# 'Special' fourth order Feynman diagram Compton Scattering - Why is it allowed?

## Main Question or Discussion Point

I thought the fundamental electromagnetic vertex is

Why is the following diagram below allowed? The 'special' feynman diagram for compton scattering ##e^- + \gamma \rightarrow e^- + \gamma## is

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Orodruin
Staff Emeritus
Homework Helper
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It is not.

It is not.
This is taken straight from Griffith's book: "Introduction to Elementary Particles".

Orodruin
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It is still not allowed, assuming the solid lines actually represent electron propagators.

It is still not allowed, assuming the solid lines actually represent electron propagators.
True. I think there is a printing error. The long vertical line is supposed to be a virtual propagator (curly line) I think.

Check http://hep.uchicago.edu/~pilcher/p237-06/hw8_sol.pdf

Orodruin
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Curly line means photon propagator, not necessarily a virtual particle.

unscientific
mfb
Mentor
True. I think there is a printing error. The long vertical line is supposed to be a virtual propagator (curly line) I think.

Check http://hep.uchicago.edu/~pilcher/p237-06/hw8_sol.pdf
I don't see your diagram there. The last one on page 1 (rotated by 90 degrees) has a curly line = photon at the place where your diagram has an error.

It is certainly virtual independent of its type because it has two ends in the diagram.

Hepth
Gold Member
Assuming its a photon connecting the bottom to the triangle loop, its the last diagram in that list. This diagram and the one with the loop's arrows in the opposite direction exactly cancel. (Write it out, take the trace, add it up). So it is not the diagram that is zero, but the sum of the two.

vanhees71