Electrodynamics and Relativity — David J. Griffiths
Physics · pair-annihilation
Problem
In a pair annihilation experiment, an electron (mass m) with momentum pe hits a positron (same mass, but opposite charge) at rest. They annihilate, producing two photons. (Why couldn't they produce just one photon?) If one of the photons emerges at 60∘ to the incident electron direction, what is its energy?