Madspin decay

Asked by SOUAD SEMLALI on 2021-06-08

Dear Olivier,

 I have a similar problem as the one mentioned in this link: https://answers.launchpad.net/mg5amcnlo/+question/696423

I am trying to compute the cross-section of gg > h2 > h3 Z* > h1 Z* Z*with h1 > b b~, Z*> jj and Z* > mu+ mu- in 2HDM type-I. For that, I used the hgg_plugin, however, there is a difference between the expected one, using the branching ratios from 2HDMC, and the one computed by madgraph and madspin. I checked 3 benchmark points, while including the total width of h, H A and H+ in the parameter Card:

  BP1: \sigma (madspin + madgraph ) = 2.5 x \sigma(expected using Brs form 2HDMC)
  BP2: \sigma (madspin + madgraph ) = 4 x \sigma(expected using Brs form 2HDMC)
  BP3: \sigma (madspin + madgraph ) = 3.5 x \sigma(expected using Brs form 2HDMC)

I tried to set the total width to Auto. But I am getting an error message :

 ERROR: Invalid command: 25 is not a valid argument for compute_widths
 Command "launch" interrupted with error:
ValueError : invalid literal for int() with base 10: 'autoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoautoauto'.

Many thanks for any help or any suggestion

Best regards

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Olivier Mattelaer (olivier-mattelaer) said :
#1

I see so many off-shell particles that I'm not even sure which branching ratio you are expecting to use.
So here the only place where the narrow width approximation has a chance to be valid is for the h1> b b~.
For all the other decay using the NWA(i.e. using branching ratio) is bound to fail.

So my guess is that your "expected" is something that does not make sense.

Cheers,

Olivier

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SOUAD SEMLALI (souad-sem) said :
#2

Dear Olivier,

 Many thanks for your help. Indeed, using the narrow width approximation
is not consistent, since it drops the off-shell contributions, but I think
it won't be different though.

 What do you think about the error message after setting the total width of
h and H to Auto?

Many thanks for any suggestions

Best regards

Le mer. 9 juin 2021 à 08:10, Olivier Mattelaer <
<email address hidden>> a écrit :

> Your question #697452 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/697452
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> I see so many off-shell particles that I'm not even sure which branching
> ratio you are expecting to use.
> So here the only place where the narrow width approximation has a chance
> to be valid is for the h1> b b~.
> For all the other decay using the NWA(i.e. using branching ratio) is bound
> to fail.
>
> So my guess is that your "expected" is something that does not make
> sense.
>
> Cheers,
>
> Olivier
>
> --
> If this answers your question, please go to the following page to let us
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> following page to enter your feedback:
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>

--
Souad SEMLALI

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Olivier Mattelaer (olivier-mattelaer) said :
#3

Hi,

If you consider the standard model case
if you take g g > h > W W > 4l

and use the Branching ratio formula for the Higgs , you have a cross-section wrong by a factor ~2.
indeed in ~50% of the case you do have
g g > h (onshell), h > w(onshell) w(offshell) > 4 lepton. where you can use the narrow-width approximation for the Higgs but not for the W
and in the other 50% you do have
g g > h (offshell), h > w(onshell) w(onshell) > 4 lepton where you can not use the NWA for the Higgs but where you can for the W.

Since you do not know in advance the ratio between the two process (this is how you can measure the total width of the Higgs actually) then in practise, you can not use the Branching ratio.

So I do not know your model but, by default, I do expect the usage of branching ratio to be wrong for the process that you described.
Now I do not have that much detail on what you do/... so I can not make any strong statement but from what you provide it does not seem evident that using BR is the way to have correct estimate of the cross-section.

For the error, I do not know, it might be an issue of the plugin or the fact that your model is such that the computation of the width depends of the value of the width itself which can lead to such issue.

I would need to have the exact model that you use (and be sure that you use either 2.9.4 or 3.1.1) to investigate that.

Cheers,

Olivier

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