More than 350 elementary particles have been discovered. Of these, the photon, electron and muon neutrino, electron, proton and their antiparticles are stable. The remaining elementary particles decay spontaneously according to an exponential law with a time constant from approximately 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10−24 to 10−22 s for resonances).
The structure and behavior of elementary particles is studied by elementary particle physics.
All elementary particles are subject to the principle of identity (all elementary particles of the same type in the Universe are completely identical in all their properties) and the principle of wave-particle duality (each elementary particle corresponds to a de Broglie wave).
All elementary particles have the property of interconvertibility, which is a consequence of their interactions: strong, electromagnetic, weak, gravitational. Interactions of particles cause transformations of particles and their collections into other particles and their collections, if such transformations are not prohibited by the laws of conservation of energy, momentum, angular momentum, electric charge, baryon charge, etc.
Main characteristics of elementary particles: mass, spin, electric charge, lifetime, parity, G-parity, magnetic moment, baryon charge, lepton charge, strangeness, isotopic spin, CP parity, charge parity.
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Subtitles
Classification
By lifetime
- Stable elementary particles are particles that have an infinitely long lifetime in a free state (proton, electron, neutrino, photon, graviton and their antiparticles).
- Unstable elementary particles are particles that decay into other particles in a free state in a finite time (all other particles).
By weight
All elementary particles are divided into two classes:
- Massless particles are particles with zero mass (photon, gluon, graviton and their antiparticles).
- Particles with non-zero mass (all other particles).
By largest back
All elementary particles are divided into two classes:
By type of interaction
Elementary particles are divided into the following groups:
Compound particles
- Hadrons are particles that participate in all types of fundamental interactions. They consist of quarks and are divided, in turn, into:
- mesons are hadrons with integer spin, that is, they are bosons;
- baryons are hadrons with half-integer spin, that is, fermions. These, in particular, include the particles that make up the nucleus of an atom - proton and neutron.
Fundamental (structureless) particles
- Leptons are fermions that have the form of point particles (that is, not consisting of anything) up to scales of the order of 10 −18 m. They do not participate in strong interactions. Participation in electromagnetic interactions was experimentally observed only for charged leptons (electrons, muons, tau leptons) and was not observed for neutrinos. There are 6 known types of leptons.
- Quarks are fractionally charged particles that are part of hadrons. They were not observed in the free state (a confinement mechanism has been proposed to explain the absence of such observations). Like leptons, they are divided into 6 types and are considered structureless, however, unlike leptons, they participate in strong interactions.
- Gauge bosons are particles through the exchange of which interactions are carried out:
- photon - a particle that carries electromagnetic interaction;
- eight gluons - particles that carry the strong interaction;
- three intermediate vector bosons W + , W− and Z 0, which tolerate weak interaction;
- graviton is a hypothetical particle that transfers gravitational interaction. The existence of gravitons, although not yet experimentally proven due to the weakness of gravitational interaction, is considered quite probable; however, the graviton is not included in the Standard Model of elementary particles.
Sizes of elementary particles
Despite the wide variety of elementary particles, their sizes fit into two groups. The sizes of hadrons (both baryons and mesons) are about 10 −15 m, which is close to the average distance between the quarks included in them. The sizes of fundamental, structureless particles - gauge bosons, quarks and leptons - within the experimental error are consistent with their point nature (the upper limit of the diameter is about 10 −18 m) ( see explanation). If in further experiments the final sizes of these particles are not discovered, then this may indicate that the sizes of gauge bosons, quarks and leptons are close to the fundamental length (which very likely may turn out to be the Planck length, equal to 1.6 10 −35 m) .
It should be noted, however, that the size of an elementary particle is a rather complex concept that is not always consistent with classical concepts. Firstly, the uncertainty principle does not allow one to strictly localize a physical particle. A wave packet, representing a particle as a superposition of precisely localized quantum states, always has finite dimensions and a certain spatial structure, and the dimensions of the packet can be quite macroscopic - for example, an electron in an experiment with interference on two slits “feels” both slits of the interferometer, separated by a macroscopic distance . Secondly, a physical particle changes the structure of the vacuum around itself, creating a “coat” of short-term virtual particles - fermion-antifermion pairs (see Polarization of vacuum) and bosons that carry interactions. The spatial dimensions of this region depend on the gauge charges possessed by the particle and on the masses of intermediate bosons (the radius of the shell of massive virtual bosons is close to their Compton wavelength, which, in turn, is inversely proportional to their mass). Thus, the radius of an electron from the point of view of neutrinos (only weak interaction is possible between them) is approximately equal to the Compton wavelength of W-bosons, ~3 × 10 −18 m, and the dimensions of the region of strong interaction of the hadron are determined by the Compton wavelength of the lightest of hadrons, the pi-meson (~10 −15 m), acting here as a carrier of interaction.
Story
Initially, the term “elementary particle” meant something absolutely elementary, the first brick of matter. However, when hundreds of hadrons with similar properties were discovered in the 1950s and 1960s, it became clear that hadrons at least have internal degrees of freedom, that is, they are not elementary in the strict sense of the word. This suspicion was later confirmed when it turned out that hadrons consist of quarks.
Thus, physicists have moved a little deeper into the structure of matter: leptons and quarks are now considered the most elementary, point-like parts of matter. For them (together with gauge bosons) the term “ fundamental particles".
In string theory, which has been actively developed since approximately the mid-1980s, it is assumed that elementary particles and their interactions are consequences of various types of vibrations of especially small “strings”.
Standard model
The Standard Model of elementary particles includes 12 flavors of fermions, their corresponding antiparticles, as well as gauge bosons (photons, gluons, W- And Z-bosons), which carry interactions between particles, and the Higgs boson, discovered in 2012, which is responsible for the presence of inertial mass in particles. However, the Standard Model is largely viewed as a temporary theory rather than a truly fundamental one, since it does not include gravity and contains several dozen free parameters (particle masses, etc.), the values of which do not follow directly from the theory. Perhaps there are elementary particles that are not described by the Standard Model - for example, such as the graviton (a particle that carries gravitational forces) or supersymmetric partners of ordinary particles. In total, the model describes 61 particles.
