BRST Symmetry and unphysical polarizations

[lornstone]

Hi,

I am reading the BRST Symmetry section of Peskin and Schroeder but I can't find anywhere
why the BRST transformation for the gauge vector,
$$\delta A_\mu^a = \varepsilon \partial_\mu c^a$$
implies that only forward polarized states can create ghosts by applying Q, Q being define by
$$\delta \phi = \varepsilon Q \phi$$
I saw in a paper that it's obvious when we go to momentum space, but unfortunately it's not for me...

Thank you!

 

[muppet]

I'm afraid I can't help, but one point about your post confuses me- why have you taken the $D^{ab}$ from Peskin and Schroeder's treatment to be diagonal? Isn't it the covariant derivative in the adjoint of the gauge group?

 

[lornstone]

Yes it is. But Peskin is considering the limit case where g is equal to zero