The antimatter counterpart of the proton. The proton forms the nucleus of the hydrogen atom for example. Antiprotons are routinely produced at Fermilab's Antiproton source by slamming high energy protons from the Main Ring into a target. The resulting nuclear collision includes antiprotons as by-products and the source accumulates them over time. After a large "stack" has been built up, the antiprotons are shot out into the Tevatron where they are brought up to the largest energies. They are also found in cosmic rays but the intensity is much smaller.
The antiproton (\bar{p}, pronounced p-bar) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy. It was discovered in 1955 by University of California, Berkeley physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics.
