War of particles and antiparticles. War of particles and antiparticles The history of the discovery of antiparticles

The antiparticle hypothesis first arose in 1928, when P. Dirac, on the basis of the relativistic wave equation, predicted the existence of the positron (see § 263), discovered four years later by K. Anderson as part of cosmic radiation.

An electron and a positron are not the only pair of particles and antiparticles. On the basis of relativistic quantum theory, they came to the conclusion that for each elementary particle there must be an antiparticle (the principle of charge conjugation). Experiments show that, with a few exceptions (for example, the photon and the p 0 -meson), indeed, each particle corresponds to an antiparticle.

It follows from the general provisions of quantum theory that particles and antiparticles must have the same masses, the same lifetimes in vacuum, the same modulus but opposite in sign electric charges (and magnetic moments), the same spins and isotopic spins, and the same other quantum numbers. , attributed to elementary particles to describe the laws of their interaction (lepton number (see § 275), baryon number (see § 275), strangeness (see § 274), charm (see § 275), etc.). Until 1956, it was believed that there is a complete symmetry between particles and antiparticles, i.e. if some process occurs between particles, then there must be exactly the same (with the same characteristics) process between antiparticles. However, in 1956 it was proved that such a symmetry is characteristic only for the strong and electromagnetic interactions and is violated for the weak one.

According to Dirac's theory, the collision of a particle and an antiparticle should lead to their mutual annihilation, as a result of which other elementary particles or photons arise. An example of this is the considered reaction (263.3) of the annihilation of an electron-positron pair (-1 0 e+ + 1 0 e® 2g).

After the theoretically predicted existence of the positron was confirmed experimentally, the question arose about the existence of the antiproton and antineutron. Calculations show that in order to create a particle-antiparticle pair, it is necessary to expend energy exceeding the double rest energy of the pair, since the particles must be imparted a very significant kinetic energy. To create a p-p̃-pair, an energy of approximately 4.4 GeV is required. The antiproton was actually discovered experimentally (1955) during the scattering of protons (accelerated at the then largest synchrophasotron at the University of California) by nucleons of target nuclei (copper served as the target), as a result of which a p - p̃ pair was born.

An antiproton differs from a proton in signs of electric charge and its own magnetic moment. An antiproton can annihilate not only with a proton, but also with a neutron:

(273.1) (273.2) (273.3)

A year later (1956), the same accelerator succeeded in obtaining an antineutron (ñ) and carrying out its annihilation. Antineutrons arose as a result of the charge exchange of antiprotons as they moved through matter. The charge exchange reaction р̃ consists in the exchange of charges between a nucleon and an antinucleon and can proceed according to the schemes

(273.4) (273.5)

The antineutron ñ differs from the neutron in the sign of its own magnetic moment. If antiprotons are stable particles, then a free antineutron, if it does not experience annihilation, eventually undergoes decay according to the scheme

Antiparticles have also been found for the p + meson, kaons and hyperons (see § 274). However, there are particles that do not have antiparticles - these are the so-called truly neutral particles. These include photon, p°-meson and η-meson (its mass is 1074m e , lifetime 7×10 -19 s; decays with the formation of p-mesons and γ-quanta). Truly neutral particles are not capable of annihilation, but they experience mutual transformations, which are the fundamental property of all elementary particles. We can say that each of the truly neutral particles is identical with its antiparticle.

Of great interest and serious difficulties were the proof of the existence of antineutrinos and the answer to the question whether neutrinos and antineutrinos are identical or different particles. Using powerful flows of antineutrinos obtained in reactors (fission fragments of heavy nuclei experience β-decay and, according to (258.1), emit antineutrinos), American physicists F. Reines and C. Cowan (1956) reliably recorded the reaction of capturing an electron antineutrino by a proton:

Similarly, the reaction of capture of an electron neutrino by a neutron is fixed:

Thus, the reactions (273.6) and (273.7) were, on the one hand, indisputable proof that v e and ṽ e, are real particles, and not fictitious concepts introduced only to explain β-decay, and on the other hand, confirmed the conclusion that v e and ṽ e- various particles.

Subsequently, experiments on the production and absorption of muon neutrinos showed that v m and ṽ m are different particles. It is also proved that the pair v e, v m are different particles, and the pair v e, ṽ e not the same as a couple v m, ṽ m According to the idea of ​​B. M. Pontecorvo (see § 271), the muon neutrino capture reaction was carried out (obtained by the decay of p + ®m + + v m (271.1)) by neutrons and the resulting particles were observed. It turned out that the reaction (273.7) does not occur, and the capture occurs according to the scheme

i.e., instead of electrons, m - -muons were born in the reaction. This confirmed the difference between v e and vm

According to modern concepts, neutrinos and antineutrinos differ from each other in one of the quantum characteristics of the state of an elementary particle - spnality, defined as the projection of the particle's spin onto the direction of its motion (per momentum). To explain the experimental data, it is assumed that the neutrino spin s is oriented antiparallel to the momentum p, i.e., the p and s directions form a left-handed screw and the neutrino has a left-handed helicity (Fig. 349, a). For antineutrinos, the p and s directions form a right screw, i.e., the antineutrino has a right spinality (Fig. 349, b). This property is equally valid for both electron and muon neutrinos (antineutrinos).

