How to evaluate a triangular fermion loop

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Say I have a scalar field which decays into to a fermion - anti fermion pair which then interact in a closed loop to produce two photons. How would I evaluate this? Can I just follow the loop backwards from any of the two photon vertices and just write vertex factor, propogator, vertex factor, propogator, vertex factor, propogator, and then take the trace of that?
 
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Yes. But the evaluation of the integral you get can have tricky subtleties related to the chiral anomaly.
 
I end up with this
Screen%20Shot%202014-10-20%20at%201.18.59%20PM.png

How in the world do I do this integral?
 
I believe you only used one diagram; you have to add the diagram with the two photons exchanged.

See any advanced QFT textbook for how to evaluate the integral.
 

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