I'm just working on a particle physics assignment and seem to be a little stuck on one concept. I have a positron-electron to two scalar particles and a photon-photon -> electron-positron problem. The issue I'm having with it is that the only way I really know how to handle them is with the casimir tricks, but the casimir tricks seem to be for when you have for example u'*gamma*u where u and u-bar are an incoming and outgoing fermion and gamma is a matrix. Is there a different trick to handle the case for having incoming photons rather than an incoming fermion? Wow, problem solved, I feel silly, antifermions have u-bar for incoming particles rather than outgoing, this is where my issue is. People of the future, read carefully.