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

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).

    upload_2017-10-18_3-34-25.png

    Applying the Feynman rules the matrix element should be:
    upload_2017-10-18_3-36-42.png
    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.

    Thanks,
    Pedro.

    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

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Problem with Feynman rules in QCD -- help understanding please
  1. Feynman diagram in QCD (Replies: 7)

  2. Scalar QCD (Replies: 1)

Loading...