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I was wondering if someone knows enough about the FormCalc package to help me out: I am using FeynArts to generate Feynman-diagrams for certain tree-level processes and FormCalc to calculate the squared amplitude of these processes (http://en.misho-web.com/phys/feynlecture.html gives some neat examples on how this is done). Now this works fine if one considers all tree-level-diagrams, i.e. the complete process at tree level, but fails when only evaluating one of several diagrams (e.g. just calculating the squared 4-vertex-diagram of the gluon+gluon -> gluon+gluon process, instead of all squared diagrams and interference terms).

My first naive idea was that FormCalc might use identities which are correct only for physical processes (i.e. all diagrams to a certain order considered) and not on the single diagram level; but I don't know if such identities exist and furthermore I didn't find anything in the code (which is to advanced for me anyway).

If someone has an idea any hint would be appreciated.

Edit: Further testing showed that the problem seems to be related just to vectorbosons in initial or final states. Because of this I was playing aroung with changing the polarizationsum (in the PolarizationSum.frm) to -d_([mu],[nu]). However even in this configuration a single squared diagram including vectorbosons does not give the result one calculates via Feyman-rules per hand using this simplified polarizationsum. So the problem isn't just the introduction of gauge-dofs in single diagrams which cancel for the whole process.

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# Using FeynArts/FormCalc for single diagram evaluation

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