In order for helicity to be used as a characteristic of neutrinos (antineutrinos), the neutrino mass must be assumed to be zero. The introduction of helicity made it possible to explain, for example, the violation of the parity conservation law (see § 274) in the case of weak interactions that cause the decay of elementary particles and β-decay. So, m - -muon is assigned right helicity, m + -muon - left.

After the discovery of such a large number of antiparticles, a new task arose - to find antinuclei, in other words, to prove the existence of antimatter, which is built from antiparticles, just like matter from particles. Antinuclei have indeed been discovered. The first antinucleus, the antideuteron (a bound state of p̃ and ñ), was obtained in 1965 by a group of American physicists led by L. Lederman. Subsequently, antihelium (1970) and antitritium (1973) nuclei were synthesized at the Serpukhov accelerator.

It should be noted, however, that the possibility of annihilation upon encountering particles does not allow antiparticles to exist among particles for a long time. Therefore, for a stable state of antimatter, it must be isolated from matter. If there were an accumulation of antimatter near the part of the Universe known to us, then powerful annihilation radiation (explosions with the release of huge amounts of energy) would have to be observed. However, astrophysicists have not registered anything of the kind so far. Research carried out to search for antinuclei (ultimately antimatter), and the first successes achieved in this direction are of fundamental importance for further knowledge of the structure of matter.

We are accustomed to using anti- to denote opposite entities. For example, the hero and anti-hero in an adventure film are in a bitter battle. However, in the microcosm, particle and antiparticle are not completely opposed to each other. A particle and an antiparticle have the same mass, lifetime, spin, only the charge differs. But not everything is so simple here either.

What are antiparticles

As a rule, from the school bench, most people understand only an electric charge as a charge. Indeed, if we consider the electron and its antiparticle - the positron, then they differ precisely in the electric charge: the electron has a negative electric charge, and the positron has a positive one. However, in addition to electromagnetic, there are also gravitational, strong and weak interactions, each of which also has its own charges. Let's say that a proton, which has a positive electric charge, and an antiproton, which has a negative electric charge, in a strong interaction acquire a baryon charge (or baryon number) equal to +1 for a proton and -1 for an antiproton. Therefore, if there is no electric charge, for example, as in the case of a neutron and an antineutron, the strongly interacting particles still differ in the baryon number, which is equal to +1 for a neutron and -1 for an antineutron.

Are there situations when both baryon and electric charges are equal to zero? Yes, for example, in the case of mesons. They are made up of a quark and an antiquark, and by definition their baryon charge is zero. Consider, for example, electrically neutral K-mesons - amazing particles in which violation of the combined spatial and charge parity was discovered. There is a K0 meson and an anti-K0 meson. The electric and baryon charges of both particles are equal to zero. Why then are they considered a particle and an antiparticle? In this case, the quark composition of mesons differs. The K0 meson consists of an anti-s quark and a d quark. The anti-K0 meson, on the contrary, consists of an s-quark and an anti-d-quark. The strange quark - s - has a new quantum number or charge - strangeness. Strangeness is different for s and anti-s quarks, just as the baryon charge is different for protons and antiprotons; d-quarks and d-antiquarks have their own quantum number, similar to strangeness. These charges make it possible to distinguish between electrically and baryon-neutral K0 and anti-K0 mesons.

However, it happens that particles and antiparticles are identical. For example, the φ meson, which consists of an anti-s-quark and an s-quark, and its antiparticle, on the contrary, consists of an s-quark and an anti-s-quark. It turns out that the φ-meson is its own antiparticle. In fact, there are many particles similar to the φ meson. The most famous of these is probably the J/ψ meson, which is made up of a charm quark and an antiquark. Photons are also identical to themselves. And carriers of weak interaction - Z0-bosons - too. But there is one elementary particle for which the answer to the question of whether it is identical to itself has not yet been clarified. This particle is a neutrino. It participates only in weak and gravitational interactions. However, the gravitational interaction at the energy scales currently available does not play any role. Therefore, we can say that the neutrino participates only in weak interactions. There are two approaches to describing neutrino states in quantum field theory. The first is the so-called Dirac approach, in which neutrinos and antineutrinos are considered non-identical to each other. In other words, from the point of view of theorists, neutrinos and antineutrinos are similar to an electron and a positron. The second is Majorana's approach, in which neutrinos and antineutrinos are considered identical to each other. The choice in favor of Majorana's concept can be given by the experimental observation of neutrinoless double beta decay of nuclei. This decay is one of the most difficult to observe experimentally. Currently, this process is still not discovered.

