Pair production

Pair production is the conversion of electromagnetic energy into a matter-antimatter particle pair, most commonly a high-energy photon producing an electron-positron pair near a heavy nucleus: γ + nucleus → e⁺ + e⁻ + nucleus. The nucleus is required as a third body to absorb momentum — a single photon cannot produce a pair in vacuum, because its four-momentum is null while the pair's combined four-momentum is timelike, and four-momentum conservation has no solution. With a recoiling nucleus to take up the spare momentum, the process is allowed, and the threshold photon energy is E_γ ≥ 2m_e c² ≈ 1.022 MeV in the limit of a nucleus much heavier than the pair.

Paul Dirac's 1928 relativistic wave equation predicted the existence of antimatter as the negative-energy solutions of his equation, reinterpreted as positive-energy "holes" in a filled negative-energy sea — the positron, an electron's antiparticle with charge +e and the same rest mass. Dirac's prediction was treated with scepticism until Carl Anderson, in 1932, observed cloud-chamber tracks of cosmic-ray-induced pairs at Caltech: a curved track with positive curvature in a magnetic field, of the same radius as electron tracks, was an electron's antiparticle. Anderson received the 1936 Nobel for the discovery. Pair production was thereafter the canonical signature of high-energy photon interactions in matter and the experimental confirmation that mass-energy equivalence operates in both directions: matter can be converted into energy, and energy into matter, with c² as the conversion factor in both.