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Hi Francesco,

there are actually two issues here:

First of all, the multiple appearances of FCGV (like FCGV["FCGV["MU"]"])

in the output is a bug in the patching code. I modified the code to

recognize when a model file has already been patchted, so it will not be

attempted to be patched again.

<https://github.com/FeynCalc/feyncalc/commit/21f5783aa3c24a2b50fb2413ee65a8daa8566a1c>

However, the appearance of DiracObject functions is a totally different

thing.

The point is that when you use operators with 4-fermion interactions,

there is no simple way to deduce the relative sign of such diagrams from

the Feynman rules. This is also described in the FeynArts manual,

chapter 6.6.

Of course, this issue appears only in this particular diagrammatic

approach, while when using e.g. Dyson-Schwinger and doing Wick

contractions by hand the signs are unambiguous. Unfortunately, FeynArts

doesn't work this way, so at the end the user has to somehow put the

correct signs by hand.

This also affects other tools, e.g. MadGraph. The trick with a heavy

scalar between two FFS vertices they recommend here:

<https://bugs.launchpad.net/mg5amcnlo/+bug/1454558>

could actually work for you, if you don't feel like resolving the

fermion flow by hand. Otherwise you need to define some way to assign

the correct signs to each of your diagrams, after which you could get

rid of DiracObjects.

Cheers,

Vladyslav

Am 02.07.2016 um 00:40 schrieb Francesco:

*> 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).
*

*>
*

*> You can download my model from here https://www.dropbox.com/sh/hm77oyedcb3p055/AABzBlEPZvcPjAstjoclCR3ya?dl=0
*

*>
*

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

*>
*

*> $LoadFeynArts = True;
*

*> << 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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