I am currently interested in 3-point QED amplitudes that involve both a photon and massless fermions. In the "limit" of real momenta these 3-point amplitudes vanish. However if we consider complex momenta they do not vanish.

That being said I try to figure out how the three point amplitudes would look like, if (via crossing symmetry) all particles are incoming. I think, that

$M(f^{h_1}\bar{f}^{h_2}\gamma^{h_3}) = (-ie) \bar{v}_{h_2}(p_2) \gamma^{\mu}u_{h_1}(p_1) \epsilon^{\mu}_{h_3}(p_3)$

($h_i$ denotes the helicity of the corresponding particle, all particles are taken to be massless)

should be the correct expression for the amplitude, but I struggle to find a source that can confirm or deny this. Therefore I would be thankful, if someone, could confirm or correct this result.


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