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Evaluating Feynman diagram in QFT without external fermion lines

  1. Dec 14, 2008 #1
    ...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. jcsd
  3. Dec 14, 2008 #2

    Vanadium 50

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    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?
  4. Dec 14, 2008 #3
    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?
  5. Dec 14, 2008 #4


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    For every closed fermion loop, there is a trace over the spin indices.
  6. Dec 15, 2008 #5
    Thanks, that helped
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