Even electrically neutral particles, such as the neutron, are not identical to their antiparticle. In the example of the neutron, the 'ordinary' particle is made out of quarks and the antiparticle out of antiquarks.
Particle-antiparticle pairs can annihilate each other if they are in appropriate quantum states. They can also be produced in various processes. These processes are used in particle accelerators to create new particles and to test theories of particle physics. High energy processes in nature can create antiparticles. These are visible in cosmic rays and in certain nuclear reactions. The word antimatter properly refers to (elementary) antiparticles, composite antiparticles made with them (such as antihydrogen) and to larger assemblies of either.
In 1932, soon after the prediction of positrons by Paul Dirac, Carl Anderson found that cosmic-ray collisions produced these particles in a cloud chamber – a particle detector in which moving electrons (or positrons) leave behind trails as they move through the gas.
The antiproton and antineutron were found by Emilio Segrè and Owen Chamberlain in 1955 at the University of California, Berkeley. Since then the antiparticles of many other subatomic particles have been created in particle accelerators. In recent years, complete atoms of antimatter have been assembled out of antiprotons and positrons, collected in electromagnetic traps.
- The exceptions are massless bosons such as the photon and the graviton.
- The laws of nature were thought to be symmetric between particles and antiparticles until CP violation experiments found that time-reversal symmetry is violated in nature. This small asymmetry is involved in baryogenesis, the process by which our universe came to consist almost entirely of matter, with almost no free antimatter.