Name: Rolf Mertig (email_not_shown)
Date: 11/11/13-12:10:40 PM Z

The || is the typesetting for StandardMatrixElement.

   DiracGamma[Momentum[k1]] .
     DiracGamma[Momentum[pf]] .
     DiracGamma[Momentum[pi]] .
         Polarization[k1, I]]] .

You can always look at that internal form by invoking InputForm on the
result, or by issuing once
before the calculation
puts the typeset output back.

Please do not use DiracGamma[0] when using OneLoop. I don't know if it will work correctly.
Rather use DiracMatrix[rho] or whatever and then you stay within Lorentz covariance.

Rolf Mertig

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