Complex "squared" amplitude

[AlessioV01]

I am trying to compute the "squared" amplitude for the process $\nu_\mu d \longrightarrow \mu u \gamma$ at tree level for massless quarks. If I specify that the final photon state is transverse with TransversePolarizationVectors -> {p5} when I use FCFAConvert, can I still use the fact that the sum over the polarizations is $-g_{\mu \nu}$? i.e. can I use DoPolarizationSums[ampSquared[0], p5, 0] or do I need to insert the auxiliary vector $n$ instead of $0$? Because my "squared" amplitude is complex if I set it to $0$, but I don't understand why...

Thanks in advance