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

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1. Oct 17, 2017

### Aruera

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.

2. Oct 18, 2017

### Orodruin

Staff Emeritus
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.