Compton scattering with off-shell photon

[Marioweee]

Homework Statement

Compute the squared modulus of the summed amplitude over the final and initial polarizations of Compton process with a virtual photon.

Relevant Equations

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How is it treated or what Feymann's rule applies to a virtual photon in an external leg? I would like to calculate the modulus of squared amplitude for the process

e-γ*→e-γ

where the * indicates that the photon is virtual. I've never dealt with virtual particles on a external leg and would like to know how to get started.
Thank you.

 

[vanhees71]

I've no idea, what the goal is. This can only be a sub-diagram in a true Feynman diagram, which in "vacuum QFT" always describes S-matrix elements with external legs symbolizing asymptotic free states. So to answer this question, we'd need the context, for which physics such an "off-shell amplitude" should be calculated.