Evaluating Feynman diagram in QFT without external fermion lines

1. Dec 14, 2008

Amanheis

...like in Photon-Photon-scattering. I know this doesn't make sense physically but the value for the diagram should still be computeable. If I want to put the expression for the matrix element together, I get a matrix, but it should be a scalar, right? Since the spinor bi-product is missing....
Do I have to take the trace?

2. Dec 14, 2008

Staff Emeritus
I'm not quite sure what you are doing, but it sounds like you are missing the polarization indices for the external photon lines. Could that be it?

3. Dec 14, 2008

Amanheis

No, contracting e_\mu with gamma^\mu gives still a matrix, since e_\mu is just a 4-vector. Or am I wrong on this part?

4. Dec 14, 2008

Avodyne

For every closed fermion loop, there is a trace over the spin indices.

5. Dec 15, 2008

Amanheis

Thanks, that helped