charge of quarks should be expressed fractional numbers, equal and elementary charge.

Numerous searches for quarks in a free state, carried out at high-energy accelerators and in cosmic rays, were unsuccessful. Scientists believe that one of the reasons for the unobservability of free quarks is, possibly, their very large masses. This prevents the production of quarks at the energies that are achieved with modern accelerators. Nevertheless, most experts are now convinced that quarks exist inside heavy particles - hadrons.