antifermion

In particle physics, an antifermion is the antiparticle of a fermion. Like fermions, antifermions have half-integer spin and obey Fermi-Dirac statistics, but they carry opposite conserved quantum numbers. The antiparticle of a given fermion has the same mass and intrinsic properties, with opposite charges and additive quantum numbers such as electric charge, lepton number, or baryon number. In vacuum, CPT symmetry implies equal masses and lifetimes for particles and antiparticles.

A familiar example is the electron and its antiparticle, the positron. Antifermions participate in the same fundamental interactions as their fermion partners (electromagnetic, weak, and, for quarks, strong interactions). The key distinction is the sign of their conserved quantum numbers; for instance, a positron has lepton number opposite to that of the electron. Quarks have corresponding antiquarks, such as the anti-up or anti-down quark, and protons have antiprotons as their antiparticles.

Antifermions are routinely produced in high-energy processes as particle–antiparticle pairs, such as electron–positron pairs or quark–antiquark pairs in collider events. When a fermion meets its antifermion, annihilation typically occurs, converting the combined mass into energy in the form of photons or other bosons.

Examples of antifermions include the positron (antielectron), antimuon, antitau, and the various antiquarks (anti-up, anti-down, etc.), as well as antineutrinos corresponding to their neutrino partners. In cosmology, the existence of antimatter raises questions about matter–antimatter asymmetry, a subject of ongoing research related to CP violation and baryogenesis.