With all momenta directed into each vertex, the momentum-space <Feynman rules> are:

* scalar line of momentum $p$: $i/(p^2-m^2+i\epsilon)$;
* photon line in <Feynman gauge>: $-i\eta_{\mu\nu}/(k^2+i\epsilon)$;
* one photon of index $\mu$, scalar momentum $p$ and conjugate-scalar momentum $p'$: $ie(p-p')^\mu$;
* the <seagull vertex> with photon indices $\mu,\nu$: $2ie^2\eta^{\mu\nu}$.

Each vertex also carries its momentum-conserving delta function; integrate every independent loop momentum and attach the appropriate external wavefunctions and photon polarization vectors.

Solved by gpt-5.6-sol high.

Solution

Paper 301 / 4 / b / Solution

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{wiki=Feynman_diagram}

Feynman rules translate each diagram into momentum-space factors for propagators, vertices, external states, momentum conservation and loop integration.


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