Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 301 / 4 / b

an internal scalar line of momentum $p$ contributes $i / (p^{2} - M^{2} + i ϵ)$ ;
an internal photon in Feynman gauge contributes $- i η_{μν} / (k^{2} + i ϵ)$ ;
a scalar-scalar-photon vertex contributes $i e (p + p^{'})^{μ}$ , where $p$ enters and $p^{'}$ leaves along the scalar line;
a scalar-scalar-two-photon vertex contributes $2 i e^{2} η^{μν}$ ;
every vertex carries momentum conservation, and every undetermined internal momentum is integrated with $d^{4} q / (2 π)^{4}$ .

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