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(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.



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.


References in periodicals archive ?
mesons, the positronium is formed by an antielectron (positron) and an electron in a semi-stable arrangement, the protonium is formed by a proton and an antiproton also semi-stable, the antiprotonic helium is formed by an antiproton and electron together with the helium nucleus (semi-stable), and muonium is formed by a positive muon and an electron.
Ulmer says the CERN measurement should improve on the precision of the current antiproton magnetic moment measurement by a factor of a thousand.
The antiprotons were spotted by the Pamela satellite - launched in 2006 to study the nature of high-energy particles from the Sun and from beyond our Solar System - so-called cosmic rays.
Each antimatter atom has antielectrons (positrons), antiprotons, and antineutrons, except the antiatom of ordinary hydrogen which has no antineutron.
Each smashup between a proton and an antiproton can create additional particles that break down into spectacular showers of debris.
Because the Tevatron collides protons with antiprotons, it's also better suited than the LHC to explore ideas about why nature contains so much more matter than antimatter (SN: 6/19/10, p.
It is currently focusing on making much bigger, colder blobs of antiprotons that can then be combined with positrons to make antihydrogen.
To make an antihelium near Earth, cosmic-ray collisions would have to create four particles--two antiprotons and two antineutrons--at nearly the same time.
After reading about the use of electrons in a particle accelerator to "cool" the antiprotons in a secondary ring ("Smashing Success: Accelerator gets cool upgrade," SN: 2/4/06, p.
In the Tevatron accelerator, discrete bunches of protons and antiprotons circulate in opposite directions around a 6-kilometer ring.
Plus, the ATRAP team took another step: They used an electric field to pry apart the exotic atoms' positrons and antiprotons.
In the new experiment, known as ATHENA, researchers trapped antiprotons in electric and magnetic fields and then mixed them with positrons in an ultracold, jar-size cylinder.