The history of the discovery of antiparticles

Already in ancient Greece, ancient thinkers asked the question of the fundamental structure of matter. According to the scientific fashion of those years, the Greeks were looking for primary elements. As a result of these searches, the Greeks had several completely different sets of primary elements and even the concept of atoms as an extravagant appendage. But the Greeks could not make a choice between different sets, since only logical arguments were not enough for the choice, and almost 2000 years remained before the idea of ​​setting up a decisive experiment.

Only at the turn of the XVII-XVIII centuries, physics was formed as a science, the main driving force of which is experiment, and remained so until the first quarter of the XX century. It was the unexpected experimental results that gave impetus to the emergence of classical electrodynamics, the special theory of relativity, and quantum mechanics.

However, in 1928 everything changed. An outstanding English theoretical physicist, one of the creators of quantum mechanics, Paul Dirac wrote a relativistic quantum equation for particles with a half-integer spin. This equation had one important feature that Dirac did not put into it: if this equation had a solution for particles with a negative electric charge, then an additional solution would inevitably appear for particles with a positive charge. In the early 1930s, only one particle with half-integer spin and negative charge was known - that was the electron - and one particle with half-integer spin and positive charge, and that was the proton. At first, physicists thought that the two solutions of the Dirac equation corresponded to these two particles. But very soon the German mathematician Hermann Weyl proved that particles from the Dirac equation with positive and negative charges must have the same masses. And then there was a problem, because the proton is about 2000 times heavier than the electron.

That is, Dirac's theory predicted a fundamentally new fact. In modern terms, Paul Dirac predicted antiparticles. Only at first no one believed in them, and Dirac himself was criticized for an allegedly erroneous equation. And in vain. After all, it has been a year since antiparticles were discovered. Only their discoverer, the talented Soviet experimental physicist Dmitry Vladimirovich Skobeltsyn, had no idea about this. The fact is that he was fascinated by the problem that was relevant for that time: the study of the composition of cosmic rays, that is, particles that fall on Earth from space. To measure the momentum of cosmic ray particles and their charge, Skobeltsyn placed a cloud chamber - the latest device for the 1930s that recorded the tracks of charged particles - in a constant magnetic field. In such a chamber, positively charged particles coming from outer space should spin in one direction, and negative ones in the other. Skobeltsyn observed several tracks similar to electron tracks, but twisting in the opposite direction. From the height of modern knowledge, we understand that such tracks were left by positrons. But the scientist suggested that these tracks are left by electrons that fly from the surface of the Earth, where they are formed as a result of natural radioactivity, and ceased to be interested in these tracks.

Therefore, Karl Anderson is considered the world's first discoverer of positrons. This brilliant American experimenter knew about Dirac's theory and wanted to experimentally test the existence of "electrons with a different charge." Anderson used the Skobeltsyn technique with a small addition that made the American experimenter a Nobel laureate: he placed a lead plate in a cloud chamber. When a charged particle hits a plate, it loses some of its energy, its momentum decreases, and the curvature of the track in a magnetic field changes. Therefore, by changing the curvature of the track, one can understand from which side of the lead plate the particle entered the chamber. This was the information that Skobeltsyn did not have in order to discover the positron. It turned out that particles whose tracks are similar to the tracks of electrons, but twisted in the opposite direction, fly from space in the same way as ordinary electrons. Anderson staged his experiment in 1932. This year is considered the year of the discovery of antiparticles and the year from which theory in particle physics began to outstrip experiment. The neutrino, the Higgs boson, the top quark were first predicted by theorists. Sometimes experiments confirmed the theory after half a century, as was the case, for example, with the Higgs boson.

We can say that at a new level we have returned to the situation that was in Ancient Greece: theorists offer many new fundamental concepts, just as the Greeks once proposed various sets of primary elements. Only now experimenters are trying to test these concepts, if there is such a technological possibility.

What about the antiproton? This is the second antiparticle that was discovered by physicists. It was discovered in 1955 at a proton accelerator by a group of talented Italian physicist Emilio Segre, who fled the Nazis to America. The discovery was awarded the Nobel Prize in 1959. Almost simultaneously with the antiproton, the antineutron was discovered.

Hundreds of antiparticles have now been discovered. Any charged particle, not necessarily with a half-integer spin, has its own antiparticle. Nobel prizes are awarded for the discovery of antiparticles. And the property of a particle and an antiparticle discovered by Anderson during the interaction to turn into photons - to annihilate - gave rise to one of the fundamental mysteries of modern physics - the baryon asymmetry of the Universe. The Dirac equation has long been recognized by all physicists and formed the basis of quantum field theory.