Fermions
The 12 flavors of fermions are divided into 3 families (generations) of 4 particles each. Six of them are quarks. The other six are leptons, three of which are neutrinos, and the remaining three carry a unit negative charge: the electron, muon, and tau lepton.
| First generation | Second generation | Third generation |
|---|---|---|
ELEMENTARY PARTICLES- primary, further indecomposable particles, of which all matter is believed to consist. In modern physics, the term "elementary particles" is usually used to designate a large group of tiny particles of matter that are not atoms (see Atom) or atomic nuclei (see Atomic nucleus); The exception is the nucleus of the hydrogen atom - the proton.
By the 80s of the 20th century, science knew more than 500 elementary particles, most of which were unstable. Elementary particles include proton (p), neutron (n), electron (e), photon (γ), pi-mesons (π), muons (μ), heavy leptons (τ +, τ -), neutrinos of three types - electronic (V e), muonic (V μ) and associated with the so-called heavy depton (V τ), as well as “strange” particles (K-mesons and hyperons), various resonances, mesons with hidden charm, “charmed” particles, upsilon particles (Υ), “beautiful” particles, intermediate vector bosons, etc. An independent branch of physics has emerged - the physics of elementary particles.
The history of particle physics dates back to 1897, when J. J. Thomson discovered the electron (see Electron radiation); in 1911, R. Millikan measured the magnitude of its electric charge. The concept of “photon” - quantum of light - was introduced by M. Planck in 1900. Direct experimental evidence of the existence of the photon was obtained by Millikan (1912-1915) and Compton (A. N. Compton, 1922). In the process of studying the atomic nucleus, E. Rutherford discovered the proton (see Proton radiation), and in 1932, J. Chadwick discovered the neutron (see Neutron radiation). In 1953, the existence of neutrinos, which W. Pauli had predicted back in 1930, was experimentally proven.
Elementary particles are divided into three groups. The first is represented by a single elementary particle - a photon, γ-quantum, or quantum of electromagnetic radiation. The second group is leptons (Greek leptos small, light), participating, in addition to electromagnetic ones, also in weak interactions. There are 6 known leptons: electron and electron neutrino, muon and muon neutrino, heavy τ-lepton and the corresponding neutrino. The third - main group of elementary particles are hadrons (Greek hadros large, strong), which participate in all types of interactions, including strong interactions (see below). Hadrons include particles of two types: baryons (Greek barys heavy) - particles with half-integer spin and a mass no less than the mass of a proton, and mesons (Greek mesos medium) - particles with zero or integer spin (see Electron paramagnetic resonance). Baryons include the proton and neutron, hyperons, some resonances and “charmed” particles, and some other elementary particles. The only stable baryon is the proton, the rest of the baryons are unstable (a neutron in a free state is an unstable particle, but in a bound state inside stable atomic nuclei it is stable. Mesons got their name because the masses of the first discovered mesons - the pi-meson and the K-meson - had values intermediate between the masses of a proton and an electron. Later, mesons were discovered whose mass exceeds the mass of a proton. Hadrons are also characterized by strangeness (S) - zero, positive or negative quantum number. Hadrons with zero strangeness are called ordinary, and with S ≠ 0 - strange. In 1964, G. Zweig and M. Gell-Mann independently suggested the quark structure of hadrons. The results of a number of experiments indicate that quarks are real material formations inside quarks. have a number of unusual properties, for example, fractional electric charge, etc. Quarks have not been observed in a free state. It is believed that all hadrons are formed due to various combinations of quarks.
Initially, elementary particles were studied in the study of radioactive decay (see Radioactivity) and cosmic radiation (see). However, since the 50s of the 20th century, studies of elementary particles have been carried out on charged particle accelerators (see), in which accelerated particles bombard a target or collide with particles flying towards them. In this case, the particles interact with each other, resulting in their interconversion. This is how most elementary particles were discovered.
Each elementary particle, along with the specifics of its inherent interactions, is described by a set of discrete values of certain physical quantities, expressed in integer or fractional numbers (quantum numbers). The common characteristics of all elementary particles are mass (m), lifetime (t), spin (J) - the intrinsic angular momentum of elementary particles, which has a quantum nature and is not associated with the movement of the particle as a whole, electric charge (Ω) and magnetic moment ( μ). The electric charges of the studied elementary particles in absolute value are integer multiples of the electron charge (e≈1.6*10 -10 k). Known elementary particles have electric charges equal to 0, ±1 and ±2.
All elementary particles have corresponding antiparticles, the mass and spin of which are equal to the mass and spin of the particle, and the electric charge, magnetic moment and other characteristics are equal in absolute value and opposite in sign. For example, the antiparticle of an electron is a positron - an electron with a positive electrical charge. An elementary particle that is identical to its antiparticle is called truly neutral, for example, a neutron and an antineutron, a neutrino and an antineutrino, etc. When antiparticles interact with each other, their annihilation occurs (see).
When an elementary particle enters a material environment, it interacts with it. There are strong, electromagnetic, weak and gravitational interactions. Strong interaction (stronger than electromagnetic interaction) occurs between elementary particles located at a distance of less than 10 -15 m (1 Fermi). At distances greater than 1.5 Fermi, the interaction force between particles is close to zero. It is the strong interactions between elementary particles that provide the exceptional strength of atomic nuclei, which underlies the stability of matter under terrestrial conditions. A characteristic feature of the strong interaction is its independence of electric charge. Hadrons are capable of strong interactions. Strong interactions cause the decay of short-lived particles (lifetime of the order of 10 -23 - 10 -24 sec.), which are called resonances.
All charged elementary particles, photons and neutral particles with a magnetic moment (for example, neutrons) are subject to electromagnetic interaction. The basis of electromagnetic interactions is the connection with the electromagnetic field. The forces of electromagnetic interaction are approximately 100 times weaker than the forces of strong interaction. The main scope of electromagnetic interaction is atoms and molecules (see Molecule). This interaction determines the structure of solids and the nature of the chemical. processes. It is not limited by the distance between elementary particles, so the size of an atom is approximately 10 4 times the size of the atomic nucleus.
Weak interactions underlie extremely slow processes involving elementary particles. For example, neutrinos, which have a weak interaction, can easily penetrate the thickness of the Earth and the Sun. Weak interactions also cause slow decays of so-called quasi-stable elementary particles, the lifetime of which is in the range of 10 8 - 10 -10 sec. Elementary particles born during strong interaction (in a time of 10 -23 -10 -24 sec.), but decaying slowly (10 -10 sec.), are called strange.
