Name: Marco Date: 06/24/12-12:36:53 PM Z
The problem is in the representation. I am sorry to have understood it
only now.
So from ?SUNDelta:
\\\\\\\“SUNDelta[a, b] is the Kronecker-delta for SU(N)
with color indices a and b.\\\\\\\”
I think it is fair to assume that it means that the delta is in the
fundamental, so the trace should be N, not N^2-1.
While actually the delta is in the adjoint. It would be nice to have it
explicitly stated.
[by the way, if you don\\\\\\\‘t know this, even simple
computations like qqbar–>qqbar with a photon exchange give wrong
results. And in general when you have QCD+EW things it is a mess.]