When I use FeynCalc's OneLoop, the output often has the sign || around expressions with gamma matrices, but not always. What is the meaning of this symbol in this case?
One amplitude that evokes this kind of result is the following (for bremsstrahlung in higher order):
num = DiracMatrix[mu].(DiracSlash[pi - k1 + k2] + m).DiracSlash[Polarization[k1]].(DiracSlash[pf + k2 - k1] + m).DiracMatrix.(DiracSlash[pi + k2] + m).DiracMatrix[mu] A
amp = num*
FeynAmpDenominator[PropagatorDenominator[pf + k2, m], PropagatorDenominator[pf + k2 + k1, m], PropagatorDenominator[pi + k2, m], PropagatorDenominator[k2, 0]]
Invocation of OneLoop[amp,k2] leads to a very large output with those ||.|| expressions.
Thanks for any help
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