From antiparticles to antimatter

If back in the 1960s physicists could obtain positrons, antiprotons and antineutrons, then it would seem that from here one step to the synthesis of antimatter, such as antihydrogen. However, there are great difficulties along the way.

To create antimatter atoms and molecules, it is not enough to obtain their building blocks - antiparticles. These antiparticles need to be slowed down. But, most importantly, antimatter must be stored in a world that consists of matter. Antiparticles cannot simply be put in a box: they will annihilate with the walls of the box. If we want to preserve antiparticles, then we must store them in a vacuum and in a "vessel without walls." For charged particles, a strong inhomogeneous magnetic field can be used as such a vessel. The task of confining neutral particles is much more difficult, but over time it was also solved using a magnetic field. At present, antihydrogen is held in magnetic Penning traps for almost 20 minutes.

Synthesis of antimatter is logical to begin with the synthesis of antinuclei. To date, however, little progress has been made in this direction. Only antihelium-3, which consists of two antiprotons and one antineutron, and antihelium-4, which consists of two antiprotons and two antineutrons, have been synthesized. (Note that antihelium-3 was synthesized at the Institute for High Energy Physics near Moscow at the U-70 accelerator, which is currently the highest-energy particle accelerator in Russia.)

Even less progress has been made in the synthesis of antiatoms. At present, only antihydrogen atoms have been synthesized. Single atoms of antihydrogen were synthesized at the European Center for Particle Physics (CERN) only in 1995. The real breakthrough came in 2002, when about 50 million antihydrogen atoms were synthesized. Since then, CERN has been a world leader in the study of the physical and chemical properties of antimatter.

Antiparticles and Fundamental Laws of Nature

In modern physics, symmetries play an exceptional role. In quantum field theory, one of the most important symmetries is the so-called CPT symmetry, that is, the symmetry with respect to the simultaneous replacement of all charges with opposite ©, the mirror reflection of space (P) and the reversal of time (T). It is believed that only CPT-symmetric theories can be realized in nature. CPT symmetry implies many properties that particles and antiparticles must obey, for example, the equality of the masses of both. What is currently interesting is not so much individual antiparticles as more complex anti-objects such as nuclei and atoms. For example, CERN actively investigates the spectroscopic properties of antihydrogen atoms. CPT symmetry requires these properties to be exactly the same as those of the hydrogen atom. Also, an antihydrogen atom must fall in the Earth's gravitational field in exactly the same way as a hydrogen atom. And such an experiment is now being carried out at CERN. So CERN is not only the Large Hadron Collider and the Higgs boson. This is also a test of the fundamental symmetries of nature. For understanding the world around us, these symmetries are even more important than the Higgs boson. So far, experiments have not been able to find a single sign of violation of the CPT symmetry.

Now let's look around and ask ourselves another natural question: why are we surrounded only by matter? And where did antimatter disappear from our world? This problem is called the baryon asymmetry of the universe. From the CPT theorem it is naive to expect that there was an equal amount of matter and antimatter after the Big Bang. This means that sooner or later global annihilation may occur. And only almost non-interacting single photons will rush through the lifeless Universe.

The riddle of baryon asymmetry has not yet been solved. Several answers can be offered here. For example, our solar system consists of matter, while another star system located far from ours consists of antimatter. But then it is not clear, for what reasons, instead of annihilation, matter and antimatter preferred to separate in space? And astronomers do not observe stellar antiworlds.

Another idea was proposed in 1967 by the Soviet academician, Nobel Peace Prize laureate Andrei Dmitrievich Sakharov. He suggested that the baryon number - the same one that we talked about at the beginning of this article - is violated, and additionally drew on the experimental fact of violation of the combined charge © and spatial (P) parity. Then unstable particles can decay somewhat differently than unstable antiparticles. And this turns out to be enough that in the end there is a little more matter than antimatter. The rest of the matter and antimatter annihilated. And all objects in the Universe consist of a small excess of matter. At present, Sakharov's theory has been supplemented and developed. But the main idea has remained unchanged.

On antimatter to the stars

It would not be an exaggeration to say that mankind dreams of flying to the stars. But even to the nearest star, Proxima Centauri, the light from the Sun takes more than three years. The rest of the stars are far away. Fantasts easily overcome such gigantic distances with the help of space-time tunnels, hyperdrives, the tenth dimension and other convenient, but, alas, just imaginary ways of transportation. In the real world, the spaceships of the first stellar explorers would have to move in the same space as light, and preferably at a speed close to the speed of light. At the same time, we want such a spacecraft to have the smallest possible mass. In this situation, there is no better fuel than antimatter for a spacecraft. Indeed, the entire mass of fuel during annihilation turns into photons that fly out of the nozzle at the speed of light. Photons must accelerate the spacecraft to very high speeds, which are fractions of the speed of light. This means that the flight to Proxima Centauri can take, say, thirty years. This is a lot, but the star explorers will have time to return to Earth within the life of one generation. What's next? It can be like in science fiction of the 1950s and 1960s: space pilots, almost ageless due to the twin paradox, and girls who are waiting for them on Earth in cryogenic chambers. Cosmic romance of the golden sixties or harsh everyday life of two thousand and fifties? But it all started with the unusual Dirac equation, which inevitably had to have two solutions, and Karl Anderson, who guessed to insert a lead plate into the cloud chamber.

