antiparticles


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antiparticles

A pair of elementary particles, such as the electron plus positron or the proton plus antiproton, that have an identical rest mass but a charge, strangeness, and other fundamental properties of equal magnitude but opposite sign (positive rather than negative and vice versa). Thus, the electron has a negative charge and the positron an equal positive charge. A reaction of a particle with its antiparticle is called annihilation. See also pair production.

Antiparticles

 

a group of elementary particles that have the same values of mass and other physical characteristics as their particle “twins” but are distinguished from them by their sign in certain characteristics of interactions—for example, electrical charge or magnetic moment. To a certain degree, the names “particle” and “antiparticle” are themselves conventional; it would be possible to call the anti-electron (a positively charged electron) the particle and the electron the antiparticle. However, atoms of the substances in the part of the universe we can observe contain electrons with precisely a negative charge, and protons have a positive charge. It is for this reason that the elementary particles known by the early 1920’s—the electron and proton (and later the neutron)—were designated “particles.”

The existence of antiparticles was first deduced in 1930 by the English physicist P. Dirac. He derived an equation describing the behavior of the electron at speeds close to that of light. As it turned out, this equation has an important property of symmetry: describing the negatively charged electron, it simultaneously compels the conclusion that there exists a particle with the same mass as the electron but with a charge of opposite sign—the anti-electron. According to Dirac’s theory, the collision of a particle and an antiparticle must lead to annihilation, the disappearance of this particle-antiparticle pair, and, as a result, the production of two or more other particles, such as photons.

In 1932 the American physicist C. Anderson discovered anti-electrons experimentally. He photographed showers formed by cosmic rays in a Wilson chamber placed in a magnetic field. A charged particle moves in a circle in a magnetic field; particles with different signs are deflected by the field in opposite directions. Along with the tracks of fast electrons, which were well known at the time, Anderson showed in photographs tracks, altogether identical in appearance, of positively charged particles of the same mass. These were called positrons. The experimental demonstration of the positron was a brilliant confirmation of Dirac’s theory. The search for other antiparticles began at this time.

In 1936 yet another particle-antiparticle pair was discovered also in cosmic rays: the positive and negative mu-meson (μ+ and μ-). In 1947 it was established that mu-mesons of cosmic rays arise as a result of the decay of certain heavier particles—pi-mesons (π+ and π-), which are particles and antiparticles both.

In 1955 the American physicists E. Segrè, O. Chamberlain, and others observed the first antiprotons obtained in the scattering of protons of very high energy (accelerated in the bevatron of the University of California) by nucleons (protons and neutrons) of target nuclei (the nuclei of copper served as the target). The physical process that resulted in the formation of antiprotons was the production of the proton-antiproton pair. The existence of antiprotons is most clearly demonstrated by their subsequent annihilation in collisions with protons of the target. It was precisely because of annihilation that somewhat later the discovery of antineutrons, which leave no track in a Wilson chamber because they have no electrical charge, was registered. In the annihilation of both the antiproton and the antineutron, four or five pi-mesons emerge, some of which are charged and leave a characteristic track in the Wilson chamber. To date, antiparticles have been experimentally detected in almost all the particles, except for certain resonances. There is no doubt, however, of the existence of these as well.

The general principles of quantum field theory permit a number of profound deductions to be drawn about the properties of particles and antiparticles. First and foremost, the mass and spin of a particle must coincide with those of an antiparticle (the same also holds regarding their isotopic spins). Furthermore, the lifetime of a particle and its antiparticle must be identical; in particular, stable antiparticles correspond to stable particles. Not only electrical charges, but all other quantities characterizing the electrical (and consequently magnetic, as well) properties—for example, magnetic moments—of particles and antiparticles must be identical in magnitude but opposite in sign. This applies as well to electrically neutral particles such as the neutron and the hyperons lambda (∧°) and sigma-zero (Σ°); their antiparticles are also electrically neutral, but they have magnetic moments that are opposite in sign. Other quantum numbers attributed to particles to describe the mechanisms of their interaction also have opposite signs: baryonic charge, lep-tonic charge, and strangeness. Only a few particles are truly neutral; they not only have no electrical properties (their charge and magnetic moment are equal to zero), but all their remaining quantum numbers that distinguish a particle from an antiparticle are equal to zero. Thus, for truly neutral particles the antiparticles coincide with the particles themselves. Such particles are the photon and the neutral pi- and eta-mesons (π0 and η0).

Until 1956 it was believed that there is complete symmetry between particles and antiparticles: if there is a certain process among particles, then the identical process must exist among antiparticles as well. In 1956 it was demonstrated that this symmetry exists only in strong interactions (nuclear) and in electromagnetic interactions. In weak interactions, which determine the decay of particles, it was discovered that particle-antiparticle symmetry is violated. Specifically, the geometric characteristics of the decay of particles turned out to be different from those of the decay of the corresponding antiparticles; if the products of the decay of a particle escape primarily to one side, then the products of the decay of an antiparticle must primarily escape to the opposite side.

Theoretically, “antimatter” can be constructed out of antiparticles exactly as matter is constructed out of particles. However, the possibility of annihilation upon encountering particles does not allow antiparticles to exist in matter for any extended time. Antimatter may “live” for a long time only in the complete absence of contact with particles of matter. Powerful annihilation radiation coming from an area of contact between matter and antimatter would be evidence of the presence of antimatter somewhere in the universe. However, there is no evidence that would testify to the existence of a region of the universe filled with antimatter.

REFERENCES

Ford, K. Mir elementarnykh chastits. Moscow, 1965. (Translated from English.)
Vlasov, N. A. Antiveshchestvo. Moscow, 1966. (Bibliography, pages 180–184.)

V. P. PAVLOV

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