# 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

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Olivier Mattelaer
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 Revision history for this message Olivier Mattelaer (olivier-mattelaer) said on 2020-11-21: #1

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:
>
> 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
>
> --

 Revision history for this message Kuntal Pal (kpal1995) said on 2020-11-21: #2

Okay, here it is

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

 Revision history for this message Olivier Mattelaer (olivier-mattelaer) said on 2020-11-21: #3

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:
>
> 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
> launch
>
> --

 Revision history for this message Kuntal Pal (kpal1995) said on 2020-11-21: #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

 Revision history for this message Olivier Mattelaer (olivier-mattelaer) said on 2020-11-21: #5

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:
>
> 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
>
> --

 Revision history for this message Kuntal Pal (kpal1995) said on 2020-11-21: #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.
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

 Revision history for this message Kuntal Pal (kpal1995) said on 2020-11-21: #7

Thanks Olivier Mattelaer, that solved my question.