· FeynCalc

Name: Maksym Date: 07/24/17-11:24:57 AM Z

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Reply: Vladyslav Shtabovenko: “Re: Error! DiracTrick has encountered a fatal problem and must abort the computation. The problem reads: Incorrect combination of dimensions and g^5 scheme!”

Hi! I’m experiencing an issue when trying to calculate the squared amplitude.

The code is

FCClearScalarProducts[];
{coef0 = u Sqrt[\[Alpha]] g/2, coef1 = 1/(32 Pi s)};
{ScalarProduct[k1, k1] = ScalarProduct[k2, k2] = 0,
  ScalarProduct[p1, p1] = ScalarProduct[p2, p2] = 0,
  ScalarProduct[p3, p3] = m^2,
  ScalarProduct[k1, k2] = s/2, ScalarProduct[k1, p1] = -t1/2,
  ScalarProduct[k1, p2] = (s1 + t1 - t2)/2,
  ScalarProduct[k1, p3] = (s - s1 + t2)/2,
  ScalarProduct[k2, p1] = (s - s2 + t1)/2,
  ScalarProduct[k2, p2] = (s2 + t2 - t1 - m^2)/2,
  ScalarProduct[k3, p3] = (m^2 - t2)/2, ScalarProduct[p1, p2] = s1/2,
  ScalarProduct[p1, p3] = (s - s1 - s2)/2,
  ScalarProduct[p2, p3] = (s2 - m^2)};
f1 = coef0 PolarizationVector[
    k2, \[Mu]] SpinorUBar[k1,
     0].GA[\[Mu]].QuarkPropagator[{k1 + k2, 0},
     Explicit -> True].GA[\[Nu]].SpinorU[p1,
     0] (-MetricTensor[\[Nu], \[Alpha]]

     FourVector[
       k1 + k2 - p1, \[Nu]] FourVector[k1 + k2 - p1, \[Alpha]]/
       mW^2) PropagatorDenominator[k1 + k2 - p1,
    mW] SpinorVBar[p2, 0].GA[\[Alpha]].(1 - GA5). SpinorU[p3, m] // FCI
f1star = ComplexConjugate[f1]
m11 = DoPolarizationSums[f1 f1star, k2, 0]
m22 = FermionSpinSum[m11] /. DiracTrace -> TR // Contract // Simplify
Mfinal = m22
dsigbardz = coef1 Mfinal

It displays the amplitude correctly and evaluates its complex conjugation, but faols when calculating the fermion polarization sum with the error
“Error! DiracTrick has encountered a fatal problem and must abort the computation. The problem reads: Incorrect combination of dimensions and g^5 scheme!”

What is the reason for this?