There was no reason to assume that the existence of a positron, or, as it is better to call it now, an antielectron, is a feature of small particles. Despite a number of specific features, the theory of interaction between nucleons develops along the same lines as the theory of interaction of electrons. In most theoretical papers, it is assumed that nucleons should be described by equations quite similar to the Dirac equations for electrons. If so, then for nucleons one should expect the existence of antiparticles located in the same

relation to the proton and neutron, in which the positron and electron are located. Experience has shown that this is exactly the case for the proton. A little later, the antineutron was also discovered, which differs from the neutron in the direction of the magnetic moment (for a neutron, the magnetic moment and the rotational momentum vector are antiparallel, and for an antineutron, they are parallel).

Rice. 246. (see scan)

The discovery of the antiproton shows the validity of the general idea - the inseparable connection of the field with particles. Just like a pair of positrons -

an electron, a proton-antiproton pair can arise by transferring a nucleon from a state of negative energy to a state with positive energy. For this purpose, energy is required no less than This is a huge energy, 1840 times greater than the energy needed to create an electron-positron pair. Billions of electron volt accelerators were needed to make the discovery of the antiproton possible.

When a proton meets an antiproton, they will annihilate. Since nucleons transfer energy through the meson field, during annihilation their mass and energy will be given to the quanta of this field - mesons.

There is no doubt that this process will be studied in detail in the coming years.

On fig. 246 shows a photograph of the annihilation of a proton and an antiproton. The process was observed in a bubble chamber filled with liquid propane. The process diagram is shown at the top left.

Considerations about the need for the existence of antiparticles apply to neutrinos as well. The "mirror" image is called an antineutrino. The difference between the particles that make up the doublet is the same as that of the neutron and antineutron.

In the form of a doublet, there are also muons, as well as other elementary particles, which we did not talk about.

Muons are a triplet: the muon occurs in the form of varieties with plus and minus charges, as well as with a charge equal to zero. Unlike the neutron and the neutrino, a spinless neutral muon cannot have an antiparticle (one can also say that it coincides with its own antiparticle). Another particle that does not have a "reflection" is a photon.

- twins of ordinary elementary particles, which differ from the latter by the sign of the electric charge and the signs of some other characteristics. Particles and antiparticles have the same masses, spins, and lifetimes. If the particle is also characterized by other internal quantum characteristics that have a sign, then the antiparticle has the same values ​​of these characteristics, but the signs are opposite. If the particle is unstable (experiencing decay), then the antiparticle is also unstable, and their lifetimes coincide and the methods of decay coincide (up to replacement in the decay schemes of particles into antiparticles).
Ordinary matter consists of protons (p), neutrons (n) and electrons (e -). Antimatter consists of their antiparticles - antiprotons (), antineutrons () and antielectrons (positrons e +). The choice of which particles to consider as particles and which as antiparticles is conditional and determined by considerations of convenience. The antiparticle of an antiparticle is a particle. When a particle and an antiparticle collide, they disappear (annihilate), turning into gamma quanta.
In some cases (for example, a photon or π 0 -meson, etc.), the particle and the antiparticle completely coincide. This is due to the fact that the photon and π 0 -meson do not have an electric charge and other internal characteristics with a sign.

Characteristic	Particle	Antiparticle
Weight	M	M
Electric charge	+(-)Q	-(+)Q
Spin	J	J
Magnetic moment	+(-)μ	-(+)μ
baryon number	+B	-B
Lepton number	+L e , +L μ , +L τ	-L e , -L μ , -L τ
Weirdness	+(-)s	-(+)s
Charm	+(-)c	-(+)c
bottomness	+(-)b	-(+)b
topness	+(-)t	-(+)t
Isospin	I	I
Isospin projection	+(-)I 3	-(+)I 3
Parity	+(-)	-(+)
Lifetime	T	T
Decay scheme		charge conjugate

Antimatter consists of antiparticles - antiprotons, antineutrons and antielectrons - positrons e +. Particles and antiparticles are equal. The choice of which particles to consider as particles and which as antiparticles is conditional and determined by considerations of convenience. In the observable part of the Universe, matter consists of negatively charged electrons, positively charged protons and neutrons.
When an electron and a positron collide, they disappear (annihilate), turning into gamma quanta. During the annihilation of strongly interacting particles, for example, a proton and an antiproton, several mesons π + , π - , π 0 , K + , K - , K 0 are formed.

