I Feynman diagram(s)

Hi guys,
And Happy new year :) I am revising for a module on relativistic quantum mechanics currently that I took some time ago.

A large portion of the module, and hence the exam involves drawing Feynman diagrams and computing invariant amplitudes. The part I am a little rusty on is how to know how many diagrams there are for a particular process, and when I must add the two together in order to obtain the invariant amplitude.

Any general advice would be appreciated but I realise this could be a little vague so here is a more concrete example from a past paper.

Consider a spin-1/2 particle ψ of charge q and mass m, and a spin-0 particle φ of the same mass and charge. Draw the lowest order Feynman diagram for the process

ψφ → ψφ

What I am not sure of is whether there is one or two diagrams here that contribute to the invariant amplitude?

Many thanks

king vitamin

Science Advisor
Gold Member
Are these two particles minimally coupled to a U(1) gauge field? (We need to know the precise Hamiltonian you're working with to give an answer)
Thanks very much for your response. Yes, I think we are supposed to assume so, though this is the full extent of the question with not extra information given.

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