Name: Lingxiao Xu Date: 05/05/15-04:54:52 PM Z
2 Aliaksandr:
In the amplitude, the color factor is just Tr[T^a.T^b] which equals to \delta^{ab}/2 exactly. So that in the amplitude squared after we sum over color indexes a and b, we have \sum_{a,b} (\delta^{ab}/2) (\delta^{ab}/2)=2, so I just multiply 2 when the amplitude get squared.
Cheers!
Lingxiao Xu