Home Vegetables The main tool for the study of elementary particles. The history of the discovery of elementary particles: atoms, hadrons, quarks, strings. A photon is an "animated" Planck quantum of light, i.e. quantum of light carrying momentum

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The main tool for the study of elementary particles. The history of the discovery of elementary particles: atoms, hadrons, quarks, strings. A photon is an "animated" Planck quantum of light, i.e. quantum of light carrying momentum

Introduction

1. Discovery of elementary particles

2. Theories of elementary particles

2.1. Quantum electrodynamics (QED)

2.2. Quark theory

2.3. Theory of the electroweak interaction

2.4. quantum chromodynamics

Conclusion

Literature

Introduction.

In the middle and second half of the 20th century, truly amazing results were obtained in those branches of physics that are occupied with the study of the fundamental structure of matter. 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 the nuclei. But there are also particles that practically do not occur in the matter surrounding us. Their lifetime is extremely short, it is the smallest fraction of a second. After this extremely short time, they decay into ordinary particles. There are astonishingly many such unstable short-lived particles: several hundred of them are already known.

In the 1960s and 1970s, physicists were completely bewildered by the abundance, variety, and unusualness of newly discovered subatomic particles. There seemed to be no end to them. It is completely incomprehensible why so many particles. 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 showed that there is no doubt about the existence of such a structure. At the end of the twentieth century. physics begins to understand what is the significance of each of the elementary particles.

The world of subatomic particles has a deep and rational order. 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 progress 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 the electron - the carrier of the 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, 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 the positive charge in atoms is concentrated in compact formations - nuclei, and in 1919 he discovered protons - particles with 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. The neutron has a mass close to that of the proton, but has no electrical 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 an electromagnetic field particle - a photon - originates from the work of M. Planck (1900). Assuming that the energy of the electromagnetic radiation of an absolutely 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 actually a stream of individual quanta (photons), and on this basis explained the laws of the photoelectric effect. Direct experimental evidence for the existence of the photon was given by R. Millikan (1912-1915) and A. Compton (1922).

The discovery of the neutrino, a particle that almost does not interact with matter, originates from the theoretical conjecture of W. Pauli (1930), which made it possible, by assuming the birth of such a particle, 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 connected with the study of cosmic rays. In 1932, in the composition of cosmic rays, K. Anderson discovered the positron (e +) - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle discovered. The existence of e+ followed directly from the relativistic theory of the electron developed by P. Dirac (1928-31) shortly before the discovery of the positron. In 1936, the American physicists K. Anderson and S. Neddermeyer discovered muons (of both signs of electric charge) in the study of 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 flows 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.

From the beginning of the 50s. accelerators have become the main tool for the study of elementary particles. In the 70s. the energies of the particles dispersed 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 shorter distances, the higher the energy of the colliding particles. Accelerators significantly increased the rate of obtaining new data and in a short time expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study their properties in more detail, 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: the antiproton (1955), the antineutron (1956), and the antisigma hyperons (1960). In 1964, the heaviest hyperon W- was discovered (with a mass of about two proton masses). In the 1960s at accelerators, a large number of extremely unstable (compared to 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 them D1 (1232) has been known since 1953. It turned out that resonances constitute the main part of elementary particles.

In 1962, it was found that there are two different neutrinos: electron and muon. In 1964, in the decays of neutral K-mesons, the so-called nonconservation was discovered. combined parity (introduced by Li Tsung-tao and Yang Chen-ning and independently by L. D. Landau in 1956), which means the need to revise the usual views on the behavior of physical processes in 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+, Lc) were discovered in 1976. In 1975, the first information about the existence of a heavy analogue of the electron and muon (heavy lepton t) was obtained. In 1977, Ў-particles with a mass on the order of 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 revealed. The world of elementary particles turned out to be rather complicated. The properties of the discovered elementary particles turned out to be unexpected in many respects. To describe them, in addition to the 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 above characteristics reflect the unusual nature of the properties of elementary particles they describe.

And the desired values. 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 elementary particle models 2.1 Three stages in the development of elementary particle physics 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: Newton's mechanics, or a far from optimal or perfect creation of technology: the Titanic liner, Tu-144 aircraft, Concorde, the Chernobyl nuclear power plant, spacecraft of the Shuttle series and much, much more. 3. Development of a systematic approach in science 3.1 Early attempts to systematize physical knowledge The first really successful attempt to systematize knowledge about ...

PLAN

Introduction

1. Discovery of elementary particles

2. Theories of elementary particles

2.1. Quantum electrodynamics (QED)

2.2. Quark theory

2.3. Theory of the electroweak interaction

2.4. quantum chromodynamics

Conclusion

Literature

Introduction.

The world of subatomic particles has a deep and rational order. This order is based on fundamental physical interactions.

1. Discovery of elementary particles.

2. Theories of elementary particles

2.1. Quantum electrodynamics (QED)

Quantum mechanics makes it possible to describe the motion of elementary particles, but not their generation or annihilation, i.e., it is used only to describe systems with a constant number of particles. A generalization of quantum mechanics is quantum field theory - it is a quantum theory of systems with an infinite number of degrees of freedom (physical fields). The need for such a theory is generated by quantum-wave dualism, the existence of wave properties in all particles. In quantum field theory, the interaction is presented as the result of the exchange of field quanta.

In the middle of the twentieth century. the theory of electromagnetic interaction was created - quantum electrodynamics of QED is a theory of interaction of photons and electrons, thought out to the smallest detail and equipped with a perfect mathematical apparatus. QED is based on the description of electromagnetic interaction using the concept of virtual photons - its carriers. This theory satisfies the basic principles of both quantum theory and relativity.

At the center of the theory is the analysis of the acts of emission or absorption of one photon by one charged particle, as well as the annihilation of an electron-positron pair into a photon or the generation of such a pair by photons.

If in the classical description electrons are represented as a solid point ball, then in QED the electromagnetic field surrounding the electron is considered as a cloud of virtual photons that relentlessly follows the electron, surrounding it with energy quanta. After an electron emits a photon, it creates a (virtual) electron-positron pore that can annihilate to form a new photon. The latter can be absorbed by the original photon, but it can give rise to a new pair, and so on. Thus, the electron is covered with a cloud of virtual photons, electrons and positrons, which are in a state of dynamic equilibrium. Photons appear and disappear very quickly, and electrons move in space along not quite definite trajectories. It is still possible to determine in one way or another the starting and ending points of the path - before and after scattering, but the path itself in the interval between the beginning and the end of the movement remains undefined.

The description of the interaction with the help of a carrier particle led to an extension of the concept of a photon. The concepts of a real (a quantum of light visible to us) and a virtual (transient, ghostly) photon are introduced, which are "seen" only by charged particles undergoing scattering.

To test whether the theory agrees with reality, physicists focused on two effects of particular interest. The first concerned the energy levels of the hydrogen atom, the simplest atom. According to QED, the levels should be slightly shifted relative to the position they would occupy in the absence of virtual photons. The second decisive test of QED concerned an extremely small correction to the electron's own magnetic moment. Theoretical and experimental results of QED verification coincide with the highest accuracy - more than nine decimal places. Such a striking correspondence gives the right to consider QED as the most perfect of the existing natural science theories.

After a similar triumph, QED was adopted as the model for the quantum description of three other fundamental interactions. Of course, the fields associated with other interactions must correspond to other carrier particles.

2.2. Quark theory

The theory of quarks is the theory of the structure of hadrons. The basic idea of this theory is very simple. All hadrons are built from smaller particles called quarks. This means that quarks are more elementary particles than hadrons. Quarks carry a fractional electrical charge: they have a charge that is either -1/3 or +2/3 of the fundamental unit, the charge of the electron. A combination of two and three quarks can have a total charge equal to zero or one. All quarks have spin S, so they are fermions. The founders of the theory of quarks Gell-Mann and Zweig, to take into account all known in the 60s. hadrons introduced three varieties (flavors) of quarks: u (from up- top), d (from down- bottom) and s (from strange - strange).

Quarks can combine with each other in one of two possible ways: either in triplets or in quark-antiquark pairs. Three quarks make up relatively heavy particles - baryons, which means "heavy particles". The best known baryons are the neutron and the proton. Lighter quark-antiquark pairs form particles called mesons - "intermediate particles". For example, a proton consists of two u- and one d-quark (uud), and a neutron consists of two d-quarks and one u-quark (udd). In order for this "trio" of quarks not to decay, a force holding them is needed, a certain " glue".

It turned out that the resulting interaction between neutrons and protons in the nucleus is simply a residual effect of a more powerful interaction between the quarks themselves. This explained why the strong force seems so complicated. When a proton "sticks" to a neutron or another proton, six quarks are involved in the interaction, each of which interacts with all the others. A significant part of the forces is spent on strong gluing of a trio of quarks, and a small part is spent on bonding two trios of quarks to each other. (But it turned out that quarks also participate in weak interactions. A weak interaction can change the flavor of a quark. This is exactly how the decay of a neutron occurs. One of the d-quarks in the neutron turns into a u-quark, and the excess charge carries away the electron that is born at the same time. Similarly, changing flavor, weak interaction leads to the decay of other hadrons.)

The fact that all known hadrons can be obtained from various combinations of the three basic particles was a triumph for the theory of quarks. But in the 70s. new hadrons were discovered (psi-particles, upsilon meson, etc.). This dealt a blow to the first version of the theory of quarks, since there was no room for a single new particle in it. All possible combinations of quarks and their antiquarks have already been exhausted.

The problem was solved by introducing three new flavors. They got the name - charm (charm), or with; b-quark (from bottom - bottom, and more often beauty - beauty, or charm); subsequently, another flavor was introduced - t (from top - top).

Quarks are held together by a strong force. The carriers of the strong interaction are gluons (color charges). The field of elementary particle physics that studies the interaction of quarks and gluons is called quantum chromodynamics. Just as quantum electrodynamics is the theory of electromagnetic interaction, so quantum chromodynamics is the theory of strong interaction.

Although there is some dissatisfaction with the quark scheme, most physicists consider quarks to be truly elementary particles - pointlike, indivisible, and without internal structure. In this respect they resemble leptons, and it has long been assumed that there must be a deep relationship between these two distinct but structurally similar families.

Thus, the most probable number of truly elementary particles (excluding carriers of fundamental interactions) at the end of the 20th century is 48. Of these: leptons (6x2) = 12 plus quarks (6x3)x2 = 36.

2.3. Theory of the electroweak interaction

In the 70s of the twentieth century, an outstanding event took place in natural science: two interactions from four physicists were combined into one. The picture of the fundamental foundations of nature has become somewhat simplified. Electromagnetic and weak interactions, seemingly very different in nature, in reality turned out to be two varieties of a single so-called. electroweak interaction. The theory of the electroweak interaction decisively influenced the further development of elementary particle physics at the end of the 20th century.

The main idea in constructing this theory was to describe the weak interaction in terms of the gauge field concept, according to which the key to understanding the nature of interactions is symmetry. One of the fundamental ideas in physics of the second half of the twentieth century. is the belief that all interactions exist only to maintain a certain set of abstract symmetries in nature. What does symmetry have to do with fundamental interactions? At first glance, the very assumption of the existence of such a connection seems paradoxical and incomprehensible.

First of all, about what is meant by symmetry. It is generally accepted that an object has symmetry if the object remains unchanged as a result of one or another operation to transform it. Thus, a sphere is symmetrical because it looks the same when rotated through any angle from its center. The laws of electricity are symmetrical with respect to the replacement of positive charges by negative ones and vice versa. Thus, by symmetry we mean invariance with respect to some operation.

There are different types of symmetries: geometric, mirror, non-geometric. Among the non-geometric ones there are the so-called gauge symmetries. Gauge symmetries are abstract and are not fixed directly. They are associated with a change in the countdown level, scale or value some physical quantity . A system has gauge symmetry if its nature remains unchanged under this kind of transformation. So, for example, in physics, the work depends on the difference in heights, and not on the absolute height; voltage - from the potential difference, and not from their absolute values, etc. Symmetries, on which the revision of the understanding of the four fundamental interactions is based, are precisely of this kind. Gauge transformations can be global or local. Gauge transformations that vary from point to point are known as "local" gauge transformations. There are a number of local gauge symmetries in nature and an appropriate number of fields are needed to compensate for these gauge transformations. Force fields can be viewed as a means by which nature creates its inherent local gauge symmetries. The significance of the concept of gauge symmetry lies in the fact that, thanks to it, all four fundamental interactions that occur in nature are theoretically modeled. All of them can be considered as gauge fields.

