# How to determine exchange particle of two-to-two process?

For the process, $$e^+e^- \rightarrow e^+e^-$$ I know that the exchange particle is a photon and for the process $$\gamma\gamma \rightarrow e^+e^-$$, the exchange particle would be an electron but I'm not sure what charge this electron would have. I have no idea how someone would figure out what the exchange particles for a two-to-two process would be.

I'd really help any sort of guidance.

• For that first one, charge should be conserved at every vertex – Triatticus Jan 17 at 12:52