I am trying to calculate the cross-section for electron-positron annihilation into a quark-antiquark pair and a gluon. I find that I need a relative minus sign between the two contributing diagrams in order to have $q_{\mu} H^{\mu\nu}=0$ with $H^{\mu\nu}$ being the hadronic trace and thus conserve the vector current. Can someone tell me where this minus sign comes from? I cannot see where any fermion positions are interchanged here.
• When you talk about two diagrams, are you grouping the ones with $\gamma$ and $Z$ in the s-channel together? I haven't performed the computation, but since for each diagram the gluons can couple to different legs (see 2 first diagrams of this figure), perhaps the gamma matrix algebra gives you the sign you want... Can you provide additional detail? – J-T Mar 3 '16 at 16:23