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I need help with conserving momentum at these vertices (this is Bhabha scattering):

So in Diagram (a), the first vertext to the left. The incoming momenta are [itex]p_{1}+p_{2}[/itex]. The outgoing momentum I'll call it [itex]p[/itex]. So...shouldnt I have [itex]p_{1}+p_{2}=p[/itex]? Furthermore, is the propagator correct if I write it like this:

[itex]\dfrac{-i\eta_{\mu\nu}}{(p_{1}+p_{2})^{2}+i\epsilon}[/itex]?

Now I dont know how to conserve momentum at the second vertex, neither do I know how to do it for any vertex in Diagram (b). It's really confusing...

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# Momentum conservation at vertex in Feynman Diagrams

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