Gravitational interactions between elementary particles produce extremely small effects due to the insignificance of the particle masses. This type of interaction has been well studied on macro-objects with large masses.
The diversity of elementary particles with different physical characteristics explains the difficulty of their systematization. Of all the elementary particles, only photons, electrons, neutrinos, protons and their antiparticles are actually stable, since they have a long lifetime. These particles are the end products of the spontaneous transformation of other elementary particles. The birth of elementary particles can occur as a result of the first three types of interactions. For strongly interacting particles, the source of creation is strong interaction reactions. Leptons, most likely, arise from the decay of other elementary particles or are born in pairs (particle + antiparticle) under the influence of photons.
Flows of elementary particles form ionizing radiation (see), causing ionization of neutral molecules of the medium. The biological effect of elementary particles is associated with the formation of substances with high chemical activity in irradiated tissues and body fluids. These substances include free radicals (see Free radicals), peroxides (see) and others. Elementary particles can also have a direct effect on biomolecules and supramolecular structures, cause the rupture of intramolecular bonds, depolymerization of high-molecular compounds, etc. The processes of energy migration and the formation of metastable compounds resulting from long-term preservation of the state of excitation in some macromolecular substrates. In cells, the activity of enzyme systems is suppressed or distorted, the structure of cell membranes and surface cell receptors changes, which leads to an increase in membrane permeability and a change in diffusion processes, accompanied by the phenomena of protein denaturation, tissue dehydration, and disruption of the internal environment of the cell. The susceptibility of cells largely depends on the intensity of their mitotic division (see Mitosis) and metabolism: with an increase in this intensity, the radiosusceptibility of tissues increases (see Radiosensitivity). Their use for radiation therapy (see), especially in the treatment of malignant neoplasms, is based on this property of flows of elementary particles - ionizing radiation. The penetrating ability of charged elementary particles depends to a large extent on the linear transfer of energy (see), that is, on the average energy absorbed by the medium at the point of passage of the charged particle, per unit of its path.
The damaging effect of the flow of elementary particles especially affects the stem cells of hematopoietic tissue, epithelium of the testicles, small intestine, and skin (see Radiation sickness, Radiation damage). First of all, systems that are in a state of active organogenesis and differentiation during irradiation are affected (see Critical organ).
The biological and therapeutic effect of elementary particles depends on their type and dose of radiation (see Doses of ionizing radiation). For example, when exposed to X-ray radiation (see X-ray therapy), gamma radiation (see Gamma therapy) and proton radiation (see Proton therapy) on the entire human body at a dose of about 100 rad, a temporary change in hematopoiesis is observed; external influence of neutron radiation (see Neutron radiation) leads to the formation in the body of various radioactive substances, for example, radionuclides of sodium, phosphorus, etc. When radionuclides that are sources of beta particles (electrons or positrons) or gamma quanta enter the body, this happens called internal irradiation of the body (see Incorporation of radioactive substances). Especially dangerous in this regard are rapidly resorbing radionuclides with a uniform distribution in the body, for example. tritium (3H) and polonium-210.
Radionuclides, which are sources of elementary particles and participate in metabolism, are used in radioisotope diagnostics (see).
Bibliography: Akhiezer A.I. and Rekalo M.P. Biography of elementary particles, Kyiv, 1983, bibliogr.; Bogolyubov N. N. and Shirokov D. V. Quantum fields, M., 1980; Born M. Atomic physics, trans. from English, M., 1965; Jones X. Physics of Radiology, trans. from English. M., 1965; Krongauz A. N., Lyapidevsky V. K. and Frolova A. V. Physical foundations of clinical dosimetry, M., 1969; Radiation therapy using high-energy radiation, ed. I. Becker and G. Schubert, trans. from German, M., 1964; Tyubiana M. et al. Physical foundations of radiation therapy and radiobiology, trans. from French, M., 1969; Shpolsky E.V. Atomic physics, vol. 1, M., 1984; Young Ch. Elementary particles, trans. from English. M., 1963.
R. V. Stavntsky.
The word atom means "indivisible." It was introduced by Greek philosophers to designate the smallest particles of which, according to their understanding, matter consists.
Nineteenth-century physicists and chemists adopted the term to refer to the smallest particles known to them. Although we have long been able to “split” atoms and the indivisible has ceased to be indivisible, nevertheless this term has been preserved. According to our current understanding, an atom consists of tiny particles, which we call elementary particles. There are also other elementary particles that are not actually part of atoms. They are usually produced using high-power cyclotrons, synchrotrons and other particle accelerators specially designed to study these particles. They also occur when cosmic rays pass through the atmosphere. These elementary particles decay within a few millionths of a second, and often within an even shorter period of time after their appearance. As a result of decay, they either change into other elementary particles or release energy in the form of radiation.
The study of elementary particles focuses on an ever-increasing number of short-lived elementary particles. Although this problem is of great importance, in particular because it is connected with the most fundamental laws of physics, nevertheless, the study of particles is currently carried out almost in isolation from other branches of physics. For this reason, we will limit ourselves to considering only those particles that are permanent components of the most common materials, as well as some particles that are very close to them. The first of the elementary particles discovered at the end of the nineteenth century was the electron, which then became an extremely useful servant. In radio tubes, the flow of electrons moves in a vacuum; and it is by adjusting this flow that incoming radio signals are amplified and converted into sound or noise. In a television, the electron beam serves as a pen that instantly and accurately copies on the receiver screen what the transmitter camera sees. In both of these cases, the electrons move in a vacuum so that, if possible, nothing interferes with their movement. Another useful property is their ability, passing through a gas, to make it glow. Thus, by allowing electrons to pass through a glass tube filled with gas under a certain pressure, we use this phenomenon to produce neon light, used at night to illuminate large cities. And here is another meeting with electrons: lightning flashed, and myriads of electrons, breaking through the thickness of the air, create a rolling sound of thunder.
However, under terrestrial conditions there are a relatively small number of electrons that can move freely, as we saw in previous examples. Most of them are securely bound in atoms. Since the nucleus of an atom is positively charged, it attracts negatively charged electrons, forcing them to remain in orbits relatively close to the nucleus. An atom usually consists of a nucleus and a number of electrons. If an electron leaves an atom, it is usually immediately replaced by another electron, which the atomic nucleus attracts with great force from its immediate environment.
