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. 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.