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Bhabha scattering

  1. May 15, 2010 #1
    I cannot figure out why there is a relative sign difference between the two diagrams for bhabha scattering. Fermi statistics is at play here but I don't see how that affects the relative sign between 2 different diagrams...
  2. jcsd
  3. May 15, 2010 #2
    Work through the Wick contractions carefully. If you have a copy of Peskin and Schroeder handy, in section 4.7 they do exactly this for an example in Yukawa theory that is similar to Bhabha scattering.
  4. May 15, 2010 #3
    The example in P&S is with t- and u-channel diagrams, for which the negative sign can come from a "heuristic" argument of antisymmetrization of the final states.

    But for Bhabha scattering, the diagrams are s- and t-channel... is there a similar heuristic argument without resorting to the Wick contractions?
  5. May 15, 2010 #4
    I thought you wanted to understand where the heuristic argument involving Fermi statistics came from. The diagrams are simply shorthand for the processes, including the Wick contractions; "resorting" to the latter is necessary to understand the former.

    Besides, it's a pretty trivial calculation; it took me three lines to check the signs of the Bhabha diagrams, and should only have taken two.
  6. May 15, 2010 #5
    okay okay :) I was hoping for something immediately obvious from just the diagrams (like the -1 for fermion loop etc.)
  7. May 15, 2010 #6
    I was wondering whether to mention the fermion loop, because its (-1) factor also comes from anticommuting the fermion operators to untangle the contractions! It's not something that I can derive from the diagram in isolation (without reference to the Feynman rules), it's just so trivial a calculation that it's easy to remember how it goes.

    PS. Where are my manners? Welcome to Physics Forums! Hope you stick around.
  8. May 16, 2010 #7
    the relative minus sign comes from the fact that you must reorder your fermion operators relative the different diagrams when you make the contraction.
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