antiparticle

An antiparticle is a subatomic particle that has the same mass as its corresponding particle but opposite electric charge and other internal quantum numbers, such as baryon or lepton number. For example, the electron has a negatively charged antiparticle, the positron, which carries a positive charge but the same mass. Other examples include the proton and the antiproton, the neutrino and the antineutrino, and the combination neutrino–antineutrino pairs in certain processes. Some particles are their own antiparticles, such as the photon and the neutral pion, while most fermions have distinct antiparticles.

Antiparticles were predicted by the Dirac equation in the early days of quantum mechanics and were experimentally confirmed with the discovery of the positron in 1932. Subsequent discoveries established the existence of antiprotons, antineutrons, and other antiparticles, reinforcing the notion that every particle has a corresponding counterpart with opposite charges and quantum numbers.

Antiparticles participate in pair production, where sufficient energy is converted into a particle and its antiparticle, and in annihilation, where a particle meets its antiparticle and their mass is converted back into energy, typically in the form of photons. These processes conserve energy, momentum, and all relevant quantum numbers, and they are central to many interactions in high-energy physics and cosmology.

In modern physics, antimatter is studied to test fundamental symmetries, such as CPT symmetry, and to explore questions about the matter–antimatter imbalance in the universe. Antiparticles are routinely produced and trapped in laboratories, and antihydrogen atoms have been created to study their properties.