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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 ?
In the Tevatron accelerator, discrete bunches of protons and antiprotons circulate in opposite directions around a 6-kilometer ring.
0i] uniformly occupied by one proton or one antiproton of every virtual couple with equal probability.
By comparing this magnetic property of protons with that of antiprotons, researchers hope to gain insight into why the universe is dominated by matter rather than antimatter.
The papers of this proceedings were presented at the Workshop on Physics with Ultra Slow Antiproton Beams, held in Wako, Saitama, Japan in March 2005.
Other categories would be (c) a matter atom with where one or more (but not all) of the electrons and/or protons are replaced by antimatter particles of the same corresponding charges, and (d) an antimatter atom such that one or more (but not all) of the antielectrons and/or antiprotons are replaced by matter particles of the same corresponding charges.
We are grateful that CERN agreed to host and give support to the experiments," says Welch, "as CERN is the only research facility in the world providing an antiproton beam with the characteristics needed for such experiments.
Some Fermilab scientists will spend months, others years, digging through the pile of information left behind: about a million billion recorded collisions between protons and their antimatter counterparts, antiprotons.
To form antihydrogen during these sessions, antiprotons were mixed with positrons inside the trap.
An atom of unmatter is formed either by (1): electrons, protons, and antineutrons, or by (2): antielectrons, antiprotons, and neutrons.
It will be used at the upcoming "Facility for Antiproton and Ion Research" (FAIR) to focus a divergent antiproton beam emerging from the pbar target.
A tri-partite contract with FAIR (Facility for Antiproton and Ion Research), Germany, and Bose Institute for supply of 600 converters worth nine million is signed by ECIL.
These include the Facility for Rare Isotope Beams at Michigan State University in East Lansing and the Facility for Antiproton and Ion Research at the GSI in Darmstadt, Germany.