Representing the weak interaction as a gauge field, physicists proceed from the fact that all particles participating in the weak interaction serve as sources of a new type of field - the field of weak forces. Weakly interacting particles, such as electrons and neutrinos, carry a "weak charge" that is analogous to an electric charge and associates these particles with a weak field.

To represent the weak interaction field as a gauge field, it is first necessary to establish the exact form of the corresponding gauge symmetry. The fact is that the symmetry of the weak interaction is much more complicated than the electromagnetic one. After all, the very mechanism of this interaction is more complex. First, in the decay of a neutron, for example, particles of at least four different types (neutron, proton, electron, and neutrino) participate in the weak interaction. Secondly, the action of weak forces leads to a change in their nature (the transformation of some particles into others due to the weak interaction). On the contrary, the electromagnetic interaction does not change the nature of the particles participating in it.

This determines the fact that the weak interaction corresponds to a more complex gauge symmetry associated with a change in the nature of the particles. It turned out that three new force fields are needed to maintain symmetry here, in contrast to a single electromagnetic field. A quantum description of these three fields was also obtained: there must be three new types of particles - interaction carriers, one for each field. All of them are called heavy vector bosons with spin 1 and are carriers of the weak interaction.

Particles W + and W - are carriers of two of the three fields associated with the weak interaction. The third field corresponds to an electrically neutral carrier particle called the Z-particle. The existence of the Z-particle means that the weak interaction may not be accompanied by the transfer of electric charge.

The concept of spontaneous symmetry breaking played a key role in the creation of the theory of electroweak interaction: not every solution of a problem must have all the properties of its initial level. Thus, particles that are completely different at low energies may actually be the same particle at high energies, but in different states. Based on the idea of spontaneous symmetry breaking, the authors of the electroweak interaction theory, Weinberg and Salam, managed to solve a great theoretical problem - they combined seemingly incompatible things (a significant mass of weak interaction carriers, on the one hand, and the idea of gauge invariance, which implies the long-range nature of the gauge field, and means zero rest mass of carrier particles, on the other) and thus combined electromagnetism and weak interaction in a unified theory of the gauge field.

In this theory, only four fields are represented: the electromagnetic field and three fields corresponding to weak interactions. In addition, a scalar field (so-called Higgs fields) that is constant throughout space has been introduced, with which particles interact in different ways, which determines the difference in their masses. (The quanta of the scalar field are new elementary particles with zero spin. They are called Higgs (after the physicist P. Higgs, who suggested their existence). The number of such Higgs bosons can reach several tens. Such bosons have not yet been experimentally discovered. physicists consider their existence optional, but a perfect theoretical model without Higgs bosons has not yet been found) Initially, W and Z quanta have no mass, but symmetry breaking causes some Higgs particles to merge with W and Z particles, endowing them with mass.

Differences in the properties of electromagnetic and weak interactions are explained by the theory as symmetry breaking. If the symmetry were not broken, then both interactions would be comparable in magnitude. Symmetry breaking entails a sharp decrease in the weak interaction. We can say that the weak interaction is so small because the W and Z particles are very massive. Leptons rarely approach such small distances (r< 1 0 n см., где n = - 1 6). Но при больших энергиях (>1 0 0 GeV), when W and Z particles can be freely produced, the exchange of W and Z bosons is just as easy as the exchange of photons (massless particles). The difference between photons and bosons is erased. Under these conditions, there should be a complete symmetry between the electromagnetic and weak interactions - the electroweak interaction.

The test of the new theory was to confirm the existence of hypothetical W and Z particles. Their discovery became possible only with the creation of very large accelerators of the latest type. The discovery in 1983 of the W and Z particles signified the triumph of the theory of the electroweak interaction. There was no more need to talk about the four fundamental interactions. There are three left.

2.4. quantum chromodynamics

The next step on the path of the Grand Unification of fundamental interactions is the merging of the strong interaction with the electroweak one. To do this, it is necessary to give the features of a gauge field to the strong interaction and introduce a generalized idea of isotopic symmetry. The strong interaction can be thought of as the result of the exchange of gluons, which ensures the bonding of quarks (in pairs or triplets) into hadrons.

The idea here is the following. Each quark has an analogue of electric charge, which serves as a source of the gluon field. It was called a color (Of course, this name has nothing to do with the usual color). If the electromagnetic field is generated by only one kind of charge, then three different color charges were required to create a more complex gluon field. Each quark is "coloured" in one of three possible colors, which, quite arbitrarily, have been called red, green, and blue. And accordingly, antiquarks are anti-red, anti-green and anti-blue.

At the next stage, the theory of the strong interaction develops along the same lines as the theory of the weak interaction. The requirement of local gauge symmetry (ie invariance with respect to color changes at each point in space) leads to the need to introduce compensating force fields. A total of eight new compensating force fields are required. The particles that carry these fields are gluons, and thus it follows from the theory that there must be as many as eight different types of gluons. (While the carrier of the electromagnetic interaction is only one (photon), and the carriers of the weak interaction are three.) Gluons have zero rest mass and spin 1. Gluons also have different colors, but not pure, but mixed (for example, blue- anti-green). Therefore, the emission or absorption of a gluon is accompanied by a change in the color of the quark ("play of colors"). So, for example, a red quark, losing a red-anti-blue gluon, turns into a blue quark, and a green quark, absorbing a blue-anti-green gluon, turns into a blue quark. In a proton, for example, three quarks are constantly exchanging gluons, changing their color. However, such changes are not arbitrary, but obey a strict rule: at any moment in time, the "total" color of the three quarks must be white light, i.e. sum "red + green + blue". This also applies to mesons, consisting of a quark-antiquark pair. Since an antiquark is characterized by anticolor, such a combination is obviously colorless ("white"), for example, a red quark in combination with an antired quark forms a colorless meson.

From the point of view of quantum chromodynamics (quantum color theory), strong interaction is nothing more than the desire to maintain a certain abstract symmetry of nature: the preservation of the white color of all hadrons while changing the color of their constituent parts. Quantum chromodynamics perfectly explains the rules that all combinations of quarks obey, the interaction of gluons with each other (a gluon can decay into two gluons or merge two gluons into one - this is why nonlinear terms appear in the gluon field equation), the complex structure of a hadron, consisting of "dressed" into clouds of quarks, etc.

It may be premature to evaluate quantum chromodynamics as the final and complete theory of the strong force, but its achievements are nevertheless promising.

Conclusion.

The origin of many properties of elementary particles and the nature of their inherent interactions remain largely unclear. Perhaps, more than one restructuring of all representations and a much deeper understanding of the relationship between the properties of microparticles and the geometric properties of space-time will be needed before the theory of elementary particles will be built.

LITERATURE

Alekseev V.P. The formation of mankind. M., 1984. Bohr N. Atomic physics and human knowledge. M., 1961 Born M. Einstein's theory of relativity. M., 1964.

Dorfman Ya.G. World history of physics from the beginning of the 19th century to the middle of the 20th century. M., 1979.

Kaempfer F. Way to modern physics. M., 1972.

Naidysh V.M. Concepts of modern natural science. Tutorial. M., 1999.

Bazhenov L.B. Structure and functions of natural science theory. M., 1978.

Rosenthal I.L. Elementary particles and structure of the Universe. M, 1984.

From electron to neutrino

Electron

Positron

Neutrino

From weirdness to charm

Discovery of strange particles

Resonances

"Charmed" Particles

Conclusion

Literature

Introduction.

The discovery of elementary particles was a natural result of the general progress 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.

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

The world of subatomic particles has a deep and rational order. This order is based on fundamental physical interactions

Elementary particles in the exact meaning of this term are primary, further indecomposable particles, of which, by assumption, all matter consists. The concept of “Elementary particles” in modern physics expresses the idea of primitive entities that determine all the known properties of the material world, an idea that originated in the early stages of the formation of natural science and has always played an important role in its development.

The concept of "elementary particles" was formed in close connection with the establishment of the discrete nature of the structure of matter at the microscopic level. Discovery at the turn of the 19th-20th centuries. the smallest carriers of the properties of matter - molecules and atoms - and the establishment of the fact that molecules are built from atoms, for the first time made it possible to describe all known substances as combinations of a finite, albeit large, number of structural components - atoms. The subsequent discovery of the presence of constituent constituents of atoms - electrons and nuclei, the establishment of the complex nature of nuclei, which turned out to be built from only two types of particles (protons and neutrons), significantly reduced the number of discrete elements that form the properties of matter, and gave reason to assume that the chain of constituent parts of matter ends with discrete structureless formations - elementary particles. Such an assumption, generally speaking, is an extrapolation of known facts and cannot be justified in any way. It is impossible to assert with certainty that particles that are elementary in the sense of the above definition exist. Protons and neutrons, for example, which for a long time were considered elementary particles, as it turned out, have a complex structure. It is possible that the sequence of structural components of matter is fundamentally infinite. It may also turn out that the statement “consists of...” at some stage of the study of matter will be devoid of content. In this case, the definition of “elementary” given above will have to be abandoned. The existence of elementary particles is a kind of postulate, and verification of its validity is one of the most important tasks of physics.

From electron to neutrino

Electron

Historically, the first discovered elementary particle was the electron - the carrier of the negative elementary electric charge in atoms

This is the "oldest" elementary particle. In ideological terms, he entered physics in 1881, when Helmholtz, in a speech in honor of Faraday, pointed out that the atomic structure of matter, together with Faraday's laws of electrolysis, inevitably leads to the idea that the electric charge must always be a multiple of some elementary charge, i.e. to the conclusion about the quantization of electric charge. The carrier of the negative elementary charge, as we now know, is the electron

Maxwell, who created the fundamental theory of electrical and magnetic phenomena and used in a significant way the experimental results of Faraday, did not accept the hypothesis of atomic electricity.

Meanwhile, the "temporary" theory of the existence of the electron was confirmed in 1897 in the experiments of J. J. Thomson, in which he identified the so-called cathode rays with electrons and measured the charge and mass of the electron. Thomson called the particles of cathode rays "corpuscles" or primordial atoms. The word "electron" was originally used to denote the magnitude of the charge of the "corpuscle". And only over time, the particle itself began to be called an electron.

However, the idea of the electron was not immediately accepted. When, in a lecture at the Royal Society, J. J. Thomson, the discoverer of the electron, suggested that the particles of cathode rays should be considered as possible components of the atom, some of his colleagues sincerely believed that he was mystifying them. Planck himself admitted in 1925 that he did not fully believe then, in 1900, in the hypothesis of the electron

We can say that after the experiments of Millikan, who measured in 1911. charges of individual electrons, this first elementary particle got the right to exist

Photon

Direct experimental proof of the existence of the photon was given by R. Millikan in 1912-1915. in his studies of the photoelectric effect, as well as A. Compton in 1922, who discovered the scattering of X-rays with a change in their frequency

A photon is, in a sense, a special particle. The fact is that its rest mass, unlike other particles (except for neutrinos), is equal to zero. Therefore, it was not immediately considered a particle: at first it was believed that the presence of a finite and non-zero rest mass is a mandatory feature of an elementary particle

A photon is an "animated" Planck quantum of light, i.e. a quantum of light that carries momentum

Light quanta were introduced by Planck in 1901 in order to explain the laws of radiation of a completely black body. But he was not particles, but only the minimum possible "portions" of light energy of one frequency or another.

Although Planck's assumption about quantizing the energy of light was absolutely contrary to all classical theory, Planck himself did not immediately understand this. The scientist wrote that he “… tried to somehow introduce the value of h into the framework of the classical theory. However, despite all such attempts, this value turned out to be very obstinate. Subsequently, this value was called Planck's constant (h \u003d 6 * 10 -27 erg.s)

After the introduction of the Planck constant, the situation did not become clearer.