In fact, the assertion that the interaction of particles and antiparticles invariably entails the creation of photons is false even with respect to electrons and positrons. A free electron-positron pair annihilates with the formation of electromagnetic quanta only if its energy is not too high. Very fast electrons and positrons are capable of generating positive and negative pi-mesons (they are also pions), plus- and minus-muons, protons and antiprotons, and even even heavier particles - only energy would be enough. Slow protons and antiprotons during annihilation give rise to charged and neutral pions (and fast ones to other particles), which decay into gamma quanta, muons and neutrinos. In principle, the collision of a particle and its anticopy can give the output of any of the combinations of particles that are not forbidden by the principles of symmetry and conservation laws.

It may seem that annihilation is no different from other interparticle interactions, but it has one fundamental feature. In order for stable particles, such as protons or electrons, to give rise to a shower of exotic inhabitants of the microcosm when they meet, they need to be properly dispersed. Slow protons will simply change their speed when they meet - this will be the end of the matter. But the proton and antiproton, approaching, either undergo elastic scattering and disperse, or annihilate and produce secondary particles.

All of the above refers to the annihilation of free particles. If at least one of them is part of a quantum system, the situation remains the same in principle, but the alternatives change. For example, the annihilation of a free electron and a free positron can never give rise to just one quantum - the law of conservation of momentum does not allow. It is easiest to see this if you work in the system of the center of inertia of the colliding pair - then the initial momentum will be equal to zero and therefore cannot coincide with the momentum of a single photon, no matter where it goes. If a positron meets an electron that is, say, part of a hydrogen atom, one-photon annihilation is also possible - in this case, part of the momentum will be transferred to the atomic nucleus.

WHAT ABOUT ANTIGRAV?

The English physicist Arthur Schuster believed that antimatter was gravitationally repelled by ordinary matter, but modern science considers this unlikely. From the most general principles of symmetry of the laws of the microworld, it follows that antiparticles should be attracted to each other by gravity, like particles without the prefix "anti". The question of what is the gravitational interaction of particles and antiparticles has not yet been fully resolved, but the answer to it is almost obvious.
Let's start with Einstein's general theory of relativity. It is based on the principle of strict equality of gravitational and inertial masses, and for ordinary matter this statement has been experimentally confirmed by many very precise measurements. Since the inertial mass of a particle is exactly equal to the mass of its antiparticle, it seems very likely that their gravitational masses are also equal. However, this is still an assumption, albeit a very plausible one, and it cannot be proved by means of general relativity.

This is the registration of radiation with an energy characteristic of annihilation, or the direct registration of antiparticles by mass and charge. Since antiprotons and antihelium nuclei cannot fly through the atmosphere, they can only be detected with the help of instruments raised into the high layers of the atmosphere on balloons, or orbital instruments, such as the AMS-01 magnetic alpha spectrometer delivered to the Mir station in 1998 , or its much improved counterpart AMS-02 (pictured), which will begin its work on the ISS.

MAIN WAYS TO SEARCH FOR ANTIMATTER

Another argument against gravitational repulsion between matter and antimatter follows from quantum mechanics. Recall that hadrons (particles that take part in strong interactions) are made up of quarks glued together by gluon bonds. Each baryon consists of three quarks, while mesons consist of paired combinations of quarks and antiquarks, and not always the same (a meson, which includes a quark and its own antiquark, is a truly neutral particle in the sense that it is completely identical to its antimeson). However, these quark structures cannot be considered absolutely stable. A proton, for example, is composed of two u-quarks, each of which carries an elementary electric charge of +2/3, and one d-quark with a charge of -1/3 (therefore, the proton's charge is +1). However, these quarks, as a result of interaction with gluons, can change their nature for a very short time - in particular, they can turn into antiquarks. If particles and antiparticles gravitationally repel each other, the weight of the proton (and also, of course, the neutron) should oscillate slightly. However, so far no such effect has been found in a single laboratory.

There is no doubt that someday His Majesty Experiment will answer this question. We need a little - to accumulate more antimatter and see how it behaves in the terrestrial gravitational field. However, technically, these measurements are incredibly complex, and it is difficult to predict when they will be able to be implemented.

SO WHAT IS THE DIFFERENCE?

