Name: Francesco (email_not_shown)
Date: 07/02/16-12:40:40 AM Z

Hi everyone.
I am trying to compute amplitudes of processes involving operators of dimension-6 (with four fermion fields) of the form QLbar Gamma[mu] T[a] QL QLbar Gamma[mu] T[a] QL.

I work with the SM.fr model of FeynRules (2.3.22) where I added a 4-fermion operator.

L81qq1:=C81qq /Lambda^2 Module[{ii,jj,kk,kk1,jj1,mu,aa,cc,sp1,sp2,sp3,sp4,cc1,cc2,cc3}, (QLbar[sp1,1,jj,cc].QL[sp2,1,jj,cc2] Ga[mu,sp1,sp2]) T[aa,cc,cc2]T[aa,cc3,cc4](QLbar[sp3,1,jj1,cc3].QL[sp4,1,jj1,cc4] Ga[mu,sp3,sp4])];

The generation of the Model and GenericModel files goes well and I use them with the Dev version of FeynCalc 9.0.1 (and FeynArts 3.9).

I read this http://www.feyncalc.org/forum/1042.html and then I did

<< FeynCalc`;
\$FAVerbose = 0;
FAPatch[PatchModelsOnly -> True]

top = CreateTopologies[0, 2 -> 2];
diags = InsertFields[
top, {F[3, {1}], F[3, {1}]} -> {F[3, {1}], F[3, {1}]},
InsertionLevel -> {Particles}, Model -> "Standard_Model_dijets_FA",
GenericModel -> "Standard_Model_dijets_FA",
ExcludeParticles -> {V[1], V[2], V[3]}];
Paint[%]

AmpList =
FCFAConvert[CreateFeynAmp[diags], ChangeDimension -> 4,
IncomingMomenta -> {p1, p2}, OutgoingMomenta -> {k1, k2},
DropSumOver -> True, List -> False, UndoChiralSplittings -> True,
FinalSubstitutions ->
Flatten[Join[M\$FACouplings, {SUNFDelta[__] :> 1}]]] // Contract;

Cmp = AmpList // ComplexConjugate // FCRenameDummyIndices;
sqAmp = AmpList Cmp // Simplify;
sqAmp // FermionSpinSum[#, ExtraFactor -> 1] &

and at this point I get the error

Something went wrong while computing the fermions spin sum! Returning \
unevaluated expression:

Specially, If I see the expression of the amplitude I get

AmpList
-I ((1/(3 Lambda^2))
2 I C81qq IndexSum[
DiracObject[DiracGamma[a], DiracGamma[7]][SequenceForm["Dir", 3],
SequenceForm[
"Dir", 2]] DiracObject[DiracGamma[a], DiracGamma[7]][
SequenceForm["Dir", 4], SequenceForm["Dir", 1]], {a, 1, 4}] + (
1/(3 Lambda^2))
2 I C81qq IndexSum[
DiracObject[DiracGamma[a], DiracGamma[7]][SequenceForm["Dir", 3],
SequenceForm[
"Dir", 1]] DiracObject[DiracGamma[a], DiracGamma[7]][
SequenceForm["Dir", 4], SequenceForm["Dir", 2]], {a, 1,
4}]) DiracObject[
Spinor[Momentum[k1], FCGV["]" "FCGV[" FCGV["]" "FCGV[" MU]], 1]][
SequenceForm[
"Dir", 3]] DiracObject[
Spinor[Momentum[k2], FCGV["]" "FCGV[" FCGV["]" "FCGV[" MU]], 1]][
SequenceForm[
"Dir", 4]] DiracObject[
Spinor[-Momentum[p1], FCGV["]" "FCGV[" FCGV["]" "FCGV[" MU]], 1]][
SequenceForm[
"Dir", 1]] DiracObject[
Spinor[-Momentum[p2], FCGV["]" "FCGV[" FCGV["]" "FCGV[" MU]], 1]][
SequenceForm["Dir", 2]]

and there is a DiracObject function which is not defined.

Where is the problem?

Regards,
Francesco

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