Photons or quanta were made “alive” by Einstein’s theory of relativity, who in 1905 showed that quanta must have not only energy, but also momentum, and that they are particles in the full sense, only special, since their rest mass is zero, and they move at the speed of light

So, the conclusion about the existence of an electromagnetic field particle - a photon - originates from the work of M. Planck (1900). Assuming that the energy of the electromagnetic radiation of an absolutely 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 actually a stream of individual quanta (photons), and on this basis explained the patterns of the photoelectric effect

Proton

The proton was discovered by E. Rutherford in 1919 in studies of the interaction of alpha particles with atomic nuclei

More precisely, the discovery of the proton is associated with the discovery of the atomic nucleus. It was made by Rutherford by bombarding nitrogen atoms with high-energy alpha particles. Rutherford concluded that "the nucleus of the nitrogen atom disintegrates as a result of the enormous forces developing in collision with a fast α-particle, and that the liberated hydrogen atom forms an integral part of the nitrogen nucleus." In 1920, the nuclei of the hydrogen atom were named protons by Rutherford (proton in Greek means the simplest, primary). There were other suggestions for a name. So, for example, the name "baron" was proposed (baros in Greek means heaviness). However, it emphasized only one feature of the hydrogen nucleus - its mass. The term "proton" was much deeper and more meaningful, reflecting the fundamental nature of the proton, because the proton is the simplest nucleus - the nucleus of the lightest isotope of hydrogen. This is undoubtedly one of the most successful terms in elementary particle physics. Thus, protons are particles with a unit positive charge and a mass 1840 times the mass of an electron.

Neutron

Another particle that makes up the nucleus, the neutron, was discovered in 1932 by J. Chadwick while studying the interaction of alpha particles with beryllium. The neutron has a mass close to that of the proton, but has no electrical charge. The discovery of the neutron completed the identification of particles - the structural elements of atoms and their nuclei

The discovery of isotopes did not clarify the question of the structure of the nucleus. By this time, only protons were known - hydrogen nuclei, and electrons, and therefore it was natural to try to explain the existence of isotopes by various combinations of these positively and negatively charged particles. One might think that the nuclei contain A protons, where A is the mass number and A ? Z electrons. In this case, the total positive charge coincides with the atomic number Z

Such a simple picture of a homogeneous nucleus at first did not contradict the conclusion about the small size of the nucleus, which followed from Rutherford's experiments. The “natural radius” of an electron r0 \u003d e 2 /mc 2 (which is obtained by equating the electrostatic energy e 2 /r0 of the charge distributed over the spherical shell to the self-energy of the electron mc 2) is r0 \u003d 2.82 * 10 -15 m. Such the electron is small enough to be inside a nucleus with a radius of 10–14 m, although it would be difficult to place a large number of particles there. In 1920 Rutherford and others considered the possibility of a stable combination of a proton and an electron, reproducing a neutral particle with a mass approximately equal to that of a proton. However, due to the lack of an electrical charge, such particles would be difficult to detect. It is unlikely that they could also knock out electrons from metal surfaces, like electromagnetic waves during the photoelectric effect.

It was not until a decade later, after natural radioactivity had been thoroughly investigated and radioactive radiation began to be widely used to cause artificial transformation of atoms, that the existence of a new constituent of the nucleus was reliably established. In 1930, W. Bothe and G. Becker from the University of Giessen irradiated lithium and beryllium with alpha particles and, using a Geiger counter, recorded the resulting penetrating radiation. Since this radiation was not affected by electric and magnetic fields, and it had a high penetrating power, the authors concluded that hard gamma radiation was emitted. In 1932, F. Joliot and I. Curie repeated experiments with beryllium, passing such penetrating radiation through a paraffin block. They found that unusually high energy protons were emitted from the paraffin and concluded that the gamma radiation passing through the paraffin produced protons as a result of scattering. (In 1923 it was found that X-rays scatter on electrons, giving the Compton effect.)

J. Chadwick repeated the experiment. He also used paraffin and, using an ionization chamber, in which the charge generated when electrons were knocked out of atoms, was collected, he measured the range of recoil protons.

Chadwick also used gaseous nitrogen (in a cloud chamber where water droplets condense along the trail of a charged particle) to absorb radiation and measure the range of nitrogen recoil atoms. Applying the laws of conservation of energy and momentum to the results of both experiments, he came to the conclusion that the detected neutral radiation is not gamma radiation, but a stream of particles with a mass close to that of a proton. Chadwick also showed that known sources of gamma radiation do not knock out protons.

Thus, the existence of a new particle, which is now called the neutron, was confirmed.

The splitting of metallic beryllium proceeded as follows:

Alpha particles 4 2 He (charge 2, mass number 4) collided with beryllium nuclei (charge 4, mass number 9), resulting in carbon and a neutron

The discovery of the neutron was an important step forward. The observed characteristics of nuclei could now be interpreted by considering neutrons and protons as constituents of nuclei

The neutron is now known to be 0.1% heavier than the proton. Free neutrons (outside the nucleus) undergo radioactive decay, turning into a proton and an electron. This is reminiscent of the original hypothesis of a compound neutral particle. However, inside a stable nucleus, neutrons are bound to protons and do not spontaneously decay.

Positron

Beginning in the 1930s and up to the 1950s, new particles were discovered mainly in cosmic rays. In 1932, in their composition, A. Anderson discovered the first antiparticle - the positron (e +) - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle discovered. The existence of e+ followed directly 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. Neddermeyer discovered muons (both signs of electric charge) in the study of cosmic rays - particles with a mass of about 200 electron masses, but otherwise surprisingly similar in properties to e-, e +

Positrons (positive electrons) cannot exist in matter, because when they slow down, they annihilate, connecting with negative electrons. In this process, which can be considered as the reverse process of pair production, positive and negative electrons disappear, and photons are formed, to which their energy is transferred. In the annihilation of an electron and a positron, in most cases two photons are formed, much less often - one photon. Single-photon annihilation can only occur when the electron is strongly bound to the nucleus; the participation of the nucleus in this case is necessary for the conservation of momentum. Two-photon annihilation, on the contrary, can also occur with a free electron. Often the annihilation process occurs after the positron has almost completely stopped. In this case, two photons with equal energies are emitted in opposite directions

The positron was discovered by Anderson while studying cosmic rays using the cloud chamber method. The figure, which is a reproduction of a photograph taken by Anderson in a cloud chamber, shows a positive particle entering a 0.6 cm thick lead plate with a momentum of 6.3 x 107 eV/s and leaving it with a momentum of 2.3 x 107 eV/s. One can set an upper limit on the mass of this particle, assuming that it only loses energy in collisions. This limit is 20 me. Based on this and other similar photographs, Anderson hypothesized the existence of a positive particle with a mass approximately equal to that of an ordinary electron. This conclusion was soon confirmed by observations by Blackett and Occhialini in a cloud chamber. Shortly thereafter, Curie and Joliot discovered that positrons are produced by the conversion of gamma rays from radioactive sources, and are also emitted by artificial radioactive isotopes. Since the photon, being neutral, forms a pair (positron and electron), it follows from the principle of conservation of electric charge that the absolute value of the charge of the positron is equal to the charge of the electron

The first quantitative determination of the mass of the positron was made by Thibaut, who measured the ratio e/m using the trochoid method and concluded that the masses of the positron and electron differ by no more than 15%. Later experiments by Spies and Zan, who used a mass spectrographic setup, showed that the masses of the electron and positron coincide to within 2%. Still later, Dumond and co-workers measured the wavelength of the annihilation radiation with great accuracy. Accurate to experimental errors (0.2%), they obtained such a value of the wavelength, which should be expected under the assumption that the positron and electron have equal masses

The law of conservation of angular momentum as applied to the process of pair production shows that positrons have a half-integer spin and, therefore, obey Fermi statistics. It is reasonable to assume that the spin of the positron is 1/2, as is the spin of the electron

Peonies and Muons. Meson discovery

The discovery of the meson, unlike the discovery of the positron, was not the result of a single observation, but rather a conclusion from a whole series of experimental and theoretical studies.

In 1932, Rossi, using the coincidence method proposed by Bothe and Kolhurster, showed that a known fraction of the cosmic radiation observed at sea level consists of particles capable of penetrating through lead plates up to 1 m thick. Shortly thereafter, he also drew attention to the existence in cosmic rays two different components. Particles of one component (the penetrating component) are able to pass through large thicknesses of matter, and the degree of their absorption by various substances is approximately proportional to the mass of these substances. Particles of the other component (shower component) are quickly absorbed, especially by heavy elements; in this case, a large number of secondary particles (showers) are formed. Cloud chamber experiments by Anderson and Neddemeyer on the passage of cosmic ray particles through lead plates also showed that there are two distinct components of cosmic rays. These experiments showed that while the average energy loss of cosmic ray particles in lead was in order of magnitude with the theoretically calculated collision loss, some of these particles experienced much greater losses.

In 1934, Bethe and Heitler published the theory of radiative loss of electrons and the production of pairs by photons. The properties of the less penetrating component observed by Anderson and Neddemeyer were in agreement with the properties of electrons predicted by the theory of Bethe and Heitler; in this case, large losses were explained by radiation processes. The properties of the shower-forming radiation discovered by Rossi could also be explained by assuming that this radiation consists of high-energy electrons and photons. On the other hand, while recognizing the validity of the theory of Bethe and Heitler, one had to conclude that "penetrating" particles in Rossi's experiments and less absorbed particles in Anderson's and Neddemeyer's experiments differ from electrons. We had to assume that the penetrating particles are heavier than electrons, since, according to the theory, the energy loss for radiation is inversely proportional to the square of the mass

In connection with this, the possibility of the collapse of the theory of radiation at high energies was discussed. As an alternative, Williams suggested in 1934 that penetrating particles of cosmic rays might have the mass of a proton. One of the difficulties associated with this hypothesis was the necessity of the existence of not only positive, but also negative protons, because cloud chamber experiments showed that the penetrating particles of cosmic rays have charges of both signs. Moreover, in some photographs taken by Anderson and Neddemeyer in a cloud chamber, one could see particles that did not radiate like electrons, but, however, were not as heavy as protons. Thus, by the end of 1936, it became almost obvious that, in addition to electrons, cosmic rays also contained particles of a hitherto unknown type, presumably particles with a mass intermediate between that of an electron and that of a proton. It should also be noted that in 1935 Yukawa, from purely theoretical considerations, predicted the existence of such particles

The existence of intermediate mass particles was directly proven in 1937 by the experiments of Neddemeyer and Anderson, Street and Stevenson.

The experiments of Neddemeyer and Anderson were a continuation (with an improved technique) of the studies mentioned above on the energy losses of cosmic ray particles. They were carried out in a cloud chamber placed in a magnetic field and divided into two halves by a platinum plate 1 cm thick. The momentum loss for individual particles of cosmic rays was determined by measuring the track curvature before and after the plate

Absorbed particles can easily be interpreted as electrons. This interpretation is supported by the fact that, unlike penetrating particles, absorbed particles often cause secondary processes in the platinum absorber and for the most part occur in groups (two or more). This is exactly what was to be expected, since many of the electrons observed in the same experimental geometry as those of Neddemeyer and Anderson are part of the showers formed in the surrounding matter. As for the nature of penetrating particles, the following two results obtained by Neddemeyer and Anderson explained a lot here:

one). Despite the fact that absorbed particles are relatively more common at low momenta, and penetrating particles are the opposite (more frequent at high momenta), there is a momentum interval in which both absorbed and penetrating particles are represented. Thus, the difference in the behavior of these two kinds of particles cannot be attributed to the difference in energies. This result excludes the possibility of considering the penetrating particles as electrons, explaining their behavior by the injustice of the theory of radiation at high energies

2). There are a number of penetrating particles with momenta less than 200 MeV/c that produce no more ionization than a singly charged particle near the minimum of the ionization curve. This means that penetrating cosmic ray particles are much lighter than protons, since a proton with a momentum less than 200 MeV/c produces a specific ionization approximately 10 times higher than the minimum

Street and Stevenson attempted to directly estimate the mass of cosmic ray particles by simultaneously measuring momentum and specific ionization. They used a cloud chamber, which was controlled by a system of Geiger-Muller counters turned on for anticoincidences. This achieved the selection of particles close to the end of their range. The chamber was placed in a magnetic field with a strength of 3500 gauss; The chamber was triggered with a delay of about 1 second, which made it possible to count droplets. Among a large number of photographs, Street and Stevenson found one of extreme interest.

