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

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1. Mar 25, 2015

### unscientific

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

2. Mar 25, 2015

### Orodruin

Staff Emeritus
It is not.

3. Mar 25, 2015

### unscientific

This is taken straight from Griffith's book: "Introduction to Elementary Particles".

4. Mar 25, 2015

### Orodruin

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

5. Mar 25, 2015

### unscientific

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

6. Mar 25, 2015

### Orodruin

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

7. Mar 25, 2015

### Staff: Mentor

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.

8. Mar 26, 2015

### Hepth

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.