# Doubt on sign of the propagator on Peskin

1. ### Einj

428
Hi everyone. I was trying to reproduce the calculation needed to include the QCD correction to the low energy electroweak Lagrangian. In particular I am looking at Peskin section 18.2. In equation 18.35 an extra minus sign appears on the numerator of the second fermion propagator.

Can anyone explain me why?

Thanks

2. ### p-brane

31
As stated in the text, all external momenta have been set to zero. Thus the only momentum is the loop momentum k. If we take the gluon momentum to be k, by conservation of momentum one of the quark momenta must be -k.

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3. ### Einj

428
Oops... stupid question! Thanks a lot

4. ### p-brane

31
Most definitely NOT a stupid question!