This photograph shows the trail of a particle with a momentum of 29 MeV/c, whose ionization is about six times the minimum. This particle has a negative charge as it moves downward. Judging by the momentum and specific ionization, its mass is about 175 electron masses; the probable error of 25% is due to the inaccuracy of the measurement of specific ionization. Note that an electron with a momentum of 29 MeV/c has practically minimal ionization. On the other hand, particles with this momentum and proton mass (either an upward moving ordinary proton or a downward moving negative proton) have a specific ionization that is about 200 times the minimum; in addition, the range of such a proton in the chamber gas must be less than 1 cm. At the same time, the trace in question is clearly visible for 7 cm, after which it leaves the illuminated volume

The experiments described above certainly proved that the penetrating particles are indeed heavier than electrons, but lighter than protons. In addition, Street and Stevenson's experiment gave the first rough estimate of the mass of this new particle, which we can now call by its common name, the meson.

So in 1936, A. Anderson and S. Neddermeyer discovered the muon (μ-meson). This particle differs from an electron only in its mass, which is about 200 times greater than the electron

In 1947 Powell observed traces of charged particles in photographic emulsions, which were interpreted as Yukawa mesons and named π mesons or pions. The decay products of charged pions, which are also charged particles, were called muons or muons. It was negative muons that were observed in Conversi's experiments: unlike pions, muons, like electrons, do not interact strongly with atomic nuclei.

Since muons of a strictly defined energy were always formed during the decay of stopped pions, it followed that when π turned into μ, one more neutral particle should be formed (its mass turned out to be very close to zero). On the other hand, this particle practically does not interact with matter, so it was concluded that it cannot be a photon. Thus, physicists are faced with a new neutral particle whose mass is zero

So, a charged Yukawa meson was discovered, decaying into a muon and a neutrino. The lifetime of the π-meson with respect to this decay turned out to be equal to 2·10 -8 s. Then it turned out that the muon is also unstable, that as a result of its decay, an electron is formed. The muon lifetime turned out to be on the order of 10 -6 s. Since the electron formed during the decay of the muon does not have a strictly defined energy, it was concluded that, along with the electron, two neutrinos are formed during the decay of the muon

Neutrino

During the β-decay of nuclei, as we have already said, in addition to electrons, neutrinos also fly out. This particle was first "introduced" into physics theoretically. It was the existence of the neutrino that was postulated by Pauli in 1929, many years before his experimental discovery (1956). Neutrino, a neutral particle with zero (or negligible) mass, was needed by Pauli in order to save the law of conservation of energy in the process of β-decay of atomic nuclei

Initially, Pauli called the hypothetical neutral particle formed during the β-decay of nuclei the neutron (this was before Chadwick's discovery) and suggested that it is part of the nucleus

How difficult it was to come to the hypothesis of neutrinos, which are formed in the very act of neutron decay, can be seen at least from the fact that just a year before the appearance of Fermi's fundamental article on the properties of the weak interaction, the researcher used the term "neutron" in a report on the current state of the physics of the atomic nucleus. to denote the two particles now called the neutron and the neutrino. “For example, according to Pauli's proposal,” says Fermi, “it would be possible to imagine that there are neutrons inside the atomic nucleus, which would be emitted simultaneously with β-particles. These neutrons could pass through large thicknesses of matter without losing their energy, and therefore would be practically unobservable. The existence of the neutron, no doubt, could simply explain some as yet incomprehensible questions, such as the statistics of atomic nuclei, the anomalous intrinsic moments of some nuclei, and also, perhaps, the nature of penetrating radiation. Indeed, when it comes to a particle emitted with β-electrons and poorly absorbed by matter, it is necessary to keep in mind the neutrino. It can be concluded that in 1932 the problems of the neutron and neutrino were extremely confused. It took a year of hard work by theorists and experimenters to resolve both fundamental and terminological difficulties.

“After the discovery of the neutron,” said Pauli, “at seminars in Rome, Fermi began to call my new particle emitted during β-decay “neutrino” to distinguish it from the heavy neutron. This Italian name has become generally accepted"

In the 1930s, Fermi's theory was generalized to positron decay (Wick, 1934) and to transitions with a change in the angular momentum of the nucleus (Gamow and Teller, 1937)

The "fate" of a neutrino can be compared with the "fate" of an electron. Both particles were initially hypothetical - the electron was introduced to bring the atomic structure of matter in line with the laws of electrolysis, and the neutrino was introduced to save the law of conservation of energy in the process of β-decay. And only much later they were discovered as real

From weirdness to charm

Discovery of strange particles

In 1947, Butler and Rochester observed two particles, called V particles, in a cloud chamber. Two tracks were observed, as if forming the Latin letter V. The formation of two tracks indicated that the particles were unstable and decayed into other, lighter ones. One of the V particles was neutral and decayed into two charged particles with opposite charges. (Later it was identified with the neutral K-meson, which decays into positive and negative pions). The other was charged and decayed into a charged particle with a smaller mass and a neutral particle. (Later it was identified with a charged K + meson, which decays into charged and neutral pions)

V -particles admit, at first glance, another interpretation: their appearance could be interpreted not as a decay of particles, but as a scattering process. Indeed, the processes of scattering of a charged particle by a nucleus with the formation of one charged particle in the final state, as well as inelastic scattering of a neutral particle by a nucleus with the formation of two charged particles, will look the same in a cloud chamber as the decay of V particles. But such a possibility was easily ruled out on the grounds that scattering processes are more probable in denser media. And V-events were observed not in lead, which was present in the cloud chamber, but directly in the chamber itself, which is filled with a gas with a lower density (compared to the density of lead)

Note that if the experimental discovery of the π meson was in some sense “expected” due to the need to explain the nature of nucleon interactions, then the discovery of V particles, like the discovery of the muon, turned out to be a complete surprise.

The discovery of V-particles and the determination of their most "elementary" characteristics stretched over more than a decade. After the first observation of these particles in 1947. Rochester and Butler continued their experiments for another two years, but they failed to observe a single particle. And only after the equipment was raised high into the mountains, V-particles were again discovered, as well as new particles were discovered.

As it turned out later, all these observations turned out to be observations of various decays of the same particle - the K-meson (charged or neutral)

The "behavior" of V-particles at birth and subsequent decay led to the fact that they began to be called strange

Strange particles were first obtained in the laboratory in 1954. Fowler, Shutt, Thorndike and Whitemore, who, using an ion beam from the Brookhaven cosmotron with an initial energy of 1.5 GeV, observed the reactions of associative production of strange particles

From 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 at 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 shorter distances, the higher the energy of the colliding particles. Accelerators significantly increased the rate of obtaining new data and in a short time expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study their properties in more detail, 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: the antiproton (1955), the antineutron (1956), and the antisigma hyperons (1960). In 1964, the heaviest hyperon W- was discovered (with a mass of about two proton masses)

Resonances.

In the 1960s at accelerators, a large number of extremely unstable (compared to 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 them D1 (1232) has been known since 1953. It turned out that resonances constitute the main part of elementary particles

The strong interaction of a π-meson and a nucleon in a state with a total isotopic spin 3/2 and a momentum 3/2 leads to the appearance of an excited state of the nucleon. This state within a very short time (of the order of 10 -23 s) decays into a nucleon and a π meson. Since this state has well-defined quantum numbers, as well as stable elementary particles, it was natural to call it a particle. To emphasize the very short lifetime of this state, it and similar short-lived states began to be called resonant.

Nucleon resonance, discovered by Fermi in 1952, was later called the ∆ 3/2 3/2 isobar (to highlight the fact that the spin and isotopic spin of the ∆-isobar are 3/2). Since the lifetime of resonances is insignificant, they cannot be observed directly, in the same way as "ordinary" protons, π-mesons and muons are observed (by their traces in track devices). Resonances are detected by the characteristic behavior of the scattering cross sections of particles, as well as by studying the properties of their decay products. Most of the known elementary particles belong to the group of resonances

The discovery of Δ-resonance was of great importance for elementary particle physics

Note that excited states or resonances are not absolutely new objects of physics. Previously, they were known in atomic and nuclear physics, where their existence is associated with the composite nature of the atom (formed from the nucleus and electrons) and the nucleus (formed from protons and neutrons). As for the properties of atomic states, they are determined only by the electromagnetic interaction. The low probabilities of their decay are associated with the smallness of the electromagnetic interaction constant

Excited states exist not only for the nucleon (in this case they speak of its isobaric states), but also for the π meson (in this case they speak of meson resonances)

“The reason for the appearance of resonances in strong interactions is incomprehensible,” Feynman writes, “at first, theorists did not assume that resonances exist in field theory with a large interaction constant. Later, they realized that if the interaction constant is large enough, then isobaric states arise. However, the true significance of the fact of the existence of resonances for fundamental theory remains unclear.

"Charmed" Particles

At the end of 1974 two groups of experimenters (Thing's group at the proton accelerator in Brookhaven and B. Richter's group, who worked at the installation with colliding electron-positron beams at Stanford) simultaneously made the most important discovery in elementary particle physics: they discovered a new particle - resonance with a mass equal to 3.1 GeV (exceeding three proton masses)

The most surprising property of this resonance was its small decay width - it is only 70 keV, which corresponds to a lifetime of about 10 -23 s

The generally accepted explanation of the nature of ψ-mesons is based on the hypothesis of the existence of a fourth, c-quark, along with the "standard" three u-, d-, and s-quarks. The c-quark differs from previously known quarks in the value of a new quantum number, called the charm. Therefore, the c-quark was called the charm - or charmed - quark.

In 1974, other 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+, Lc) were discovered in 1976. In 1975, the first information was obtained about the existence of a heavy analogue of the electron and muon (heavy lepton t)

Ting and Richter were awarded the Nobel Prize in Physics in 1976 for the discovery of ψ particles.

In 1977 heavier (compared to ψ-particles) neutral mesons with masses of the order of 10 GeV were discovered, i.e. more than ten times heavier than nucleons. As in the case of ψ-mesons, these mesons, called "upsilon" mesons, were observed in the reaction of production of muon pairs in proton-nuclear collisions

Conclusion

Thus, over the years that have passed since the discovery of the electron, a huge number of various microparticles of matter have been revealed. All elementary particles are characterized by exceptionally small dimensions: the linear dimensions of a nucleon and a pion are approximately equal to 10 -15 m. The theory predicts that the size of an electron should be of the order of 10 -19 m

The mass of the vast majority of particles is comparable to the mass of a proton, which in energy units is close to 1 GeV (1000 MeV)

The world of elementary particles turned out to be rather complicated. The properties of the discovered elementary particles turned out to be unexpected in many respects. To describe them, in addition to the 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 nature of the properties of elementary particles described by them

The study of the internal structure of matter and the properties of elementary particles from its first steps was accompanied by a radical revision of many established concepts and ideas. The regularities governing the behavior of matter in the small turned out to be so different from the regularities of classical mechanics and electrodynamics that they required completely new theoretical constructions for their description. Such new fundamental constructions in the theory were private (special) and general relativity (A. Einstein, 1905 and 1916; Relativity theory, Gravity) and quantum mechanics (1924-27; N. Bohr, L. de Broglie, W. Heisenberg, E. Schrödinger, M. Born). The theory of relativity and quantum mechanics marked a true revolution in the science of nature and laid the foundations for describing the phenomena of the microworld. However, to describe the processes occurring with elementary particles, quantum mechanics turned out to be insufficient. It took the next step - the quantization of classical fields (the so-called secondary quantization) and the development of quantum field theory. The most important stages on the path of its development were: the formulation of quantum electrodynamics (P. Dirac, 1929), the quantum theory of b-decay (E. Fermi, 1934), which laid the foundation for the modern theory of weak interactions, quantum mesodynamics (Yukawa, 1935). The immediate predecessor of the latter was the so-called. b-theory of nuclear forces (I. E. Tamm, D. D. Ivanenko, 1934; Strong interactions). This period ended with the creation of a consistent computing apparatus for quantum electrodynamics (S. Tomonaga, R. Feynman, J. Schwinger; 1944-49), based on the use of renormalization techniques (Quantum Field Theory). This technique was later generalized to other versions of quantum field theory.

Quantum field theory continues to develop and improve and is the basis for describing the interactions of elementary particles This theory has a number of significant successes, and yet it is still very far from completeness and cannot claim the role of a comprehensive theory of elementary particles. The origin of many properties of elementary particles and the nature of the inherent their interactions remain largely unclear. It is possible that more than one restructuring of all representations and a much deeper understanding of the relationship between the properties of microparticles and the geometric properties of space-time will be needed before the theory of elementary particles will be built.

