Definition
Antimatter is matter composed of antiparticles, each of which has the same mass as its ordinary-matter partner but opposite values for charge, lepton number, baryon number, and other quantum numbers.
When a particle meets its antiparticle they annihilate completely, converting their rest mass into photons or other particle pairs according to E = mc². The reverse — pair production — creates a particle-antiparticle pair from sufficiently energetic photons.
Why it matters
How it works
Dirac's relativistic equation for the electron has two sets of solutions — one with positive energy, one with negative. To avoid an unstable universe in which all electrons cascade into negative-energy states, Dirac reinterpreted the negative-energy solutions as positively charged particles with positive energy moving backward in time, or equivalently, as a new species of particle: the positron.
Anderson saw exactly such a particle in a cloud chamber four years later. Since then every fermion in the Standard Model — quarks, electrons, muons, taus, neutrinos — has been found to have an antiparticle. Force-carrying bosons either have antiparticles (the W⁺ and W⁻) or are self-conjugate (the photon, the Z, the gluon up to color).
Annihilation is the antimatter signature. An electron meeting a positron at rest produces two 511 keV gamma rays back-to-back — a process exploited in positron-emission tomography. At higher energies the annihilation can produce heavier particle-antiparticle pairs, which is how colliders like LEP and the Tevatron searched the energy frontier.
The matter-antimatter asymmetry of the cosmos remains one of physics' open problems. Tiny CP-violating differences observed in kaon and B-meson decays are not enough to explain why our galaxy is matter rather than half-and-half. Some grand unified theories and leptogenesis models try to fill the gap.