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A Problem with Feynman rules in QCD -- help understanding please

  1. Oct 17, 2017 #1
    Hi, I'm trying to compute the cross section of a gluon emission from quarks in a electron-positron annihilation at first order (I attach the diagrams below).


    Applying the Feynman rules the matrix element should be:
    My problem is that I have no idea where this minus sign comes from. (I obtain a plus sign).

    If anyone can help me I would be very grateful.


    P.S. In the equation I posted, k1, k2 and k3 are the 4-momentum of the quark, antiquark and the gluon respectively and p1 and p2 of the electron and positron.
  2. jcsd
  3. Oct 18, 2017 #2


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    The momenta ##k_2## and ##k_3## are defined in the direction opposite to the fermion flow. In the other diagram ##k_1## and ##k_3## are in the same direction as the fermion flow.
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