MadGraph5_aMC@NLO

partial width of particle go lower than QCD scale

Asked by pattara angkinun

Dear Madgraph team,

I am working on the some susy model where stop is heavier than gluino, so gluino can decay via some thing like 3 body decay mode, i.e., go -> t t~ n1. Since the width from this decay mode is lower than QCD scale, Madgraph always set it to 0. However, I would like to save that decay table to param card, so my question is that is it possible to do that. I understand that the gluino should hadronize to R-hadron before decaying, however, R-hadron might decay inside the detector. The decay of such R-hadron can be handled with pythia8 but the decay table for gluino is require that why I would like to keep it.

Best regards,
Pattara Angkinun

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

That's something interesting. At the same time I'm reluctant to do that by default since this will lead to many mistake by people not realizing that the width is so small...

Actually, if you do not use the auto-width but direct computation of the width (via the generate command) you will not get such issue. So this could be a work around. Alternatively you can hack the code to remove such setting to 0.
I could also add an option within the MG5 interface to force to keep such contribution but this will not be directly called via the auto width (since you do not have access to any of the option with the auto keyword.

Since I guess you are not running a scan with this, neither PY8 automatically from MG5aMC, I guess it should be something workable for you.

What do you think?

Olivier

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pattara angkinun (pattara.angkinun) said : 		#2

Dear Oliver,

thank you for your reply. Could you explain more what is the width computation via the generate command, i.e. what is the exact syntax, I did it with the auto-width and compute_widths but both have the same issue. Indeed, I use PY8 from madgraph interface and I want to run a scan over a parameter space, however, this issue appears only in a small area on parameter space. So, even if there is no direct way in madgraph I can compute the width first and then put it to param card manually for each point. However, please let me know if you have any other suggestion.

Best regards,
Pattara Angkinun

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

In this case you can do
generate go > t t~ n1
output
launch

You can also do
generate go > t t~ n1
output
launch -i
calculate_decay_widths

The second method is nicer since it will create a new param_card automatically.

Cheers,

Olivier

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pattara angkinun (pattara.angkinun) said : 		#4

Thank you Olivier, that is very helpful.

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pattara angkinun (pattara.angkinun) said : 		#5

Thanks Olivier Mattelaer, that solved my question.

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pattara angkinun (pattara.angkinun) said : 		#6

Dear Olivier,

I have one more question. If I want to include 4-body decay, e.g. go > t w- b~ n1, Is it correct that I have to put $t~ in the end to avoid double counting with 3-body decay.

Best regards,
Pattara Anginun

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Olivier Mattelaer (olivier-mattelaer) said : 		#7

I do not know, this depend too much of your model. including N+1 body iin top of N body is not something trivial since it is easy to break gauge invariance (the $ syntax is bad for that).
So please be carefull on what you do.

What you can try is to use the (hidden) syntax
decay_diagram go --body_decay=4
and then do display diagrams if this seems reasonable or not.
(if it is you can use that syntax instead of the "generate" one)

Cheers,

Olivier

> On 21 May 2019, at 15:37, pattara angkinun <email address hidden> wrote:
>
> Question #680837 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/680837
>
> pattara angkinun posted a new comment:
> Dear Olivier,
>
> I have one more question. If I want to include 4-body decay, e.g. go >
> t w- b~ n1, Is it correct that I have to put $t~ in the end to avoid
> double counting with 3-body decay.
>
> Best regards,
> Pattara Anginun
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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pattara angkinun (pattara.angkinun) said : 		#8

Dear Olivier,

thank you for the answer.

Best regards,
Pattara Angkinun