After the discovery of the positron for a quarter of a century, almost all physicists were sure that nature does not distinguish between particles and antiparticles. More specifically, it was believed that any physical process involving particles corresponds to exactly the same process involving antiparticles, and both of them are carried out with the same probability. Available experimental data testified that this principle is observed for all four fundamental interactions - strong, electromagnetic, weak and gravitational.
And then all at once everything changed dramatically. In 1956, American physicists Li Jundao and Yang Jenning published a Nobel Prize-winning paper in which they discussed the difficulty of two seemingly identical particles, the theta meson and tau meson, decaying into different numbers of pions. The authors emphasized that this problem can be solved if we assume that such decays are associated with processes whose character changes when going from right to cool, in other words, with mirror reflection (a little later, physicists realized that in general terms, we need to talk about reflections in each of the three coordinate planes - or, what is the same, about the change of signs of all spatial coordinates, spatial inversion). This means that the mirrored process may be banned or occur with a different probability than before the mirroring. A year later, American experimenters (belonging to two independent groups and working by different methods) confirmed that such processes do exist.
This was just the beginning. At the same time, theoretical physicists from the USSR and the USA realized that the violation of mirror symmetry makes possible the violation of symmetry with respect to the replacement of particles by antiparticles, which was also repeatedly proved in experiments. It is worth noting that not long before Lee and Yang, but still in the same 1956, the possibility of breaking mirror symmetry was discussed by the experimental physicist Martin Block and the great theorist Richard Feynman, but they never published these considerations.

During one of the last shuttle missions (STS-134) in 2010, a new scientific instrument, the Alpha Magnetic Spectrometer (AMS-02, Alpha Magnetic Spectrometer), will be delivered to the ISS. Its AMS-01 prototype was delivered aboard the Mir space station in 1998 and confirmed the concept's performance. The main goal of the scientific program will be to study and measure with high precision the composition of cosmic rays, as well as to search for exotic forms of matter - dark matter, strange matter (particles that contain strange (s) quarks), as well as antimatter - in particular, antihelium nuclei .

AMS TO ISS

Physicists traditionally designate mirror reflection with the Latin letter P, and the replacement of particles with their antiparticles with the letter C. Both symmetries are violated only in processes involving the weak interaction, the one that is responsible for the beta decay of atomic nuclei. It follows that it is due to weak interactions that there are differences in the behavior of particles and antiparticles.
A strange violation of mirror symmetry brought to life attempts to compensate for it in some way. Already in 1956, Lee and Yang, and independently Lev Landau, suggested that nature does not distinguish between systems that are obtained from each other by applying the C and P transformations together (the so-called CP symmetry). From the point of view of theory, this hypothesis looked very convincing and, moreover, fit well with the experimental data. However, just eight years later, employees of the Brookhaven National Laboratory discovered that one of the uncharged K-mesons (or, as they are also called, kaons) can decay into a pion pair. With strict adherence to CP-symmetry, such a transformation is impossible - and therefore, this symmetry is not universal! True, the share of supposedly forbidden decays did not exceed 0.2%, but they still took place! The discovery earned Brookhaven team leaders James Cronin and Val Fitch the Nobel Prize in Physics.

SYMMETRY AND ANTIMATTER

CP-symmetry violations are directly related to the difference between matter and antimatter. In the late 1990s, a very beautiful experiment was done at CERN with K 0 neutral kaons, each of which consists of a d quark and a more massive strange antiquark. The laws of nature allow the antiquark to lose some of its energy and turn into an anti-d. The released energy can be used to decay the kaon, but it is possible that the neighboring d-quark will absorb it and turn into a strange quark. As a result of this, a particle will appear, consisting of an anti-d-quark and a strange quark, that is, a neutral antikaon. Formally, this transformation can be described as the result of applying the CP transformation to the kaon!
Thus, if the CP symmetry is observed absolutely strictly, then the neutral kaons K 0 transform into their antiparticles with exactly the same probability as they undergo reverse transformations. Any violation of CP-symmetry will entail a change in one of these probabilities. If we prepare a beam of an equal number of neutral kaons and antikaons and follow the dynamics of the concentration of both particles, we can find out whether their quantum oscillations respect CP symmetry.

This is exactly what CERN physicists have done. They found that neutral antikaons become kaons a little faster than they turn into antikaons. In other words, a process was discovered during which antimatter turns into matter faster than matter into antimatter! In a mixture with initially equal parts of matter and antimatter, over time, even a small, but still measurable excess of matter is formed. The same effect was revealed in experiments with other heavy neutral particles - D 0 -mesons and B 0 -mesons.
Thus, by the end of the 20th century, experimenters had convincingly proved that weak interactions have different effects on particles and antiparticles. Although these differences are very small in themselves and only come to light in the course of certain transformations of very exotic particles, they are all quite real. This means the presence of physical asymmetry between matter and antimatter.
To complete the picture, one more circumstance should be noted. In the 1950s, the most important proposition of relativistic quantum mechanics, the CPT theorem, was proved. It says that particles and antiparticles are strictly symmetric with respect to the CP transformation followed by time reversal (strictly speaking, this theorem is true only without taking into account gravity, otherwise the question remains open). Therefore, if CP-symmetry is not respected in some processes, their speed in the "forward" and "reverse" directions (what to consider as both, of course, is a matter of agreement) should not be the same. This is precisely what the experiments at CERN with neutral kaons proved.

WHERE IS THE ANTI-WORLD?