What does this wonderful electron look like? No one has seen him and will never see him; and yet we know its properties so well that we can predict in great detail how it will behave in the most varied situations. We know its mass (its "weight") and its electrical charge. We know that most often he behaves as if the person in front of us is very small particle, in other cases it exhibits properties waves. An extremely abstract, but at the same time very precise theory of the electron was proposed in complete form several decades ago by the English physicist Dirac. This theory gives us the opportunity to determine under what circumstances an electron will be more similar to a particle, and under what circumstances its wave character will predominate. This dual nature - particle and wave - makes it difficult to give a clear picture of the electron; therefore, a theory that takes both these concepts into account and yet gives a complete description of the electron must be very abstract. But it would be unwise to limit the description of such a wonderful phenomenon as the electron to such earthly images as peas and waves.
One of the premises of Dirac's theory of the electron was that there must be an elementary particle that has the same properties as the electron, except that it is positively charged and not negatively charged. Indeed, such an electron twin was discovered and named positron. It is part of cosmic rays, and also arises as a result of the decay of certain radioactive substances. Under terrestrial conditions, the life of a positron is short. As soon as it finds itself in the vicinity of an electron, and this happens in all substances, the electron and positron “destroy” each other; The positive electric charge of the positron neutralizes the negative charge of the electron. Since, according to relativity, mass is a form of energy, and since energy is "indestructible", the energy represented by the combined masses of the electron and positron must be conserved somehow. This task is performed by a photon (quantum of light), or usually two photons that are emitted as a result of this fateful collision; their energy is equal to the total energy of the electron and positron.
We also know that the reverse process also occurs; a Photon can, under certain conditions, for example, flying close to the nucleus of an atom, create an electron and a positron “out of nothing.” For such creation it must have an energy at least equal to the energy corresponding to the total mass of the electron and positron.
Therefore, elementary particles are not eternal or constant. Both electrons and positrons can appear and disappear; however, the energy and resulting electrical charges are conserved.
Except for the electron, the elementary particle known to us much earlier than any other particle is not the positron, which is relatively rare, but proton- the nucleus of a hydrogen atom. Like a positron, it is positively charged, but its mass is approximately two thousand times greater than the mass of a positron or electron. Like these particles, the proton sometimes exhibits wave properties, but only under extremely special conditions. The fact that its wave nature is less pronounced is actually a direct consequence of its possession of much greater mass. The wave nature, which is characteristic of all matter, does not become important to us until we begin to work with exclusively light particles such as electrons.
A proton is a very common particle. A hydrogen atom consists of a proton, which is its nucleus, and an electron, which orbits around it. The proton is also part of all other atomic nuclei.
Theoretical physicists predicted that the proton, like the electron, has an antiparticle. Opening negative proton or antiproton, which has the same properties as the proton but is negatively charged, confirmed this prediction. The collision of an antiproton with a proton “destroys” them both in the same way as in the case of a collision of an electron and a positron.
Another elementary particle neutron, has almost the same mass as a proton, but is electrically neutral (no electric charge at all). Its discovery in the thirties of our century - approximately simultaneously with the discovery of the positron - was extremely important for nuclear physics. The neutron is part of all atomic nuclei (with the exception, of course, of the ordinary nucleus of the hydrogen atom, which is simply a free proton); When an atomic nucleus collapses, it releases one (or more) neutrons. An atomic bomb explodes due to neutrons released from uranium or plutonium nuclei.
Since protons and neutrons together form atomic nuclei, both are called nucleons. After some time, the free neutron turns into a proton and an electron.
We are familiar with another particle called antineutron, which, like the neutron, is electrically neutral. It has many of the properties of a neutron, but one of the fundamental differences is that the antineutron decays into an antiproton and an electron. When colliding, a neutron and an antineutron destroy each other,
Photon, or light quantum, is an extremely interesting elementary particle. Wanting to read a book, we turn on the light bulb. So, a switched-on light bulb generates a huge number of photons that rush to the book, as well as to all other corners of the room, at the speed of light. Some of them, hitting the walls, die immediately, others hit and bounce off the walls of other objects again and again, but after less than one millionth of a second from the moment of their appearance, they all die, with the exception of only a few who manage to escape through the window and slip away into space. The energy needed to generate photons is supplied by electrons flowing through the light bulb when it is turned on; dying, the photons give off this energy to a book or other object, heating it, or to the eye, causing stimulation of the optic nerves.
The energy of a photon, and therefore its mass, does not remain unchanged: there are very light photons along with very heavy ones. Photons that produce ordinary light are very light, their mass is only a few millionths of the mass of an electron. Other photons have a mass approximately the same as the mass of an electron, and even much greater. Examples of heavy photons are x-rays and gamma rays.
Here is a general rule: the lighter the elementary particle, the more expressive its wave nature. The heaviest elementary particles - protons - exhibit relatively weak wave characteristics; they are somewhat stronger for electrons; the strongest are photons. In fact, the wave nature of light was discovered much earlier than its corpuscular characteristics. We have known that light is nothing more than the movement of electromagnetic waves since Maxwell demonstrated this throughout the second half of the last century, but it was Planck and Einstein, at the dawn of the twentieth century, who discovered that light also has corpuscular characteristics, that it sometimes is emitted in the form of individual “quanta”, or, in other words, in the form of a stream of photons. It cannot be denied that it is difficult to unite and fuse together in our minds these two apparently dissimilar concepts of the nature of light; but we can say that, like the "dual nature" of the electron, our concept of such an elusive phenomenon as light must be very abstract. And only when we want to express our idea in rough images, we must sometimes liken light to a flow of particles, photons, or wave motion of an electromagnetic nature.
There is a relationship between the corpuscular nature of a phenomenon and its “wave” properties. The heavier the particle, the shorter the corresponding wavelength; the longer the wavelength, the lighter the corresponding particle. X-rays, consisting of very heavy photons, have a correspondingly very short wavelength. Red light, which has a longer wavelength than blue light, is made up of photons that are lighter than the photons that carry blue light. The longest electromagnetic waves in existence, radio waves, are made up of tiny photons. These waves do not exhibit the properties of particles in the slightest; their wave nature is entirely the predominant characteristic.
And finally, the smallest of all small elementary particles is neutrino. It has no electrical charge, and if it has any mass, it is close to zero. With some exaggeration, we can say that the neutrino is simply devoid of properties.
