Antiproton


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antiproton

[′an·tē¦prō‚tän]
(particle physics)
The antiparticle to the proton; a strongly interacting baryon which is stable, carries unit negative charge, has the same mass as the proton (938.3 MeV), and has spin ½.

Antiproton

 

symbol ρ̄ or p̄), antiparticle with respect to the proton. The mass and spin of an antiproton are equal to those of a proton, but the electrical charge and magnetic momentum, although identical in absolute value, are opposite in sign. The existence of the antiproton was predicted by contemporary theories of elementary particles, and research for its existence in cosmic rays was conducted for about 20 years. Antiprotons were experimentally discovered in 1955 by O. Chamberlain, E. Segrè, C. Wiegand, and T. Ypsilantis at Berkeley (USA), using a proton accelerator with a maximum energy of 6.3 giga electron volts (GeV).

In accordance with the law of conservation of heavy particles (baryons), antiprotons can be bred only in pairs with protons (or with neutrons, if the law of conservation of electric charge is satisfied). The threshold (minimum) energy for creation of a proton-antiproton pair through the collision of two free protons in the laboratory systems of coordinates—that is, in a system in which one of the protons in the collision is at rest—is ~6.6GeV; for collision with a proton or a neutron that is bound in the atomic nucleus, it is about 4 GeV. Therefore, formation of antiprotons on nuclei was expected in a 6.3 GeV proton accelerator.

In the experiment by Chamberlain and the others, antiprotons were bred through the collisions of protons from the accelerator with a copper target. At least 1011 collisions are required for a minimal antiproton generation. A magnetic deflection system removes the negatively charged particles, the great majority of which are negatively charged pi-mesons. To detect antiprotons—that is, to differentiate them from other negatively charged particles—the mass must be determined. This is done by determining the momentum (by deflection in a magnetic field) and velocity (with the use of a Cherenkov counter) of the particles.

Another notable feature of antiprotons was observed in experiments—their annihilation in collisions with protons and neutrons of nuclear matter. It turns out that four or five high-energy pi-mesons are created as a result of antiproton annihilation.

With the help of accelerators it is now possible to obtain quite intensive beams of antiprotons. In experiments with such beams in the 1960’s a number of short-lived elementary particles (meson resonances) were discovered.

V. P. PAVLOV

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