Literature

Akhiezer A.I., Rekalo M.P. Biography of elementary particles. -K.: Naukova Dumka, 1983

Dorfman Ya.G. World history of physics from the beginning of the 19th century to the middle of the 20th century. -M.: 1979

Zisman G.A., Todes O.M. Course of general physics. -K.: Ed. Edelweiss, 1994

Kaempfer F. Way to modern physics. -M.: 1972

Kreychi. The world through the eyes of modern physics. -M.: Mir, 1974

Myakishev G.Ya. Elementary particles. -M.: Enlightenment, 1977

Pasichny A.P. Physics of elementary particles. -K.: Vishcha school, 1980

Saveliev I.V. Physics course. -M.: Nauka, 1989

The notion that the world is made up of fundamental particles has a long history. For the first time, the idea of the existence of the smallest invisible particles that make up all the surrounding objects was expressed 400 years before our era 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 it was possible to explain a number of chemical phenomena on this basis. In the 30s of the 19th century, in the theory of electrolysis developed by M. Faraday, the concept of an ion appeared and the elementary charge was measured. But around the middle of the 19th century, experimental facts began to appear that cast doubt on the idea of the indivisibility of atoms. The results of these experiments suggested that atoms have a complex structure and that they contain electrically charged particles. This was confirmed by the French physicist Henri Becquerel, who in 1896 discovered the phenomenon of radioactivity.

This was followed by the discovery of the first elementary particle by the English physicist Thomson in 1897. It was an electron that finally acquired the status of a real physical object and became the first known elementary particle in the history of mankind. Its mass is about 2000 times less than the mass of a hydrogen atom and is equal to:

m = 9.11*10^(-31) kg.

The negative electric charge of an electron is called elementary and is equal to:

e = 0.60*10^(-19) Cl.

An analysis of the atomic spectra shows that the spin of an electron is 1/2, and its magnetic moment is equal to one Bohr magneton. Electrons obey Fermi statistics because they have a half-integer spin. This agrees with experimental data on the structure of atoms and on the behavior of electrons in metals. Electrons participate in electromagnetic, weak and gravitational interactions.

The second discovered elementary particle was the proton (from the Greek protos - the first). This elementary particle was discovered in 1919 by Rutherford while studying the fission products of atomic nuclei of various chemical elements. In a literal sense, a proton is the nucleus of an atom of the lightest isotope of hydrogen - protium. The proton spin is 1/2. The proton has a positive elementary charge +e. Its mass is:

m = 1.67*10^(-27) kg.

or about 1836 electron masses. Protons are part of the nuclei of all atoms of chemical elements. After that, in 1911, Rutherford proposed a planetary model of the atom, which helped scientists in further studies of the composition of atoms.

In 1932, J. Chadwick discovered the third elementary particle, the neutron (from Latin neuter - neither one nor the other), which has no electric charge and has a mass of approximately 1839 electron masses. The neutron spin is also 1/2.

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

The discovery of the neutrino, a particle that hardly interacts with matter, originates from the theoretical conjecture of W. Pauli in 1930, which made it possible, by assuming the birth of such a particle, 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 by F. Reines and K. Cowen.

But the substance consists not only of particles. There are also antiparticles - elementary particles that have the same mass, spin, lifetime and some other internal characteristics as their "twins" particles, but differ from particles in signs of electric charge and magnetic moment, baryon charge, lepton charge, strangeness and etc. All elementary particles, except for absolutely neutral ones, have their own antiparticles.

The first discovered antiparticle was the positron (from Latin positivus - positive) - a particle with an electron mass, but a positive electric charge. This antiparticle was discovered in cosmic rays by the American physicist Carl David Anderson in 1932. Interestingly, the existence of the positron was theoretically predicted by the English physicist Paul Dirac almost a year before the experimental discovery. Moreover, Dirac predicted the so-called processes of annihilation (disappearance) and the birth of an electron-positron pair. Pair annihilation itself is one of the types of transformations of elementary particles that occurs when a particle collides with an antiparticle. During annihilation, the particle and antiparticle disappear, turning into other particles, the number and type of which are limited by conservation laws. The reverse process of annihilation is the birth of a pair. The positron itself is stable, but in matter, due to annihilation with electrons, there is a very short time. The annihilation of an electron and a positron is that when they meet, they disappear, turning into γ- quanta (photons). And in the event of a collision γ- a quantum with some massive nucleus, an electron-positron pair is born.

In 1955, another antiparticle was discovered - the antiproton, and a little later - the antineutron. The antineutron, like the neutron, does not have an electric charge, but it undoubtedly belongs to the antiparticles, since it participates in the process of annihilation and the birth of a neutron-antineutron pair.

The possibility of obtaining antiparticles led scientists to the idea of creating antimatter. Atoms of antimatter should be built in such a way: in the center of the atom there is a negatively charged nucleus, consisting of antiprotons and antineutrons, and positrons with a positive charge revolve around the nucleus. In general, the atom also turns out to be neutral. This idea has received brilliant experimental confirmation. In 1969, at the proton accelerator in the city of Serpukhov, Soviet physicists obtained the nuclei of antihelium atoms. Also in 2002, 50,000 antihydrogen atoms were produced at the CERN accelerator in Geneva. But, despite this, accumulations of antimatter in the Universe have not yet been discovered. It also becomes clear that at the slightest interaction of antimatter with any substance, their annihilation will occur, which will be accompanied by a huge release of energy, several times greater than the energy of atomic nuclei, which is extremely unsafe for people and the environment.

At present, antiparticles of almost all known elementary particles have been experimentally discovered.

An important role in the physics of elementary particles is played by conservation laws that establish equality between certain combinations of quantities that characterize the initial and final state of the system. The arsenal of conservation laws in quantum physics is greater than in classical physics. It was supplemented by the laws of conservation of various parities (spatial, charge), charges (lepton, baryon, etc.), internal symmetries inherent in one or another type of interaction.

Identification of the characteristics of individual subatomic particles is an important, but only the initial stage in the knowledge of their world. At the next stage, it is still necessary to understand what is the role of each individual particle, what are its functions in the structure of matter.

Physicists have found that, first of all, the properties of a particle are determined by its ability (or inability) to participate in a strong interaction. The particles participating in the strong interaction form a special class and are called hadrons. Particles that participate in the weak interaction and do not participate in the strong one are called leptons. In addition, there are interaction-carrier particles.

Leptons.

Leptons are considered to be true elementary particles. Although leptons may or may not have an electrical charge, they all have a spin of 1/2. Among the leptons, the most famous is the electron. The electron is the first of the discovered elementary particles. Like all other leptons, the electron, apparently, is an elementary (in the proper sense of the word) object. As far as we know, the electron does not consist of any other particles.

Another well-known lepton is the neutrino. Neutrinos are the most common particles in the universe. The Universe can be imagined as a boundless neutrino sea, in which islands in the form of atoms are occasionally found. But despite such a prevalence of neutrinos, it is very difficult to study them. As we have noted, neutrinos are almost elusive. Not participating in either strong or electromagnetic interactions, they penetrate matter as if it does not exist at all. Neutrinos are some "ghosts of the physical world".

Muons are fairly widespread in nature, accounting for a significant portion of cosmic radiation. In many ways, the muon resembles an electron: it has the same charge and spin, participates in those interactions, but has a large mass (about 207 electron masses) and is unstable. In about two millionths of a second, a muon decays into an electron and two neutrinos. At the end of the 1970s, a third charged lepton was discovered, which was called the "tau lepton". This is a very heavy particle. Its mass is about 3500 electron masses. But in all other respects it behaves like an electron and a muon.

In the 1960s, the list of leptons expanded significantly. It was found that there are several types of neutrinos: electron neutrino, muon neutrino and tau neutrino. Thus, the total number of neutrino varieties is three, and the total number of leptons is six. Of course, each lepton has its own antiparticle; thus the total number of distinct leptons is twelve. Neutral leptons participate only in the weak interaction; charged - in the weak and electromagnetic. All leptons participate in gravitational interaction, but are not capable of strong ones.

Hadrons.

If there are just over a dozen leptons, then there are hundreds of hadrons. Such a multitude of hadrons suggests that hadrons are not elementary particles, but are built from smaller particles. All hadrons are found in two varieties - electrically charged and neutral. Among the hadrons, the neutron and proton are the most well-known and widespread, which in turn belong to the class of nucleons. The remaining hadrons are short-lived and rapidly decay. Hadrons participate in all fundamental interactions. They are divided into baryons and mesons. Baryons include nucleons and hyperons.

To explain the existence of nuclear forces of interaction between nucleons, quantum theory required the existence of special elementary particles with a mass greater than the mass of an electron, but less than the mass of a proton. These particles predicted by quantum theory were later called mesons. Mesons were discovered experimentally. They turned out to be a whole family. All of them turned out to be short-lived unstable particles living in a free state in billionths of a second. For example, a charged pi meson or pion has a rest mass of 273 electron masses and a lifetime:

t = 2.6*10^(-8) s.

Further, in studies at charged particle accelerators, particles with masses exceeding the mass of a proton were discovered. These particles were called hyperons. They were found even more than mesons. The family of hyperons includes: lambda-, sigma-, xy- and omega-minus-hyperons.

The existence and properties of most of the known hadrons have been established in experiments on accelerators. The discovery of many different hadrons in the 1950s and 1960s puzzled physicists extremely. But over time, hadrons were classified according to their mass, charge, and spin. Gradually, a more or less clear picture began to emerge. Concrete ideas appeared on how to systematize the chaos of empirical data, to reveal the secret of hadrons in scientific theory. The decisive step here was taken in 1963, when the theory of quarks was proposed.

Theory of quarks.

The theory of quarks is the theory of the structure of hadrons. The basic idea of this theory is very simple. All hadrons are built from smaller particles called quarks. This means that quarks are more elementary particles than hadrons. Quarks are hypothetical particles, because were not observed in the free state. The baryon charge of quarks is 1/3. They carry a fractional electrical charge: they have a charge that is either -1/3 or +2/3 of the fundamental unit, the charge of the electron. A combination of two and three quarks can have a total charge equal to zero or one. All quarks have spin S, so they are fermions. The founders of the theory of quarks Gell-Mann and Zweig, in order to take into account all hadrons known in the 60s, introduced three types (colors) of quarks: u (from up - top), d (from down - bottom) and s (from strange - strange) .

Quarks can combine with each other in one of two possible ways: either in triplets or in quark-antiquark pairs. Comparatively heavy particles - baryons - are composed of three quarks. The best known baryons are the neutron and the proton. Lighter quark-antiquark pairs form particles called mesons - "intermediate particles". For example, a proton is made up of two u-quarks and one d-quark (uud), while a neutron is made up of two d-quarks and one u-quark (udd). In order for this "trio" of quarks not to decay, a force holding them, a kind of "glue", is needed.

It turned out that the resulting interaction between neutrons and protons in the nucleus is simply a residual effect of a more powerful interaction between the quarks themselves. This explained why the strong force seems so complicated. When a proton "sticks" to a neutron or another proton, six quarks are involved in the interaction, each of which interacts with all the others. A significant part of the forces is spent on strong gluing of a trio of quarks, and a small part is spent on bonding two trios of quarks to each other. But later it turned out that quarks also participate in the weak interaction. The weak force can change the color of a quark. This is how neutron decay occurs. One of the d-quarks in the neutron turns into a u-quark, and the excess charge carries away the electron that is born at the same time. Similarly, by changing the flavor, the weak interaction leads to the decay of other hadrons.

The fact that all known hadrons can be obtained from various combinations of the three basic particles was a triumph for the theory of quarks. But in the 1970s, new hadrons were discovered (psi-particles, upsilon meson, etc.). This dealt a blow to the first version of the theory of quarks, since there was no room for a single new particle in it. All possible combinations of quarks and their antiquarks have already been exhausted.

The problem was solved by introducing three new colors. They got the name - c - quark (charm - charm), b - quark (from bottom - bottom, and more often beauty - beauty, or charm), and subsequently another color was introduced - t (from top - top).

