Four-fermion Operator

Asked by Jeong Han Kim on 2018-04-16

Dear MadGraph team,

I'm interested in implementing a four-fermion operator through the FeynRules-MadGraph chain. By reading through old questions posted here, I understood that there is some issue regarding the four-fermion operator. I am wondering whether this issue has been resolved or not in the current versions of FeynRules (> 2.3) and MadGraph (> MG5_aMC_v2_3_3).

1. In particular, I'm trying to implement a simple four-fermion operator:

Lag := 2 Pi Eta /(cutoff^2) (ubar.Ga[mu].ProjM.u) (ubar.Ga[mu].ProjM.u);

FeynRules successfully generates UFO file, with some suspicious warning:

``Warning: Multi-Fermion operators are not yet fully supported!``

Nevertheless, this UFO can successfully generate events for the following process:

generate u u > u u NP^2==2

The resulting cross section matches to a manual calculation. So the thing seems to be working well.

2. I'm further wondering whether MadGraph can deal with a full QCD + NP process as well (i.e. |QCD + NP|^2):

generate p p > j j NP <= 1

where NP is an interaction vertex of the four-fermion operator. I am especially wondering whether signs of interference terms are correctly taken care of by MadGraph.

Thank you in advance!

Best regards

Jeong-han Kim

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
2018-04-16
Last reply:
2018-04-16

Hi,

MG5aMC does not have full support for four fermion interactions.
But we are fully secure on it. If we do not support the four-fermion interaction that you are using
we are going to crash (either when loading the model or when doing the "output" command)

Concerning FR, I thought that the warning was removed (they should do it actually).
They actually provide fully consistent ouptut since at least 4 years. (I dod not know which version number this correspond to)

Cheers,

Olivier

> On 16 Apr 2018, at 06:02, Jeong Han Kim <email address hidden> wrote:
>
> New question #667977 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/667977
>
> Dear MadGraph team,
>
> I'm interested in implementing a four-fermion operator through the FeynRules-MadGraph chain. By reading through old questions posted here, I understood that there is some issue regarding the four-fermion operator. I am wondering whether this issue has been resolved or not in the current versions of FeynRules (> 2.3) and MadGraph (> MG5_aMC_v2_3_3).
>
> 1. In particular, I'm trying to implement a simple four-fermion operator:
>
> Lag := 2 Pi Eta /(cutoff^2) (ubar.Ga[mu].ProjM.u) (ubar.Ga[mu].ProjM.u);
>
> FeynRules successfully generates UFO file, with some suspicious warning:
>
> ``Warning: Multi-Fermion operators are not yet fully supported!``
>
> Nevertheless, this UFO can successfully generate events for the following process:
>
> generate u u > u u NP^2==2
>
> The resulting cross section matches to a manual calculation. So the thing seems to be working well.
>
>
> 2. I'm further wondering whether MadGraph can deal with a full QCD + NP process as well (i.e. |QCD + NP|^2):
>
> generate p p > j j NP <= 1
>
> where NP is an interaction vertex of the four-fermion operator. I am especially wondering whether signs of interference terms are correctly taken care of by MadGraph.
>
> Thank you in advance!
>
> Best regards
>
> Jeong-han Kim
>
>
>
>
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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