Our knowledge of elementary particles is the modern frontier of physics. The atom was discovered in the nineteenth century, and scientists of that time discovered an increasing number of different kinds of atoms; in a similar way, today we are finding more and more elementary particles. And although it has been proven that atoms consist of elementary particles, we cannot expect that, by analogy, it will be found that elementary particles consist of even smaller particles. The problem facing us today is very different, and there is not the slightest sign that we will be able to split elementary particles. Rather, the hope is that all elementary particles will be shown to be manifestations of one even more fundamental phenomenon. And if it were possible to establish this, we would be able to understand all the properties of elementary particles; could calculate their masses and methods of their interaction. Many attempts have been made to approach the solution of this problem, which is one of the most important problems in physics.
There is no clear definition of the concept “elementary particle”; usually only a certain set of values of physical quantities characterizing these particles and their some very important distinctive properties are indicated. Elementary particles have:
1) electric charge
2) intrinsic angular momentum or spin
3) magnetic moment
4) own mass - “rest mass”
In the future, other quantities characterizing particles may be discovered, so this list of the main properties of elementary particles should not be considered complete.
However, not all elementary particles (a list of them is given below) have the full set of the above properties. Some of them have only an electric charge and mass, but no spin (charged pions and kaons); other particles have mass, spin and magnetic moment, but do not have an electric charge (neutron, lambda hyperon); still others have only mass (neutral pions and kaons) or only spin (photons, neutrinos). It is mandatory for elementary particles to have at least one of the properties listed above. Note that the most important particles of matter - runs and electrons - are characterized by a full set of these properties. It must be emphasized: electric charge and spin are fundamental properties of particles of matter, i.e. their numerical values remain constant under all conditions.
PARTICLES AND ANTI-PARTICLES
Each elementary particle has its opposite - an “antiparticle”. The mass, spin and magnetic moment of the particle and antiparticle are the same, but if the particle has an electric charge, then its antiparticle has a charge of the opposite sign. The proton, positron and antineutron have the same magnetic moments and spins, while the electron, neutron and antiproton have opposite orientations.
The interaction of a particle with its antiparticle is significantly different from the interaction with other particles. This difference is expressed in the fact that a particle and its antiparticle are capable of annihilation, that is, a process as a result of which they disappear, and other particles appear in their place. So, for example, as a result of the annihilation of an electron and a positron, photons, protons and antiprotons-pions, etc. appear.
LIFE TIME
Stability is not a mandatory feature of elementary particles. Only the electron, proton, neutrino and their antiparticles, as well as photons, are stable. The remaining particles are transformed into stable ones either directly, as happens, for example, with a neutron, or through a chain of successive transformations; for example, an unstable negative pion first turns into a muon and a neutrino, and then the muon turns into an electron and another neutrino:
The symbols indicate “muon” neutrinos and antineutrinos, which are different from “electronic” neutrinos and antineutrinos.
The instability of particles is assessed by the length of time they exist from the moment of “birth” to the moment of decay; both of these moments in time are marked by particle tracks in measuring installations. If there are a large number of observations of particles of a given “type”, either the “average lifetime” or the half-life of decay is calculated. Let us assume that at some point in time the number of decaying particles is equal, and at that moment this number becomes equal. Assuming that the decay of particles obeys a probabilistic law
you can calculate the average lifetime (during which the number of particles decreases by a factor) and the half-life
(during which this number is halved).
It's interesting to note that:
1) all uncharged particles, except neutrinos and photons, are unstable (neutrinos and photons stand out among other elementary particles in that they do not have their own rest mass);
2) of the charged particles, only the electron and proton (and their antiparticles) are stable.
Here is a list of the most important particles (their number continues to increase at the present time) indicating the designations and main
properties; electric charge is usually indicated in elementary units mass - in units of electron mass spin - in units
(see scan)
PARTICLE CLASSIFICATION
The study of elementary particles has shown that grouping them according to the values of their basic properties (charge, mass, spin) is insufficient. It turned out to be necessary to divide these particles into significantly different “families”:
1) photons, 2) leptons, 3) mesons, 4) baryons
and introduce new characteristics of particles that would show that a given particle belongs to one of these families. These characteristics are conventionally called “charges” or “numbers”. There are three types of charges:
1) lepton-electron charge;
2) lepton-muon charge
3) baryon charge
These charges are given numerical values: and -1 (particles have a plus sign, antiparticles have a minus sign; photons and mesons have zero charges).
Elementary particles obey the following two rules:
Each elementary particle belongs to only one family and is characterized by only one of the above charges (numbers).
For example:
However, one family of elementary particles may contain a number of different particles; for example, the group of baryons includes the proton, neutron and a large number of hyperons. Let us present the division of elementary particles into families:
leptons “electronic”: These include electron positron electron neutrino and electron antineutrino
leptons “muonic”: These include muons with negative and positive electrical charge and muon neutrinos and antineutrinos. These include the proton, neutron, hyperons and all their antiparticles.
The existence or absence of an electric charge is not associated with membership in any of the listed families. It is noticed that all particles whose spin is equal to 1/2 necessarily have one of the charges indicated above. Photons (whose spin is equal to unity), mesons - pions and kaons (whose spin is equal to zero) have neither leptonic nor baryon charges.
In all physical phenomena in which elementary particles participate - in decay processes; birth, annihilation and mutual transformations, the second rule is observed:
algebraic sums of numbers for each type of charge separately are always kept constant.
This rule is equivalent to the three conservation laws:
These laws also mean that mutual transformations between particles belonging to different families are prohibited.