In 1933, Paul Dirac was sure that in our Universe there are entire islands of antimatter, which he mentioned in his Nobel lecture. However, modern scientists believe that there are no such islands either in our Galaxy or beyond. Of course, antimatter as such exists. Antiparticles are generated by many high-energy processes - say, the thermonuclear burning of stellar fuel and supernova explosions. They arise in magnetized plasma clouds surrounding neutron stars and black holes, during collisions of fast cosmic particles in interstellar space, when the earth's atmosphere is bombarded by cosmic rays, and, finally, in accelerator experiments. In addition, the decay of some radionuclides is accompanied by the formation of antiparticles - namely, positrons. But all this is only antiparticles, and by no means antimatter. So far, no one has been able to detect even cosmic antihelium, let alone heavier elements. The search for gamma radiation with a specific spectrum, caused by annihilation at the boundaries of cosmic clusters of matter and antimatter, was also unsuccessful.

WORLD OR ANTI-WORLD?

Let's imagine that we are flying on an interstellar ship that is approaching a planet with intelligent life. How to find out what our brothers in mind are made of - matter or antimatter? You can send a reconnaissance probe, but if it explodes in the atmosphere, we may be considered space aggressors, as in Krzysztof Borun's science fiction novel Antiworld. This can be avoided by using the same neutral kaons and antikaons. As already mentioned, they are able not only to turn into each other, but also to disintegrate, and in different ways. In such decays, neutrinos can be produced accompanied either by positive pions and electrons, or by negative pions and positrons. Due to the asymmetry between matter and antimatter, the rates of such reactions are somewhat different. This circumstance can be used as "litmus paper". To test a planet for antimateriality, it is convenient to take not pure kaons and antikaons, but their mixed states; they are designated as K S and K L (S - short, and L - long). The fact is that in state L, the lifetime of a particle is 570 times longer than in state S (5.12 x 10 -8 s versus 8.95 x 10 -11 s). In the long-lived version of kaons, the symmetry of matter and antimatter is much stronger - for every 10,000 decays of the desired type, approximately 5015 produce positrons, and 4985 electrons. By the way, the historical experiment of Cronin and Finch was also made on K-mesons. Now let's start the conversation. Kaons have a characteristic mass slightly more than half that of a proton. Let's explain to the brothers in mind that we need an unstable neutral particle, the mass of which is slightly larger than the mass of the nucleus of the simplest of atoms. Alien physicists will make K-mesons and determine the characteristics of their decays. We will ask whether the sign of the electric charge of the lightest of the charged particles, generated in these decays a little more often than a similar particle of the opposite sign, coincides with the sign of the particles that make up the atoms of their world. In the case of a positive answer, it will become clear to us that positrons are part of their atoms and, therefore, the alien consists of antimatter. And if the answer is negative - you can prepare for landing!

WORLD OR ANTI-WORLD?

Reports periodically appear in the scientific literature about the discovery of non-standard primary sources of cosmic antiparticles of unknown origin. In April 2009, data were published on a mysterious excess of extremely fast positrons detected by the PAMELA detector complex. This equipment is placed on board the Russian satellite Resurs-DK, which was sent into near-Earth orbit on June 15, 2006 from the Baikonur cosmodrome. Some experts interpreted this result as possible evidence of the annihilation of hypothetical dark matter particles, but a less exotic explanation soon emerged. This hypothesis was commented on by the well-known cosmic ray specialist Veniamin Berezinsky from the Gran Sasso National Laboratory, which is part of the Italian National Institute of Nuclear Physics: “The standard model for the production of galactic cosmic rays rests on three positions. Supernova remnants are considered the first and main source of charged particles. The second idea - particles are accelerated to ultrarelativistic speeds at the fronts of post-explosive shock waves, and in this acceleration the role of their own magnetic field is very large. The third position is that cosmic rays propagate by diffusion. My former student, and now professor at the National Institute of Astrophysics, Pasquale Blasi showed that the excess of positrons detected by the PAMELA complex is quite consistent with this model.Protons accelerated in shock waves collide with particles of cosmic gas and it is in this zone of their acceleration that they turn into positive pions, which decay tsya with the formation of positrons and neutrinos. According to Blazy's calculations, this process could very well produce exactly the same concentration of positrons that PAMELA found. Such a mechanism for generating positrons looks absolutely natural, but for some reason it has never occurred to anyone until now. Blasi also showed that the same processes should also generate excess antiprotons. However, the cross section of their production is much smaller than the corresponding value for positrons, because of which they can be detected only at higher energies. I think it will become possible over time."
In general, so far everything speaks for the fact that there are no antistars, no antiplanets, or even the smallest antimeteors in space. On the other hand, conventional Big Bang models state that, shortly after birth, our universe contained the same number of particles and antiparticles. So why did the former survive and the latter disappear?