# How to know what type of diagram contributes to a two-to-two process?

There are 3 types of diagrams that can contribute to a two-to-two process; the $$s$$-channel, $$u$$-channel and $$t$$-channel. How do I know what diagrams can contribute to a process?

I know that in QED, only $$\gamma \rightarrow f\bar{f}$$ vertices are allowed, where $$\gamma$$ is a photon, $$f$$ is a fermion and $$\bar{f}$$ is the anti-fermion.

So for the process $$e^+e^- \rightarrow \mu^+ \mu^-$$, only the $$s$$-channel is allowed and not the $$t$$- or $$u$$-channels.

• Read Peskin chapters 3,4,5 – InertialObserver Jan 16 '19 at 10:42
• All diagrams contribute to a process, upto an imposed cutoff. – Mozibur Ullah Jan 16 '19 at 19:59

For the process $$e^{+}e^{-} \to \mu^{+}\mu^{-}$$ the Feynman rules eliminate all but the s-channel diagram, because the rest contain a $$\gamma \mu^{\pm} e^{\mp}$$ vertex, and that doesn't exist in the theory.
If instead we considered $$e^{+}e^{-} \to e^{+}e^{-}$$ then only the u-channel would be eliminated since it requires a vertex $$\gamma e^{\pm} e^{\pm}$$.