For some particles - kaons and hyperons - it turned out to be necessary to additionally introduce another characteristic, called strangeness and denoted by Kaons have lambda and sigma hyperons - xi-hyperons - (upper sign for particles, lower sign for antiparticles). In processes in which the appearance (birth) of particles with strangeness is observed, the following rule is observed:
Law of conservation of strangeness. This means that the appearance of one strange particle must necessarily be accompanied by the appearance of one or more strange antiparticles, so that the algebraic sum of the numbers before and after
the birth process remained constant. It is also noted that during the decay of strange particles, the law of conservation of strangeness is not observed, i.e., this law operates only in processes of the birth of strange particles. Thus, for strange particles the processes of creation and decay are irreversible. For example, a lambda hyperon (strangeness equals decays into a proton and a negative pion:
In this reaction, the law of conservation of strangeness is not observed, since the proton and pion obtained after the reaction have strangeness equal to zero. However, in the reverse reaction, when a negative pion collides with a proton, a single lambda hyperon does not appear; the reaction proceeds with the formation of two particles having oddities of opposite signs:
Consequently, in the reaction of the creation of a lambda hyperon, the law of conservation of strangeness is observed: before and after the reaction, the algebraic sum of “strange” numbers is equal to zero. Only one decay reaction is known in which the constancy of the sum of strange numbers is observed - this is the decay of a neutral sigma hyperon into a lambda hyperon and a photon:
Another feature of strange particles is the sharp difference between the duration of the birth processes (of the order of ) and the average time of their existence (about ); for other (non-strange) particles these times are of the same order.
Note that the need to introduce lepton and baryon numbers or charges and the existence of the above conservation laws suggest that these charges express a qualitative difference between particles of different types, as well as between particles and antiparticles. The fact that particles and antiparticles must be assigned charges of opposite signs indicates the impossibility of mutual transformations between them.
Further penetration into the depths of the microworld is associated with the transition from the level of atoms to the level of elementary particles. As the first elementary particle at the end of the 19th century. the electron was discovered, and then in the first decades of the 20th century. – photon, proton, positron and neutron.
After the Second World War, thanks to the use of modern experimental technology, and above all powerful accelerators, in which conditions of high energies and enormous speeds are created, the existence of a large number of elementary particles was established - over 300. Among them there are both experimentally discovered and theoretically calculated, including resonances, quarks and virtual particles.
Term elementary particle originally meant the simplest, further indecomposable particles that underlie any material formations. Later, physicists realized the entire convention of the term “elementary” in relation to micro-objects. Now there is no doubt that particles have one structure or another, but, nevertheless, the historically established name continues to exist.
The main characteristics of elementary particles are mass, charge, average lifetime, spin and quantum numbers.
Resting mass elementary particles are determined in relation to the rest mass of the electron. There are elementary particles that do not have a rest mass - photons. The remaining particles according to this criterion are divided into leptons– light particles (electron and neutrino); mesons– medium-sized particles with a mass ranging from one to a thousand electron masses; baryons– heavy particles whose mass exceeds a thousand electron masses and which includes protons, neutrons, hyperons and many resonances.
Electric charge is another important characteristic of elementary particles. All known particles have a positive, negative or zero charge. Each particle, except the photon and two mesons, corresponds to antiparticles with opposite charges. Around 1963–1964 a hypothesis was put forward about the existence quarks– particles with a fractional electric charge. This hypothesis has not yet been confirmed experimentally.
By lifetime particles are divided into stable And unstable . There are five stable particles: the photon, two types of neutrinos, the electron and the proton. It is stable particles that play the most important role in the structure of macrobodies. All other particles are unstable, they exist for about 10 -10 -10 -24 s, after which they decay. Elementary particles with an average lifetime of 10–23–10–22 s are called resonances. Due to their short lifetime, they decay before they even leave the atom or atomic nucleus. Resonant states were calculated theoretically; they could not be detected in real experiments.
In addition to charge, mass and lifetime, elementary particles are also described by concepts that have no analogues in classical physics: the concept back . Spin is the intrinsic angular momentum of a particle that is not associated with its movement. Spin is characterized by spin quantum number s, which can take integer (±1) or half-integer (±1/2) values. Particles with integer spin – bosons, with a half-integer – fermions. Electrons are classified as fermions. According to the Pauli principle, an atom cannot have more than one electron with the same set of quantum numbers n,m,l,s. Electrons, which correspond to wave functions with the same number n, are very close in energy and form an electron shell in the atom. Differences in the number l determine the “subshell”, the remaining quantum numbers determine its filling, as mentioned above.
In the characteristics of elementary particles there is another important idea interactions. As noted earlier, four types of interactions between elementary particles are known: gravitational,weak,electromagnetic And strong(nuclear).
All particles having a rest mass ( m 0), participate in gravitational interaction, and charged ones also participate in electromagnetic interaction. Leptons also participate in weak interactions. Hadrons participate in all four fundamental interactions.
According to quantum field theory, all interactions are carried out due to the exchange virtual particles , that is, particles whose existence can only be judged indirectly, by some of their manifestations through some secondary effects ( real particles can be directly recorded using instruments).
It turns out that all four known types of interactions - gravitational, electromagnetic, strong and weak - have a gauge nature and are described by gauge symmetries. That is, all interactions are, as it were, made “from the same blank.” This gives us hope that it will be possible to find “the only key to all known locks” and describe the evolution of the Universe from a state represented by a single supersymmetric superfield, from a state in which the differences between the types of interactions, between all kinds of particles of matter and field quanta have not yet appeared.
There are a huge number of ways to classify elementary particles. For example, particles are divided into fermions (Fermi particles) - particles of matter and bosons (Bose particles) - field quanta.
According to another approach, particles are divided into 4 classes: photons, leptons, mesons, baryons.
Photons (electromagnetic field quanta) participate in electromagnetic interactions, but do not have strong, weak, or gravitational interactions.
Leptons got their name from the Greek word leptos- easy. These include particles that do not have strong interaction: muons (μ – , μ +), electrons (е – , у +), electron neutrinos (v e – ,v e +) and muon neutrinos (v – m, v + m). All leptons have a spin of ½ and are therefore fermions. All leptons have a weak interaction. Those that have an electrical charge (that is, muons and electrons) also have an electromagnetic force.
Mesons – strongly interacting unstable particles that do not carry the so-called baryon charge. Among them is r-mesons, or pions (π + , π – , π 0), TO-mesons, or kaons (K +, K –, K 0), and this-mesons (η) . Weight TO-mesons is ~970me (494 MeV for charged and 498 MeV for neutral TO-mesons). Life time TO-mesons has a magnitude of the order of 10 –8 s. They disintegrate to form I-mesons and leptons or only leptons. Weight this-mesons is 549 MeV (1074me), the lifetime is about 10–19 s. This-mesons decay to form π-mesons and γ-photons. Unlike leptons, mesons have not only a weak (and, if they are charged, electromagnetic) interaction, but also a strong interaction, which manifests itself when they interact with each other, as well as during the interaction between mesons and baryons. All mesons have zero spin, so they are bosons.
