Doubt on sign of the propagator on Peskin

[Einj]

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

 

[p-brane]

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.

 

[Einj]

Oops... stupid question! Thanks a lot

 

[p-brane]

Oops... stupid question! Thanks a lot

Most definitely NOT a stupid question!