Two-photon decay of P-wave positronium: A tutorial

A detailed exposition of two-photon decays of P-wave positronium is given to fill an existing gap in the pedagogical literature. Annihilation decay rates of P-wave positronium are negligible compared to the rates of radiative electric dipole transitions to the lower energy S-states. This circumstance makes such decays experimentally inaccessible. However, the situation is different for quarkonium and the experimental and theoretical research of two-photon and two-gluon decays of P-wave quarkonia is a still a flourishing field.