antiparticles

Antiparticles are particles that have the same rest mass as their corresponding particles but opposite electric charge and other conserved quantum numbers, such as lepton or baryon number. In relativistic quantum theory, every particle has a corresponding antiparticle, and particle–antiparticle pairs can be created or annihilated together.

Common examples include the positron, the antiparticle of the electron; the antiproton of the proton; and the antineutron of the neutron. Each antiparticle carries the same mass as its partner but opposite charges and quantum numbers. For neutral particles, some are their own antiparticles (self-conjugate), such as the photon, the Z boson, and the neutral pion, while others, like the neutral kaons, have distinct antiparticles.

Antiparticles are produced in high-energy processes, such as particle collisions and certain radioactive decays, and in pair production where energy converts into a particle–antiparticle pair. When a particle meets its antiparticle, annihilation occurs, releasing energy—typically photons in the form of gamma rays—and sometimes other particles. The total energy of the final products equals the rest mass energy of the annihilating pair.

Historically, antiparticles were predicted by Paul Dirac in 1928 and first observed in 1932 as the positron by Carl Anderson. Today antiparticles are a standard element of the Standard Model; their existence has implications for fundamental symmetries, including CPT invariance, and for questions about the matter–antimatter asymmetry of the universe. In medicine and high-energy physics, antiparticles are used in imaging techniques such as positron emission tomography (PET) and in collider experiments.