Four-fermion interaction and vertices

Asked by Krystsina on 2020-10-06

Dear MadGraph experts,

I was going to derive the 4-fermion vertices in Feynrules and to pass the generated model further to MadGraph. According to the most usable model, the contact interaction is described by four-fermions Lagrangian of the general form as follows: f¯ γμ f f¯ γμ f.

Eventually, after trying few more straightforward ways, obtaining the Lagrangian in the form Llqlq=(l γμ l¯)(q¯ γμ q)

from SMEFT package seemed to have succeeded. This is two-lepton two-quark interaction defined as "lq1" term in the SMEFT model.

However, the FeynRules producing outputs to MadGraph ended up with warnings: "Multi-Fermion operators are not yet fully supported!"

My question is: Is there a way to implement 4-fermion interaction with point-like propagator in MadGraph? Can it be done with FeynRules or any other tool?

Thanks a lot for any your help. Kristina

Question information

Language:
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MadGraph5_aMC@NLO Edit question
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Last query:
2020-10-06
Last reply:
2020-10-06

Hi,

We support most of the cases but not all of them.
We do not support the case where we have identical particle and fermion flow violation.
FeynRules output should be working (as far as I know even in those cases) but madgraph will crash either at the time of importing the model or when generating the diagram if you face a case not supported by MG5aMC.

Cheers,

Olivier

PS: For more details on the FeynRules side, you should obviously ask the FR author and not us.

> On 6 Oct 2020, at 22:00, Krystsina <email address hidden> wrote:
>
> New question #693315 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/693315
>
> Dear MadGraph experts,
>
> I was going to derive the 4-fermion vertices in Feynrules and to pass the generated model further to MadGraph. According to the most usable model, the contact interaction is described by four-fermions Lagrangian of the general form as follows: fL¯γμfLfL¯γμqL.
>
> Eventually, after trying few more straightforward ways, obtaining the Lagrangian in the form Llqlq=(lpγμlr¯)(qs¯γμqt)
>
> from SMEFT package seemed to have succeeded. This is two-lepton two-quark interaction defined as "lq1" term in the SMEFT model.
>
> However, the FeynRules producing outputs to MadGraph ended up with warnings: "Multi-Fermion operators are not yet fully supported!"
>
> My question is: Is there a way to implement 4-fermion interaction with point-like propagator in MadGraph? Can it be done with FeynRules or any other tool?
>
> Thanks a lot for any your help. Kristina
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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