What is the difference between these two Feynman diagrams? They should both describe the same physical process, annihilation between an electron and a positron.
Answer
The first process corresponds to e−e+→e−e+ (Bhabha scattering), where the final and initial states are the same, consisting of an electron and positron. However, the second process is e−e+→γγ, where instead the final state is that of two photons. The scattering amplitudes will be different. Notice that the first diagram requires an insertion of the photon propagator,
−iημνq2+iϵ
whereas the second diagram has a fermionic internal line, requiring a propagator,
i(q+mf)q2−m2f+iϵ
In addition, the second diagram will contain polarization vectors, as the photons are not internal lines but rather external. For a comprehensive overview of QED, see Peskin and Schroeder's text.
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