Class baryons combines nucleons (p,n) and unstable particles with a mass greater than the mass of nucleons, called hyperons. All baryons have a strong interaction and, therefore, actively interact with atomic nuclei. The spin of all baryons is ½, so the baryons are fermions. With the exception of the proton, all baryons are unstable. During the decay of baryons, along with other particles, a baryon is necessarily formed. This pattern is one of the manifestations baryon charge conservation law.
In addition to the particles listed above, a large number of strongly interacting short-lived particles have been discovered, which are called resonances . These particles are resonant states formed by two or more elementary particles. The resonance lifetime is only ~ 10 –23 –10 –22 s.
Elementary particles, as well as complex microparticles, can be observed thanks to the traces that they leave as they pass through matter. The nature of the traces allows us to judge the sign of the particle’s charge, its energy, momentum, etc. Charged particles cause ionization of molecules along their path. Neutral particles do not leave traces, but they can reveal themselves at the moment of decay into charged particles or at the moment of collision with any nucleus. Consequently, neutral particles are ultimately also detected by the ionization caused by the charged particles they generate.
Particles and antiparticles. In 1928, the English physicist P. Dirac managed to find a relativistic quantum mechanical equation for the electron, from which a number of remarkable consequences follow. First of all, from this equation, in a natural way, without any additional assumptions, the spin and numerical value of the electron’s own magnetic moment are obtained. Thus, it turned out that spin is both a quantum and a relativistic quantity. But this does not exhaust the significance of the Dirac equation. It also made it possible to predict the existence of the electron’s antiparticle – positron. From the Dirac equation, not only positive but also negative values are obtained for the total energy of a free electron. Studies of the equation show that for a given particle momentum, there are solutions to the equation corresponding to the energies: .
Between the greatest negative energy (– m e With 2) and the least positive energy (+ m e c 2) there is an interval of energy values that cannot be realized. The width of this interval is 2 m e With 2. Consequently, two regions of energy eigenvalues are obtained: one begins with + m e With 2 and extends to +∞, the other starts from – m e With 2 and extends to –∞.
A particle with negative energy must have very strange properties. Transitioning into states with less and less energy (that is, with negative energy increasing in magnitude), it could release energy, say, in the form of radiation, and, since | E| unconstrained, a particle with negative energy could emit an infinitely large amount of energy. A similar conclusion can be reached in the following way: from the relation E=m e With 2 it follows that a particle with negative energy will also have a negative mass. Under the influence of a braking force, a particle with a negative mass should not slow down, but accelerate, performing an infinitely large amount of work on the source of the braking force. In view of these difficulties, it would seem that the state with negative energy should be excluded from consideration as leading to absurd results. This, however, would contradict some general principles of quantum mechanics. Therefore, Dirac chose a different path. He proposed that transitions of electrons to states with negative energy are usually not observed for the reason that all available levels with negative energy are already occupied by electrons. 
According to Dirac, a vacuum is a state in which all levels of negative energy are occupied by electrons, and levels with positive energy are free. Since all levels lying below the forbidden band are occupied without exception, electrons at these levels do not reveal themselves in any way. If one of the electrons located at negative levels is given energy E≥ 2m e With 2, then this electron will go into a state with positive energy and will behave in the usual way, like a particle with positive mass and negative charge. This first theoretically predicted particle was called the positron. When a positron meets an electron, they annihilate (disappear) - the electron moves from a positive level to a vacant negative one. The energy corresponding to the difference between these levels is released in the form of radiation. In Fig. 4, arrow 1 depicts the process of creation of an electron-positron pair, and arrow 2 – their annihilation. The term “annihilation” should not be taken literally. Essentially, what occurs is not a disappearance, but a transformation of some particles (electron and positron) into others (γ-photons).
There are particles that are identical with their antiparticles (that is, they do not have antiparticles). Such particles are called absolutely neutral. These include the photon, π 0 meson and η meson. Particles identical with their antiparticles are not capable of annihilation. This, however, does not mean that they cannot be transformed into other particles at all.
If baryons (that is, nucleons and hyperons) are assigned a baryon charge (or baryon number) IN= +1, antibaryons – baryon charge IN= –1, and all other particles have a baryon charge IN= 0, then all processes occurring with the participation of baryons and antibaryons will be characterized by conservation of charge baryons, just as processes are characterized by conservation of electric charge. The law of conservation of baryon charge determines the stability of the softest baryon, the proton. The transformation of all quantities that describe a physical system, in which all particles are replaced by antiparticles (for example, electrons with protons, and protons with electrons, etc.), is called the conjugation charge.
Strange particles.TO-mesons and hyperons were discovered as part of cosmic rays in the early 50s of the XX century. Since 1953, they have been produced at accelerators. The behavior of these particles turned out to be so unusual that they were called strange. The unusual behavior of the strange particles was that they were clearly born due to strong interactions with a characteristic time of the order of 10–23 s, and their lifetimes turned out to be of the order of 10–8–10–10 s. The latter circumstance indicated that the decay of particles occurs as a result of weak interactions. It was completely unclear why the strange particles lived for so long. Since the same particles (π-mesons and protons) are involved in both the creation and decay of the λ-hyperon, it was surprising that the rate (that is, the probability) of both processes is so different. Further research showed that strange particles are born in pairs. This led to the idea that strong interactions cannot play a role in particle decay due to the fact that the presence of two strange particles is necessary for their manifestation. For the same reason, the single creation of strange particles turns out to be impossible.
To explain the prohibition of the single production of strange particles, M. Gell-Mann and K. Nishijima introduced a new quantum number, the total value of which, according to their assumption, should be conserved under strong interactions. This is a quantum number S was named the strangeness of the particle. In weak interactions, the strangeness may not be preserved. Therefore, it is attributed only to strongly interacting particles - mesons and baryons.
Neutrino. Neutrino is the only particle that does not participate in either strong or electromagnetic interactions. Excluding the gravitational interaction, in which all particles participate, neutrinos can only take part in weak interactions.
