Antiproton
The antiproton is the antimatter particle (antiparticle) to the proton. It has the same mass as the proton (mass-proton/mass-antiproton = 1.000.000.001(69)), but negative charge.
Antiprotons are part of cosmic radiation. They do not occur naturally on Earth and can only be produced artificially in particle accelerators; other antiparticles, however, namely the antineutrino and the positron, are also produced naturally in beta decays.
Antiprotons were first artificially produced in 1955 at the Bevatron at Lawrence Berkeley National Laboratory with a proton beam of 6.3 GeV striking a copper target. The energy available at this projectile energy in the center-of-mass system is just enough for nucleon pair production (proton and antiproton), so the antiproton moves slowly. Magnetic deflection of the particles allowed "sorting out" of the antiprotons. Pulse and velocity analysis in two scintillation counters provided evidence that negatively charged particles with proton mass had been created; the meson background was suppressed by suitable coincidence circuits. Contrary to expectations, the antiproton does not annihilate into photons with one proton, but several free pions are produced.
Emilio Segrè, together with Owen Chamberlain, received the Nobel Prize in Physics in 1959 "for their discovery of the antiproton". Clyde E. Wiegand and Thomas Ypsilantis were also involved in the experiment.
The Antiproton Decelerator storage ring at CERN is used for antiproton research. A facility for antiproton research, called Facility for Antiproton and Ion Research (FAIR), is under construction at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt.
One application of antiprotons could be in antiprotonic stereography, which would allow radiation therapy to be performed much more efficiently. However, generating an antiproton beam is much more complex than generating a proton beam. So far (2019), only a few accelerators worldwide are capable of doing so.
The antiproton forms the nucleus of the simplest antiatom: antihydrogen.
