Name: D. Azevedo (email_not_shown)
Date: 05/29/18-06:46:47 PM Z

Hey Vladyslav,

Thanks for the reply, though the problem persists. I actually tried your solution before but I ran into the same problem.

after doing:

res=((M12 // FermionSpinSum) /. DiracTrace -> Tr) // Contract

I still get a product of traces, because there are still dummy indexes inside them:

tr(...GA[alpha]...GA[alphalin]..) tr (... GA[alpha]...GA[alphalin]...)

Everything is multiplying, so there are no "free indexes" M12 is a Lorentz scalar.

Best,
Duarte

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