So far, free quarks and antiquarks have not been observed. However, there is practically no doubt about the reality of their existence. Moreover, searches are underway for "real" elementary particles following quarks - gluons, which are carriers of interactions between quarks, because quarks are held together by a strong interaction, and gluons (color charges) are carriers of the strong interaction. The field of elementary particle physics that studies the interaction of quarks and gluons is called quantum chromodynamics. Just as quantum electrodynamics is the theory of electromagnetic interaction, so quantum chromodynamics is the theory of strong interaction. Quantum chromodynamics is a quantum field theory of the strong interaction of quarks and gluons, which is carried out by exchanging between them - gluons (analogues of photons in quantum electrodynamics). Unlike photons, gluons interact with each other, which leads, in particular, to an increase in the strength of interaction between quarks and gluons as they move away from each other. It is assumed that it is this property that determines the short action of nuclear forces and the absence of free quarks and gluons in nature.

According to modern concepts, hadrons have a complex internal structure: baryons consist of 3 quarks, mesons - from a quark and an antiquark.

Thus, the most probable number of truly elementary particles (excluding carriers of fundamental interactions) at the end of the 20th century is 48. Of these: leptons (6x2) = 12 and quarks (6x3)x2 = 36.

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KALININGRAD BORDER INSTITUTE OF THE FEDERAL SECURITY SERVICE OF THE RUSSIAN FEDERATION

CENTER FOR ADDITIONAL AND PROFESSIONAL EDUCATION

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"The concept of modern natural science"

"The history of the discovery of elementary particles"

Content

Introduction

Electron
Photon
Proton
Neutron
Positron
Neutrino
Discovery of strange particles
"Charming" particles
Conclusion
List of used literature

Introduction

Elementary particles in the exact meaning of this term are primary, further indecomposable particles, of which, by assumption, all matter consists. The concept of elementary particles in modern physics expresses the idea of primitive entities that determine all known properties of the material world, an idea that originated in the early stages of the formation of natural science and has always played an important role in its development.

The existence of elementary particles was discovered by physicists in the study of nuclear processes, therefore, until the middle of the 20th century, elementary particle physics was a branch of nuclear physics. At present, elementary particle physics and nuclear physics are close, but independent branches of physics, 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.

The concept of “elementary particles” was formed in close connection with the establishment of the discrete nature of the structure of matter at the microscopic level. all known substances as combinations of a finite, albeit large, number of structural components - atoms.Revealing in the future the presence of constituent constituents of atoms - electrons and nuclei, the establishment of the complex nature of nuclei, which turned out to be built from only two types of particles (protons and neutrons), significantly reduced the number discrete elements that form the properties of matter, and gave reason to assume that the chain of constituent parts of matter ends with discrete structureless formations - elementary particles. Such an assumption, generally speaking, is an extrapolation of known facts and cannot be rigorously substantiated. Tew to assert that particles that are elementary in the sense of the above definition exist. Protons and neutrons, for example, which for a long time were considered elementary particles, as it turned out, have a complex structure. It is possible that the sequence of structural components of matter is fundamentally infinite. The existence of elementary particles is a kind of postulate, and verification of its validity is one of the most important tasks of physics.

The history of the discovery of elementary particles

The notion that the world is made up of fundamental particles has a long history. For the first time, the idea of the existence of the smallest invisible particles that make up all the surrounding objects was expressed 400 years before our era 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 it was possible to explain a number of chemical phenomena on this basis. In the 30s of the 19th century, in the theory of electrolysis developed by M. Faraday, the concept of an ion appeared and the elementary charge was measured. The end of the 19th century was marked by the discovery of the phenomenon of radioactivity (A. Becquerel, 1896), as well as the discoveries of electrons (J. Thomson, 1897) and b-particles (E. Rutherford, 1899). In 1905, in physics, an idea arose about the 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 the nuclei of atoms 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 year, 1932, the positron was discovered in cosmic rays (K. Anderson). A positron is a positively charged particle that has 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 studied, and it became clear that these particles are also not unchanging elementary "bricks" of nature. In 1937, particles with a mass of 207 electron masses, called muons (m-mesons), were discovered in cosmic rays. Then, in 1947-1950, pions (that is, p-mesons) were discovered, 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 the study of cosmic rays, the development of accelerator technology, and the study of nuclear reactions.

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 undergo spontaneous transformations into other particles at certain intervals. 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 an m-meson is 2.2·10 - 6 s, and that of a neutral p-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. In terms of the scale of the microcosm, 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 for mutual transformations is the most important property of all elementary particles. Elementary particles 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 is the annihilation (that is, the disappearance) of an electron and a positron, accompanied by the birth of photons of high energy. The reverse process can also occur - the birth of an electron-positron pair, for example, in the collision of a photon with a sufficiently high energy with a nucleus. Such a dangerous twin, as the positron is for the electron, the proton also has. It's called an antiproton. The electric charge of the antiproton is negative. At present, antiparticles have been found in 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 the neutron has an antiparticle. The neutron and antineutron differ only in the signs of the magnetic moment and the so-called baryon charge. The existence of antimatter atoms is possible, the nuclei of which consist 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 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 grouped into three groups: photons, leptons and hadrons.

The group of photons includes the only particle - the photon, which is the carrier of the electromagnetic interaction.

The next group consists of light lepton particles. This group includes two types of neutrinos (electronic and muon), electron and m-meson.

The third large group is made up of heavy particles called hadrons. This group is divided into two subgroups. Lighter particles make up a subgroup of mesons. The lightest of them are positively and negatively charged, as well as neutral p-mesons with masses of the order of 250 electron masses. Pions are quanta of the nuclear field, just as photons are quanta of the electromagnetic field. This subgroup also includes four K mesons and one Z0 meson. All mesons have spin equal to zero.

The second subgroup - 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. The omega-minus-hyperon, discovered in 1964, closes the table.

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 fundamental 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. So, each baryon consists of three quarks, antibaryon - from 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. Numerous searches for quarks in the free state, carried out in high-energy accelerators and in cosmic rays, turned out to be unsuccessful. Scientists believe that one of the reasons for the unobservability of free quarks is, perhaps, their very large masses. This prevents the creation of quarks at the energies that are achieved at modern accelerators. However, most experts are now confident that quarks exist inside heavy particles - hadrons.

Fundamental interactions. The processes in which various elementary particles participate differ greatly in their characteristic times and energies. According to modern concepts, there are four types of interactions in nature that cannot be reduced to other, simpler types of interactions: strong, electromagnetic, weak and gravitational. These types of interactions are called fundamental.

Strong (or nuclear) interaction is the most intense of all types of interactions. They cause an exceptionally strong bond between protons and neutrons in the nuclei of atoms. Only heavy particles - hadrons (mesons and baryons) can take part in strong interaction. Strong interaction manifests itself at distances of the order of and less than 10 - 15 m. Therefore, it is called short-range.

Electromagnetic interaction. Any electrically charged particles can take part in this type of interaction, as well as photons - quanta of the electromagnetic field. The electromagnetic interaction is responsible, in particular, for the existence of atoms and molecules. It determines many properties of substances in solid, liquid and gaseous states. The Coulomb repulsion of protons leads to the instability of nuclei with large mass numbers. The electromagnetic interaction determines the processes of absorption and emission of photons by atoms and molecules of matter and many other processes in the physics of the micro- and macroworld.

Weak interaction is the slowest of all interactions occurring in the microcosm. Any elementary particles, except for photons, can take part in it.

Gravitational interaction is inherent in all particles without exception, however, due to the smallness of the masses of elementary particles, the forces of gravitational interaction between them are negligibly small and their role in the processes of the microcosm is insignificant. Gravitational forces play a decisive role in the interaction of space objects (stars, planets, etc.) with their huge masses.

In the 1930s, a hypothesis arose that interactions in the world of elementary particles are carried out through the exchange of quanta of some field. This hypothesis was originally put forward by our compatriots I.E. Tamm and D.D. Ivanenko. They suggested that fundamental interactions arise from the exchange of particles, just as the covalent chemical bond of atoms arises from the exchange of valence electrons, which are combined on empty electron shells.

The interaction carried out by the exchange of particles has received in physics the name of the exchange interaction. So, for example, the electromagnetic interaction between charged particles arises as a result of the exchange of photons - quanta of the electromagnetic field.

The exchange interaction theory gained recognition after the Japanese physicist H. Yukawa theoretically showed in 1935 that the strong interaction between nucleons in the nuclei of atoms can be explained by assuming that nucleons exchange hypothetical particles called mesons. Yukawa calculated the mass of these particles, which turned out to be approximately equal to 300 electron masses. Particles with such a mass were subsequently actually discovered. These particles are called p-mesons (pions). Three types of pions are currently known: p + , p - and p 0 .

In 1957, the existence of heavy particles, the so-called vector bosons W + , W - and Z 0 , was theoretically predicted, causing the exchange mechanism of weak interaction. These particles were discovered in 1983 in colliding beam experiments with protons and high-energy antiprotons. The discovery of vector bosons was a very important achievement in elementary particle physics. This discovery marked the success of a theory that unified the electromagnetic and weak forces into a single so-called electroweak force. This new theory considers the electromagnetic field and the field of weak interaction as different components of the same field, in which, along with the quantum of the electromagnetic field, vector bosons participate.

After this discovery, in modern physics, the confidence that all types of interaction are closely related to each other and, in essence, are different manifestations of a certain unified field, has significantly increased. However, the unification of all interactions is still only an attractive scientific hypothesis.

Theoretical physicists make considerable efforts in attempts to consider on a unified basis not only the electromagnetic and weak, but also the strong interaction. This theory is called the Great Unification. Scientists suggest that the gravitational interaction must also have its own carrier - a hypothetical particle called the graviton. However, this particle has not yet been discovered.

At present, it is considered proven that a unified field that unites all types of interaction can exist only at extremely high particle energies that are unattainable with modern accelerators. Particles could possess such large energies only at the earliest stages of the existence of the Universe, which arose as a result of the so-called Big Bang. Cosmology - the science of the evolution of the universe - suggests that the Big Bang happened 18 billion years ago. The standard model of the evolution of the Universe assumes that in the first period after the explosion, the temperature could reach 10 32 K, and the particle energy E = kT could reach values of 10 19 GeV. During this period, matter existed in the form of quarks and neutrinos, while all types of interactions were combined into a single force field. Gradually, as the Universe expanded, the energy of the particles decreased, and the gravitational interaction first separated from the unified field of interactions (at particle energies of ≈ 1019 GeV), and then the strong interaction separated from the electroweak one (at energies of the order of 1014 GeV). At energies of the order of 10 3 GeV, all four types of fundamental interactions turned out to be separated. Simultaneously with these processes, the formation of more complex forms of matter - nucleons, light nuclei, ions, atoms, etc. went on. Cosmology in its model tries to trace the evolution of the Universe at different stages of its development from the Big Bang to the present day, based on the laws of elementary particle physics, as well as nuclear and atomic physics.

Electron

Perhaps these electrons Worlds, where there are five continents, Arts, knowledge, wars, thrones And the memory of forty centuries!

Valery Bryusov's poem "The World of the Electron" was written on August 13, 1922.

Historically, the first discovered elementary particle was the electron - the carrier of the negative elementary electric charge in atoms.

This is the "oldest" elementary particle. In ideological terms, he entered physics in 1881, when Helmholtz, in a speech in honor of Faraday, pointed out that the atomic structure of matter, together with Faraday's laws of electrolysis, inevitably leads to the idea that the electric charge must always be a multiple of some elementary charge, i.e. . to the conclusion about the quantization of electric charge. The carrier of the negative elementary charge, as we now know, is the electron.

Maxwell, who created the fundamental theory of electrical and magnetic phenomena and made significant use of Faraday's experimental results, did not accept the hypothesis of atomic electricity.

Meanwhile, the "temporary" theory of the existence of the electron was confirmed in 1897 in the experiments of JJ Thomson, in which he identified the so-called cathode rays with electrons and measured the charge and mass of the electron. Thomson called the particles of cathode rays "corpuscles" or primordial atoms. The word "electron" was originally used to denote the magnitude of the charge of the "corpuscle". And only over time, the particle itself began to be called an electron. However, the idea of the electron was not immediately accepted. When, in a lecture at the Royal Society, J. J. Thomson, the discoverer of the electron, suggested that the particles of cathode rays should be considered as possible components of the atom, some of his colleagues sincerely believed that he was mystifying them. Planck himself admitted in 1925 that he did not fully believe then, in 1900, in the hypothesis of the electron.

We can say that after the experiments of Millikan, who measured in 1911. charges of individual electrons, this first elementary particle got the right to exist.

