Name: Daniel Date: 10/11/18-03:03:33 PM Z

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I´m trying to calculate the matrix element for a ff->ff mediated by a scalar field. The matrix looks like:

Ma = 1/(SP[k1 - p1] - m^2) Spinor[k1, m1].Spinor[-p1, m1].Spinor[-p2, m].Spinor[k2, m]

But when I request the contract as,

Ma2 = FermionSpinSum[Ma*MaC] // Contract

it does not work, saying “DiracSimplify::spinorsleft: Error! After applying FermionSpinSum to all spinor chains the output
still contains spinors. Evaluation aborted.”

What I´m doing wrong? Thanks in advance!

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