In Griffith's Elementary Particles we are asked to draw all four vertex Feynman Diagrams for Compton Scattering. Most are straightforward but one puzzles me.
As has been said here: https://www.physicsforums.com/threads/special-fourth-order-feynman-diagram-compton-scattering-why-is-it-allowed.805039/
But my question is, does this not represent pair annihilation instead of Compton scattering? All other fourth-order Feyman Diagrams feature electron and photon going in and electron and photon going out.
This one however an electron-positron pair going in and two photons going out. I understand that the two processes are related by crossing symmetry, but are they considered to be the same process or am i missing something important?
Best Answer
Time in this diagram (as the arrow shows) goes from left to right instead of bottom to top. Thus the particles on the left (an electron and photon) are the initial particles and those on the right (an electron and photon) are the final particles. Hence this is Compton scattering instead of pair production.