I encountered some problems when contracting Lorentz indeces while having Dirac matrices and spinors in my amplitudes. Here is my program:
Tau1[p_, q_, m_, a_, b_] := -I*kappa/2*(1/4*(GA[a]*(FV[p, b] + FV[q, b]) + GA[b]*(FV[p, a] + FV[q, a])) - 1/2*MT[a,b] *(1/2*(DiracSlash[p] + DiracSlash[q]) - m));
P[a_, b_, c_, d_] := 1/2*(MT[a, c]MT[b, d] + MT[a, d]MT[b, c] - MT[a, b]MT[c, d]);
iDF[q_, a_, b_, c_, d_] := I*P[a, b, c, d] / (ScalarProduct[q,q] + ieps);
M = -I*SpinorUBar[p - q, m1].Tau1[p, p - q, m1, a, b] . SpinorU[p, m1] * iDF[q, a, b, c, d] * SpinorUBar[p+q,m2] . Tau1[p, p + q, m2, c, d] . SpinorU[p, m2]
If I now enter Contract[M], some Lorentz indeces are not contracted. If I enter Calc[M] or DiracSimplify[M], Mathematica gives strange results involving gamma matrices with index $MU(1).
Does anyone know why this problem shows up and how I can get around it???
Thank you very much,
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