Antiparticle

The existence of the positron strongly suggested to scientists that other antiparticles might exist. If there was a positively-charged electron, they asked, why could there also not be a negatively-charged proton... the antiproton. The search for the antiproton took much longer than the search for the antielectron. In fact, it was not until 1955 that Emilio Segre and Owen Chamberlain were able to prove that antiprotons are produced when protons from a powerful cyclotron collide with each other.

The antineutron is a fundamentally different kind of antiparticle than the antielectron or antiproton. Since neutrons have no electrical charge, an antineutron could not differ in this respect from its mirror image. Instead, an antineutron is a particle whose direction of spin is opposite that of the neutron. Since a particle's spin is expressed in the magnetic field that it generates—its magnetic moment—the antineutron is defined as the antiparticle with a magnetic moment equal in magnitude, but opposite in sign, to that of the neutron.

Other antiparticles also exist. For example, the electron is a member of a group of fundamental particles known as the leptons. Other leptons include the mu neutrino (muon) and the tau neutrino (tauon), electron-like particles that exist only at very high energy levels not observed under circumstances of our ordinary everyday world. Both muons and tauons have their own antiparticles, the antimuon and the antitauon.