For a long time, it remained unclear how a neutrino differs from an antineutrino. The discovery of the law of conservation of combined parity made it possible to answer this question: they differ in helicity. Under helicity a certain relationship between the directions of the impulse is understood R and back S particles. Helicity is considered positive if spin and momentum are in the same direction. In this case, the direction of particle motion ( R) and the direction of “rotation” corresponding to the spin form a right-handed screw. When the spin and momentum are oppositely directed, the helicity will be negative (the translational movement and “rotation” form a left-handed screw). According to the theory of longitudinal neutrinos developed by Yang, Lee, Landau and Salam, all neutrinos existing in nature, regardless of the method of their origin, are always completely longitudinally polarized (that is, their spin is directed parallel or antiparallel to the momentum R). Neutrino has negative(left) helicity (corresponding to the ratio of directions S And R, shown in Fig. 5 (b), antineutrino – positive (right-handed) helicity (a). Thus, helicity is what distinguishes neutrinos from antineutrinos.
Rice. 5. Scheme of helicity of elementary particles
Systematics of elementary particles. The patterns observed in the world of elementary particles can be formulated in the form of conservation laws. Quite a lot of such laws have already accumulated. Some of them turn out to be not exact, but only approximate. Each conservation law expresses a certain symmetry of the system. Laws of conservation of momentum R, angular momentum L and energy E reflect the properties of symmetry of space and time: conservation E is a consequence of the homogeneity of time, the preservation R due to the homogeneity of space, and the preservation L– its isotropy. The law of conservation of parity is associated with the symmetry between right and left ( R-invariance). Symmetry with respect to charge conjugation (symmetry of particles and antiparticles) leads to the conservation of charge parity ( WITH-invariance). The laws of conservation of electric, baryon and lepton charges express a special symmetry WITH-functions. Finally, the law of conservation of isotopic spin reflects the isotropy of isotopic space. Failure to comply with one of the conservation laws means a violation of the corresponding type of symmetry in this interaction.
In the world of elementary particles there is a rule: everything that is not prohibited by conservation laws is permitted. The latter play the role of exclusion rules governing the interconversion of particles. First of all, let us note the laws of conservation of energy, momentum and electric charge. These three laws explain the stability of the electron. From the conservation of energy and momentum it follows that the total rest mass of the decay products must be less than the rest mass of the decaying particle. This means that an electron could only decay into neutrinos and photons. But these particles are electrically neutral. So it turns out that the electron simply has no one to transfer its electric charge to, so it is stable.
Quarks. There have become so many particles called elementary that serious doubts have arisen about their elementary nature. Each of the strongly interacting particles is characterized by three independent additive quantum numbers: charge Q, hypercharge U and baryon charge IN. In this regard, a hypothesis arose that all particles are built from three fundamental particles - carriers of these charges. In 1964, Gell-Mann and, independently of him, the Swiss physicist Zweig put forward a hypothesis according to which all elementary particles are built from three particles called quarks. These particles are assigned fractional quantum numbers, in particular, an electric charge equal to +⅔; –⅓; +⅓ respectively for each of the three quarks. These quarks are usually designated by the letters U,D,S. In addition to quarks, antiquarks are considered ( u,d,s). To date, 12 quarks are known - 6 quarks and 6 antiquarks. Mesons are formed from a quark-antiquark pair, and baryons are formed from three quarks. For example, a proton and a neutron are composed of three quarks, which makes the proton or neutron colorless. Accordingly, three charges of strong interactions are distinguished - red ( R), yellow ( Y) and green ( G).
Each quark is assigned the same magnetic moment (μV), the value of which is not determined from theory. Calculations made on the basis of this assumption give the value of the magnetic moment μ p for the proton = μ kv, and for a neutron μ n = – ⅔μ sq.
Thus, for the ratio of magnetic moments the value μ p is obtained / μn = –⅔, in excellent agreement with the experimental value.
Basically, the color of the quark (like the sign of the electric charge) began to express the difference in the property that determines the mutual attraction and repulsion of quarks. By analogy with quanta of fields of various interactions (photons in electromagnetic interactions, r-mesons in strong interactions, etc.) particles that carried the interaction between quarks were introduced. These particles were called gluons. They transfer color from one quark to another, causing the quarks to be held together. In quark physics, the confinement hypothesis was formulated (from the English. confinements– capture) of quarks, according to which it is impossible to subtract a quark from the whole. It can only exist as an element of the whole. The existence of quarks as real particles in physics is reliably substantiated.
The idea of quarks turned out to be very fruitful. It made it possible not only to systematize already known particles, but also to predict a whole series of new ones. The situation that has developed in the physics of elementary particles is reminiscent of the situation created in atomic physics after the discovery of the periodic law in 1869 by D. I. Mendelev. Although the essence of this law was clarified only about 60 years after the creation of quantum mechanics, it made it possible to systematize the chemical elements known by that time and, in addition, led to the prediction of the existence of new elements and their properties. In the same way, physicists have learned to systematize elementary particles, and the developed taxonomy has, in rare cases, made it possible to predict the existence of new particles and anticipate their properties.
So, at present, quarks and leptons can be considered truly elementary; There are 12 of them, or together with anti-chatits - 24. In addition, there are particles that provide four fundamental interactions (interaction quanta). There are 13 of these particles: graviton, photon, W± - and Z-particles and 8 gluons.
Existing theories of elementary particles cannot indicate what is the beginning of the series: atoms, nuclei, hadrons, quarksIn this series, each more complex material structure includes a simpler one as a component. Apparently, this cannot continue indefinitely. It was assumed that the described chain of material structures is based on objects of a fundamentally different nature. It is shown that such objects may not be pointlike, but extended, albeit extremely small (~10‑33 cm) formations, called superstrings. The described idea is not realizable in our four-dimensional space. This area of physics is generally extremely abstract, and it is very difficult to find visual models that help simplify the perception of the ideas inherent in the theories of elementary particles. Nevertheless, these theories allow physicists to express the mutual transformation and interdependence of the “most elementary” micro-objects, their connection with the properties of four-dimensional space-time. The most promising is the so-called M-theory (M – from mystery- riddle, secret). She's operating twelve-dimensional space . Ultimately, during the transition to the four-dimensional world that we directly perceive, all “extra” dimensions are “collapsed.” M-theory is so far the only theory that makes it possible to reduce four fundamental interactions to one - the so-called Superpower. It is also important that M-theory allows for the existence of different worlds and establishes the conditions that ensure the emergence of our world. M-theory is not yet sufficiently developed. It is believed that the final "theory of everything" based on M-theory will be built in the 21st century.
