Pair production

$\gamma\gamma \to e^+e^-$

Two photons produce an electron-positron pair. The time-reversed process of pair annihilation. Related by crossing symmetry to Compton scattering. The two tree diagrams correspond to the two orderings of photon absorption.

First observed by Breit and Wheeler (1934) in theory; extremely difficult to observe from real photons alone. Routinely produced at colliders from virtual photons in the Coulomb fields of heavy nuclei (Bethe-Heitler process).

Order $\alpha$ diagrams

2 diagrams.