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QED vertex

  1. May 4, 2009 #1
    If the momenta on the three external legs p(incoming fermion), p'(outgoing fermion) and
    p-p' (photon) of a vertex diagram are replaced by -p, -p' and p'-p respectively (i.e., all the external momenta are multiplied by -1) does the amplitude remain unchanged?
  2. jcsd
  3. May 4, 2009 #2


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    yes due to crossing symmetry
  4. May 7, 2009 #3
    The analytic continuation argument used to derive crossing symmetry (Itzykson & Zuber)
    seems to require that there are no massless particles (vacuum is an isolated point). Further
    it appears that the external legs must be on mass-shell in the crossing symmetry derivation.
    In the 3-point vertex function all the external legs cannot be on mass-shell. I think the
    answer to the question is still yes, but short of a lengthy analytic continuation argument
    that handles massless particles and doesn't require mass-shell condition (which may not even work), it's unclear how crossing symmetry can be applied directly. Any thoughts?
  5. May 7, 2009 #4


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    ah yes, a photon is involved, didn't thought of that =/
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