Photon

Direct experimental proof of the existence of the photon was given by R. Millikan in 1912-1915. in his studies of the photoelectric effect, as well as A. Compton in 1922, who discovered the scattering of x-rays with a change in their frequency.

A photon is, in a sense, a special particle. The fact is that its rest mass, unlike other particles (except for neutrinos), is equal to zero. Therefore, it was not immediately considered a particle: at first it was believed that the presence of a finite and non-zero rest mass is a mandatory feature of an elementary particle.

A photon is an "animated" Planck quantum of light, i.e. a quantum of light that carries momentum.

Light quanta were introduced by Planck in 1901 in order to explain the laws of radiation of a completely black body. But he was not particles, but only the minimum possible "portions" of light energy of one frequency or another.

Although Planck's assumption about quantizing the energy of light was absolutely contrary to all classical theory, Planck himself did not immediately understand this. The scientist wrote that he "... tried to somehow introduce the value of h into the framework of the classical theory. However, despite all such attempts, this value turned out to be very obstinate." Subsequently, this value was called Planck's constant (h=6*10 -27 erg. s).

After the introduction of Planck's constant, the situation did not become clearer.

Photons or quanta were made "alive" by the theory of relativity of Einstein, who in 1905 showed that quanta must have not only energy, but also momentum, and that they are particles in the full sense, only special, since their rest mass is zero, and they move at the speed of light.

So the conclusion about the existence of an electromagnetic field particle - a photon - originates from the work of M. Planck (1900). Assuming that the energy of the electromagnetic radiation of an absolutely 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 actually a stream of individual quanta (photons), and on this basis explained the laws of the photoelectric effect.

Proton

The proton was discovered by E. Rutherford in 1919 in studies of the interaction of alpha particles with atomic nuclei.

More precisely, the discovery of the proton is associated with the discovery of the atomic nucleus. It was made by Rutherford by bombarding nitrogen atoms with high-energy b-particles. Rutherford concluded that "the nucleus of the nitrogen atom disintegrates as a result of the enormous forces developing upon collision with a fast 6-particle, and that the liberated hydrogen atom forms an integral part of the nitrogen nucleus." In 1920, the nuclei of the hydrogen atom were named protons by Rutherford (proton in Greek means the simplest, primary). There were other suggestions for a name. So, for example, the name "baron" was proposed (baros in Greek means heaviness). However, it emphasized only one feature of the hydrogen nucleus - its mass. The term "proton" was much deeper and more meaningful, reflecting the fundamental nature of the proton, because the proton is the simplest nucleus - the nucleus of the lightest isotope of hydrogen. This is undoubtedly one of the most successful terms in elementary particle physics. Thus, protons are particles with a unit positive charge and a mass 1840 times the mass of an electron.

Neutron

Another particle that makes up the nucleus, the neutron, was discovered in 1932 by J. Chadwick while studying the interaction of 6 particles with beryllium. The neutron has a mass close to that of the proton, but has no electrical charge. The discovery of the neutron completed the identification of particles - the structural elements of atoms and their nuclei.

The discovery of isotopes did not clarify the question of the structure of the nucleus. By this time, only protons were known - hydrogen nuclei, and electrons, and therefore it was natural to try to explain the existence of isotopes by various combinations of these positively and negatively charged particles. One might think that the nuclei contain A protons, where A is the mass number, and A?Z electrons. In this case, the total positive charge coincides with the atomic number Z.

Such a simple picture of a homogeneous nucleus at first did not contradict the conclusion about the small size of the nucleus, which followed from Rutherford's experiments. The “natural radius” of an electron r0 \u003d e 2 /mc 2 (which is obtained by equating the electrostatic energy e 2 /r0 of the charge distributed over the spherical shell to the self-energy of the electron mc 2) is r0 \u003d 2.82 * 10 - 15 m. Such the electron is small enough to be inside a nucleus with a radius of 10 - 14 m, although it would be difficult to place a large number of particles there. In 1920 Rutherford and others considered the possibility of a stable combination of a proton and an electron, reproducing a neutral particle with a mass approximately equal to that of a proton. However, due to the lack of an electrical charge, such particles would be difficult to detect. It is unlikely that they could also knock out electrons from metal surfaces, like electromagnetic waves during the photoelectric effect.

It was not until a decade later, after natural radioactivity had been thoroughly investigated and radioactive radiation began to be widely used to cause artificial transformation of atoms, that the existence of a new constituent of the nucleus was reliably established. In 1930, W. Bothe and G. Becker of the University of Giessen irradiated lithium and beryllium with alpha particles and, using a Geiger counter, recorded the resulting penetrating radiation. Since this radiation was not affected by electric and magnetic fields, and it had a high penetrating power, the authors concluded that hard gamma radiation was emitted. In 1932, F. Joliot and I. Curie repeated experiments with beryllium, passing such penetrating radiation through a paraffin block. They found that unusually high energy protons were emitted from the paraffin and concluded that the gamma radiation passing through the paraffin produced protons as a result of scattering. (In 1923 it was found that X-rays scatter on electrons, giving the Compton effect)

J. Chadwick repeated the experiment. He also used paraffin and, using an ionization chamber, in which the charge generated when electrons were knocked out of atoms, was collected, he measured the range of recoil protons.

The fission of metallic beryllium proceeded as follows: alpha particles of 4 2 He (charge 2, mass number 4) collided with beryllium nuclei (charge 4, mass number 9), resulting in carbon and a neutron. The discovery of the neutron was an important step forward. The observed characteristics of nuclei could now be interpreted by considering neutrons and protons as constituents of nuclei. The neutron is now known to be 0.1% heavier than the proton. Free neutrons (outside the nucleus) undergo radioactive decay, turning into a proton and an electron. This is reminiscent of the original hypothesis of a compound neutral particle. However, inside a stable nucleus, neutrons are bound to protons and do not spontaneously decay.

Positron

Beginning in the 1930's and up to the 1950's, new particles were discovered mainly in cosmic rays. In 1932, in their composition, A. Anderson discovered the first antiparticle - the positron (e +) - a particle with the mass of an electron, but with a positive electric charge. The positron was the first antiparticle discovered. The existence of e+ followed directly 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. Neddermeyer discovered muons (of both signs of electric charge) in the study of cosmic rays - particles with a mass of about 200 electron masses, but otherwise surprisingly similar in properties to e-, e +.

The positron was discovered by Anderson while studying cosmic rays using the cloud chamber method. The figure, which is a reproduction of a photograph taken by Anderson in a cloud chamber, shows a positive particle entering a 0.6 cm thick lead plate with a momentum of 6.3 * 107 eV / s and leaving it with a momentum of 2.3 * 107 eV / s from. One can set an upper limit on the mass of this particle, assuming that it only loses energy in collisions. This limit is 20 me. Based on this and other similar photographs, Anderson hypothesized the existence of a positive particle with a mass approximately equal to that of an ordinary electron. This conclusion was soon confirmed by observations by Blackett and Occhialini in a cloud chamber. Shortly thereafter, Curie and Joliot discovered that positrons are produced by the conversion of gamma rays from radioactive sources, and are also emitted by artificial radioactive isotopes. Since the photon, being neutral, forms a pair (positron and electron), it follows from the principle of conservation of electric charge that the absolute value of the charge of the positron is equal to the charge of the electron.

The first quantitative determination of the mass of the positron was made by Thiebaud, who measured the ratio e/m using the trochoid method and concluded that the masses of the positron and electron differ by no more than 15%. Later experiments by Spies and Zan, who used a mass spectrographic setup, showed that the masses of the electron and positron coincide to within 2%. Still later, Dumond and co-workers measured the wavelength of the annihilation radiation with great accuracy. Up to experimental errors (0.2%), they obtained such a value of the wavelength, which should be expected under the assumption that the positron and electron have equal masses.

The law of conservation of angular momentum as applied to the process of pair production shows that positrons have a half-integer spin and, therefore, obey Fermi statistics. It is reasonable to assume that the spin of the positron is 1/2, as is the spin of the electron.

pions and muons. Meson discovery

The discovery of the meson, unlike the discovery of the positron, was not the result of a single observation, but rather a conclusion from a whole series of experimental and theoretical studies.

In 1932, Rossi, using the coincidence method proposed by Bothe and Kolhurster, showed that a known fraction of the cosmic radiation observed at sea level consists of particles capable of penetrating through lead plates up to 1 m thick. Shortly thereafter, he also drew attention to the existence in cosmic rays two different components. Particles of one component (the penetrating component) are able to pass through large thicknesses of matter, and the degree of their absorption by various substances is approximately proportional to the mass of these substances. Particles of the other component (shower component) are quickly absorbed, especially by heavy elements; in this case, a large number of secondary particles (showers) are formed. Cloud chamber experiments by Anderson and Neddemeyer on the passage of cosmic ray particles through lead plates also showed that there are two distinct components of cosmic rays. These experiments showed that while the average energy loss of cosmic ray particles in lead was in order of magnitude the theoretically calculated collision loss, some of these particles experienced much greater losses.

In 1934, Bethe and Heitler published the theory of radiative loss of electrons and the production of pairs by photons. The properties of the less penetrating component observed by Anderson and Neddemeyer were in agreement with the properties of electrons predicted by the theory of Bethe and Heitler; in this case, large losses were explained by radiation processes. The properties of the shower-forming radiation discovered by Rossi could also be explained by assuming that this radiation consists of high-energy electrons and photons. On the other hand, while recognizing the validity of the theory of Bethe and Heitler, one had to conclude that "penetrating" particles in Rossi's experiments and less absorbed particles in Anderson's and Neddemeyer's experiments differ from electrons. We had to assume that the penetrating particles are heavier than electrons, since, according to the theory, energy losses for radiation are inversely proportional to the square of the mass.

In connection with this, the possibility of the collapse of the theory of radiation at high energies was discussed. As an alternative, Williams suggested in 1934 that penetrating particles of cosmic rays might have the mass of a proton. One of the difficulties associated with this hypothesis was the necessity of the existence of not only positive, but also negative protons, because cloud chamber experiments showed that the penetrating particles of cosmic rays have charges of both signs. Moreover, in some photographs taken by Anderson and Neddemeyer in a cloud chamber, one could see particles that did not radiate like electrons, but, however, were not as heavy as protons. Thus, by the end of 1936, it became almost obvious that, in addition to electrons, cosmic rays also contained particles of a hitherto unknown type, presumably particles with a mass intermediate between that of an electron and that of a proton. It should also be noted that in 1935, Yukawa, from purely theoretical considerations, predicted the existence of such particles.

The existence of intermediate mass particles was directly proven in 1937 by the experiments of Neddemeyer and Anderson, Street and Stevenson.

The experiments of Neddemeyer and Anderson were a continuation (with an improved technique) of the studies mentioned above on the energy losses of cosmic ray particles. They were carried out in a cloud chamber placed in a magnetic field and divided into two halves by a platinum plate 1 cm thick. The momentum loss for individual cosmic ray particles was determined by measuring the track curvature before and after the plate.

Absorbed particles can easily be interpreted as electrons. This interpretation is supported by the fact that, unlike penetrating particles, absorbed particles often cause secondary processes in the platinum absorber and for the most part occur in groups (two or more). This is exactly what was to be expected, since many of the electrons observed in the same experimental geometry as those of Neddemeyer and Anderson are part of the showers formed in the surrounding matter. As regards the nature of the penetrating particles, the following two results obtained by Neddemeyer and Anderson explained a lot here.

This photograph shows the trail of a particle with a momentum of 29 MeV/c, whose ionization is about six times the minimum. This particle has a negative charge as it moves downward. Judging by the momentum and specific ionization, its mass is about 175 electron masses; a probable error of 25% is due to the inaccuracy of the measurement of specific ionization. Note that an electron with a momentum of 29 MeV/c has practically minimal ionization. On the other hand, particles with this momentum and proton mass (either an upward moving ordinary proton or a downward moving negative proton) have a specific ionization that is about 200 times the minimum; in addition, the range of such a proton in the chamber gas must be less than 1 cm. At the same time, the trace in question is clearly visible for 7 cm, after which it leaves the illuminated volume.

The experiments described above certainly proved that the penetrating particles are indeed heavier than electrons, but lighter than protons. In addition, Street and Stevenson's experiment gave the first rough estimate of the mass of this new particle, which we can now call by its common name, the meson.

So in 1936 A. Anderson and S. Neddermeyer discovered the muon (m - meson). This particle differs from the electron only in its mass, which is approximately 200 times greater than the electron.

