Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    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

    Avodyne

    User Avatar
    Science Advisor

    For every closed fermion loop, there is a trace over the spin indices.
     
  6. Dec 15, 2008 #5
    Thanks, that helped
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Evaluating Feynman diagram in QFT without external fermion lines
  1. Feynman diagrams (Replies: 5)

  2. Feynman diagram (Replies: 2)

  3. Feynman Diagrams (Replies: 2)

Loading...