Top decay and 4 Fermion operators

Asked by Kuntal Pal on 2020-11-21

Hello Everyone!!

So I have been trying to study the effect of Dimension 6 4 fermion operators in top quark decay. Everything runs fine but there is no interference term between the SM diagrams and 4 fermion operators. The decay width has the SM part and contribution from 4 fermion operators which go as 1/Lambda^4. The 1/Lambda^2 term is missing.

Any reason why this is happening?

Thanks

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Solved by:
Olivier Mattelaer
Solved:
2020-11-21
Last query:
2020-11-21
Last reply:
2020-11-21

This question was reopened

Either they are no interference or you use a wrong syntax.
So what syntax did you use?

Cheers,

Olivier
> On 21 Nov 2020, at 05:35, Kuntal Pal <email address hidden> wrote:
>
> New question #694114 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/694114
>
> Hello Everyone!!
>
> So I have been trying to study the effect of Dimension 6 4 fermion operators in top quark decay. Everything runs fine but there is no interference term between the SM diagrams and 4 fermion operators. The decay width has the SM part and contribution from 4 fermion operators which go as 1/Lambda^4. The 1/Lambda^2 term is missing.
>
> Any reason why this is happening?
>
> Thanks
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Kuntal Pal (kpal1995) said : #2

Okay, here it is

define l+ = l+ ta+
generate t > b j j QED<=99
add process t > b l+ vl QED<=99
output madevent
launch

So here you do not have any interference between
> generate t > b j j QED<=99
and
> add process t > b l+ vl QED<=99
None are possible anyway in this case since the final state is not the same.

But otherwise, you will have all interference term included within each of those two computation.

Cheers,

Olivier

> On 21 Nov 2020, at 09:15, Kuntal Pal <email address hidden> wrote:
>
> Question #694114 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/694114
>
> Kuntal Pal posted a new comment:
> Okay, here it is
>
> define l+ = l+ ta+
> generate t > b j j QED<=99
> add process t > b l+ vl QED<=99
> output madevent
> launch
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Kuntal Pal (kpal1995) said : #4

Yeah each of the individual processes should have interference terms and should be getting it using

Inteference = Total width - SM width - NP width.

This value should go over as 1/Lambda^2 which is not happening. Let me check in case i am missing any diagrams.

But thank you !!

Best,
Kuntal

A better way to have the interference only can be to do
> generate t > b j j NP^2==1

Depending of the UFO model author choices of pa, it might be
> generate t > b j j NP^2==2

This will compute the interference alone

Cheers,

olivier

> On 21 Nov 2020, at 09:45, Kuntal Pal <email address hidden> wrote:
>
> Question #694114 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/694114
>
> Kuntal Pal posted a new comment:
> Yeah each of the individual processes should have interference terms and
> should be getting it using
>
> Inteference = Total width - SM width - NP width.
>
> This value should go over as 1/Lambda^2 which is not happening. Let me
> check in case i am missing any diagrams.
>
> But thank you !!
>
> Best,
> Kuntal
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Kuntal Pal (kpal1995) said : #6

I tried it and the output for the width is close to my expectation. But what is this warning all about?

Computation of interference term with decay is not 100% validated.
                Please check carefully your result.
                One suggestion is also to compare the generation of your process with and without
                set group_subprocesses True
                (to write Before the generate command)

Thanks,
Kuntal

Kuntal Pal (kpal1995) said : #7

Thanks Olivier Mattelaer, that solved my question.