In 1947 Powell observed traces of charged particles in photographic emulsions, which were interpreted as Yukawa mesons and named p mesons or pions. The decay products of charged pions, which are also charged particles, were called m-mesons or muons. It was negative muons that were observed in Conversi's experiments: unlike pions, muons, like electrons, do not interact strongly with atomic nuclei.

Since the decay of stopped pions always produced muons of a strictly defined energy, it followed that the transition of p into m should produce one more neutral particle (its mass turned out to be very close to zero). On the other hand, this particle practically does not interact with matter, so it was concluded that it cannot be a photon. Thus, physicists have encountered a new neutral particle whose mass is zero. So, a charged Yukawa meson was discovered, decaying into a muon and a neutrino. The p-meson lifetime with respect to this decay turned out to be 2×10 -8 s. Then it turned out that the muon is also unstable, that as a result of its decay, an electron is formed. The muon lifetime turned out to be on the order of 10 -6 s. Since the electron formed during the decay of the muon does not have a strictly defined energy, it was concluded that, along with the electron, two neutrinos are formed during the decay of the muon. 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.

Neutrino

During the beta decay of nuclei, as we have already said, in addition to electrons, neutrinos also fly out. This particle was first "introduced" into physics theoretically. It was the existence of the neutrino that was postulated by Pauli in 1929, many years before his experimental discovery (1956). Neutrino, a neutral particle with zero (or negligibly small) mass, was needed by Pauli in order to save the law of conservation of energy in the process of beta-decay of atomic nuclei.

Initially, Pauli called the hypothetical neutral particle formed during the beta decay of nuclei the neutron (this was before Chadwick's discovery) and suggested that it was part of the nucleus.

How difficult it was to come to the hypothesis of neutrinos, which are formed in the very act of neutron decay, can be seen at least from the fact that just a year before the appearance of Fermi's fundamental article on the properties of the weak interaction, the researcher used the term "neutron" in a report on the current state of nuclear physics to denote the two particles now called the neutron and the neutrino. “For example, according to Pauli's proposal,” says Fermi, “it would be possible to imagine that inside the atomic nucleus there are neutrons that would be emitted simultaneously with β-particles. These neutrons could pass through large thicknesses of matter, practically without losing their energy, and therefore, they would be practically unobservable. The existence of the neutron, undoubtedly, could simply explain some as yet incomprehensible questions, such as the statistics of atomic nuclei, the anomalous proper moments of some nuclei, and also, perhaps, the nature of penetrating radiation. " Indeed, when it comes to a particle emitted with β-electrons and poorly absorbed by matter, it is necessary to keep in mind the neutrino. It can be concluded that in 1932 the problems of the neutron and neutrino were extremely confused. It took a year of hard work by theorists and experimenters to resolve both fundamental and terminological difficulties.

“After the discovery of the neutron,” Pauli said, “at seminars in Rome, Fermi began to call my new particle emitted during beta decay “neutrino” to distinguish it from the heavy neutron. This Italian name has become generally accepted.”

In the 1930s, Fermi's theory was generalized to positron decay (Wick, 1934) and to transitions with a change in the angular momentum of the nucleus (Gamow and Teller, 1937).

The "fate" of a neutrino can be compared with the "fate" of an electron. Both particles were initially hypothetical - the electron was introduced to bring the atomic structure of matter in line with the laws of electrolysis, and the neutrino - to save the law of conservation of energy in the process of beta-decay. And only much later they were discovered as real ones.

In 1962, it was found that there are two different neutrinos: electron and muon. In 1964, in the decays of neutral K-mesons, the so-called nonconservation was discovered. combined parity (introduced by Li Tsung-dao and Yang Chen-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.

Discovery of strange particles

Late 40s - early 50s. were marked by the discovery of a large group of particles with unusual properties, called “strange.” were made on accelerators - installations that create intense flows 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.

In 1947, Butler and Rochester observed two particles, called V particles, in a cloud chamber. Two tracks were observed, as if forming the Latin letter V. The formation of two tracks indicated that the particles were unstable and decayed into other, lighter ones. One of the V-particles was neutral and decayed into two charged particles with opposite charges. (Later it was identified with the neutral K-meson, which decays into positive and negative pions). The other was charged and decayed into a charged particle with a smaller mass and a neutral particle. (Later it was identified with the charged K+ meson, which decays into charged and neutral pions).

V-particles allow, at first glance, another interpretation: their appearance could be interpreted not as a decay of particles, but as a scattering process. Indeed, the processes of scattering of a charged particle by a nucleus with the formation of one charged particle in the final state, as well as inelastic scattering of a neutral particle by a nucleus with the formation of two charged particles, will look the same in a cloud chamber as the decay of V-particles. But such a possibility was easily ruled out on the grounds that scattering processes are more probable in denser media. And V-events were observed not in lead, which was present in the cloud chamber, but directly in the chamber itself, which is filled with a gas with a lower density (compared to the density of lead).

We note that if the experimental discovery of the p-meson was in some sense "expected" in connection with the need to explain the nature of nucleon interactions, then the discovery of V-particles, like the discovery of the muon, turned out to be a complete surprise.

As it turned out later, all these observations turned out to be observations of various decays of the same particle - the K-meson (charged or neutral).

The "behavior" of V-particles at birth and subsequent decay led to the fact that they were called strange.

Strange particles were first obtained in the laboratory in 1954. Fowler, Shutt, Thorndike and Whitemore, who, using an ion beam from the Brookhaven cosmotron with an initial energy of 1.5 GeV, observed the reactions of associative production of strange particles.

From 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 at 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 shorter distances, the higher the energy of the colliding particles. Accelerators significantly increased the rate of obtaining new data and in a short time expanded and enriched our knowledge of the properties of the microworld. The use of accelerators to study strange particles made it possible to study their properties in more detail, 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: the antiproton (1955), the antineutron (1956), and the antisigma hyperons (1960). In 1964, the heaviest hyperon W - (with a mass of about two proton masses) was discovered.

Resonances.

In the 1960s a large number of extremely unstable (compared to other unstable elementary particles) particles, called “resonances”, were discovered at accelerators. The masses of most resonances exceed the mass of a proton. The first of them, D1 (1232), has been known since 1953. make up the bulk of elementary particles.

The strong interaction of a p meson and a nucleon in a state with a total isotopic spin of 3/2 and a moment of 3/2 leads to the appearance of an excited state of the nucleon. This state decays into a nucleon and a p meson within a very short time (on the order of 10 -23 s). Since this state has well-defined quantum numbers, as well as stable elementary particles, it was natural to call it a particle. To emphasize the very short lifetime of this state, it and similar short-lived states came to be called resonant.

Nucleon resonance, discovered by Fermi in 1952, was later called the D 3/2 3/2 isobar (to highlight the fact that the spin and isotopic spin of the D isobar are 3/2). Since the lifetime of resonances is insignificant, they cannot be observed directly, in the same way as the "ordinary" proton, p-mesons and muons are observed (by their traces in track devices). Resonances are detected by the characteristic behavior of the scattering cross sections of particles, as well as by studying the properties of their decay products. Most of the known elementary particles belong to the group of resonances.

The discovery of D-resonance was of great importance for the physics of elementary particles.

Excited states exist not only for the nucleon (in this case they speak of its isobaric states), but also for the p meson (in this case they speak of meson resonances).

“The reason for the appearance of resonances in strong interactions is incomprehensible,” Feynman writes, “at first, theorists did not assume that there were resonances in field theory with a large coupling constant. Later, they realized that if the coupling constant is large enough, then isobaric states arise. However, the true meaning of the fact the existence of resonances for the fundamental theory remains unclear."

Introduction.

From electron to neutrino

Electron

Photon

Proton

Neutron

Positron

Peonies and Muons. Meson discovery

Neutrino

From weirdness to charm

Discovery of strange particles

Resonances.

"Charmed" Particles

Conclusion

Literature

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Introduction

The history of the discovery of elementary particles

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. Most short-lived elementary particles have lifetimes of the order of 10 - 22 -10 - 23 s.

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

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

The group of photons includes the only particle - the photon, which is the carrier of the electromagnetic interaction.

The next group consists of light lepton particles. This group includes two types of neutrinos (electronic and muon), electron and m-meson.

The second subgroup - 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. The omega-minus-hyperon, discovered in 1964, closes the table.

Weak interaction is the slowest of all interactions occurring in the microcosm. Any elementary particles, except for photons, can take part in it.

The interaction carried out by the exchange of particles has received in physics the name of the exchange interaction. So, for example, the electromagnetic interaction between charged particles arises as a result of the exchange of photons - quanta of the electromagnetic field.

Electron

Perhaps these electrons Worlds, where there are five continents, Arts, knowledge, wars, thrones And the memory of forty centuries!

Valery Bryusov's poem "The World of the Electron" was written on August 13, 1922.

Historically, the first discovered elementary particle was the electron - the carrier of the negative elementary electric charge in atoms.

Maxwell, who created the fundamental theory of electrical and magnetic phenomena and made significant use of Faraday's experimental results, did not accept the hypothesis of atomic electricity.

We can say that after the experiments of Millikan, who measured in 1911. charges of individual electrons, this first elementary particle got the right to exist.

Photon

Direct experimental proof of the existence of the photon was given by R. Millikan in 1912-1915. in his studies of the photoelectric effect, as well as A. Compton in 1922, who discovered the scattering of x-rays with a change in their frequency.

A photon is an "animated" Planck quantum of light, i.e. a quantum of light that carries momentum.

Light quanta were introduced by Planck in 1901 in order to explain the laws of radiation of a completely black body. But he was not particles, but only the minimum possible "portions" of light energy of one frequency or another.

After the introduction of Planck's constant, the situation did not become clearer.

Photons or quanta were made "alive" by the theory of relativity of Einstein, who in 1905 showed that quanta must have not only energy, but also momentum, and that they are particles in the full sense, only special, since their rest mass is zero, and they move at the speed of light.

Proton

The proton was discovered by E. Rutherford in 1919 in studies of the interaction of alpha particles with atomic nuclei.

Neutron

J. Chadwick repeated the experiment. He also used paraffin and, using an ionization chamber, in which the charge generated when electrons were knocked out of atoms, was collected, he measured the range of recoil protons.

Positron

The law of conservation of angular momentum as applied to the process of pair production shows that positrons have a half-integer spin and, therefore, obey Fermi statistics. It is reasonable to assume that the spin of the positron is 1/2, as is the spin of the electron.

pions and muons. Meson discovery

The discovery of the meson, unlike the discovery of the positron, was not the result of a single observation, but rather a conclusion from a whole series of experimental and theoretical studies.

The existence of intermediate mass particles was directly proven in 1937 by the experiments of Neddemeyer and Anderson, Street and Stevenson.

So in 1936 A. Anderson and S. Neddermeyer discovered the muon (m - meson). This particle differs from the electron only in its mass, which is approximately 200 times greater than the electron.

Neutrino

Initially, Pauli called the hypothetical neutral particle formed during the beta decay of nuclei the neutron (this was before Chadwick's discovery) and suggested that it was part of the nucleus.

“After the discovery of the neutron,” Pauli said, “at seminars in Rome, Fermi began to call my new particle emitted during beta decay “neutrino” to distinguish it from the heavy neutron. This Italian name has become generally accepted.”

In the 1930s, Fermi's theory was generalized to positron decay (Wick, 1934) and to transitions with a change in the angular momentum of the nucleus (Gamow and Teller, 1937).

Discovery of strange particles

We note that if the experimental discovery of the p-meson was in some sense "expected" in connection with the need to explain the nature of nucleon interactions, then the discovery of V-particles, like the discovery of the muon, turned out to be a complete surprise.

As it turned out later, all these observations turned out to be observations of various decays of the same particle - the K-meson (charged or neutral).

The "behavior" of V-particles at birth and subsequent decay led to the fact that they were called strange.

Strange particles were first obtained in the laboratory in 1954. Fowler, Shutt, Thorndike and Whitemore, who, using an ion beam from the Brookhaven cosmotron with an initial energy of 1.5 GeV, observed the reactions of associative production of strange particles.

Resonances.

The discovery of D-resonance was of great importance for the physics of elementary particles.

Excited states exist not only for the nucleon (in this case they speak of its isobaric states), but also for the p meson (in this case they speak of meson resonances).

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