Particles stored in LHE

Asked by Yizhou Cai

Dear experts,

I am generating ttbar via a resonance state like "generate g g > H > t t~". At some points there will be 5 particles (g g H t t~) stored in LHE files, but at the other points only g g t t~ will be stored. I am wondering if it is because of the H lifetime.

Many thanks for your help!

Regards,
Yizhou

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

The decision to write (or not) the Higgs follow the LHE description.
The important point is that the presence (or absence) of the particle allows/force the parton-shower to handle the event in a different way. Therefore forbidding (or forcing) MG5aMC to always (or never) write this particle will have an impact on the parton-shower description of the sample.

Obviously in this choice the value of the width of the particle is one of the main consideration (with the mass)

Cheers,

Olivier

> On 10 Aug 2021, at 16:35, Yizhou Cai <email address hidden> wrote:
>
> New question #698310 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698310
>
> Dear experts,
>
> I am generating ttbar via a resonance state like "generate g g > H > t t~". At some points there will be 5 particles (g g H t t~) stored in LHE files, but at the other points only g g t t~ will be stored. I am wondering if it is because of the H lifetime.
>
> Many thanks for your help!
>
> Regards,
> Yizhou
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Yizhou Cai (yzcai) said :
#2

Hi Olivier!

Thanks for your kindly reply! Is there a way to change the cutoff value?

Regards,
Yizhou

Revision history for this message
Yizhou Cai (yzcai) said :
#3

Thanks Olivier Mattelaer, that solved my question.

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Yizhou Cai (yzcai) said :
#4

Hi Olivier!

Sorry to bother you again, I have a further question.

Could you please elaborate on the difference between the two parton shower treatments. I am not sure if it is more correct to ignore H when its width/mass > 20%, while force it to shower when width/mass ~7%. I am wondering if you could clarify.

Regards,
Yizhou

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

Hi,

This is actually more a question for the parton-shower people than from us. They should be able to give more precise answer to this.
In principle the effect is double
1) This break the dipole configuration for the parton-shower in various starting dipole when the particle is writtten. This is likely not an effect that mattter here if your "H" is scalar under QCD (but it would matter if it is a scalar octet).
2) The invariant mass of the written particle should be preserved by the parton-shower which therefore limit the reshuffling method used by the parton-shower to preserve the energy-momenta. If your particle is too far-offshell or not really resonant (width/mass >=10%, then asking to preserve the invariant mass of that particle is not appropriate and the particle should not be written within the event record.

As said above, this is my understanding of the situation if you need more details/... please contact parton-shower author.

Cheers,

Olivier

Revision history for this message
Yizhou Cai (yzcai) said :
#6

Thanks Olivier! I will contact the Pythia authors for more details. But in the MG side, is there any way to change the cutoff value if I do not want to store H when width/mass ~7%?

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Best Olivier Mattelaer (olivier-mattelaer) said :
#7

My advise would be to postprocess the file and to remove the line then.
In principle have the bwcutoff parameter but setting it too very small value will impact the phase-space integrator and I would therefore prefer that you use post-processing.

Cheers,

Olivier

> On 11 Aug 2021, at 04:20, Yizhou Cai <email address hidden> wrote:
>
> Question #698310 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/698310
>
> Yizhou Cai posted a new comment:
> Thanks Olivier! I will contact the Pythia authors for more details. But
> in the MG side, is there any way to change the cutoff value if I do not
> want to store H when width/mass ~7%?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Yizhou Cai (yzcai) said :
#8

Thanks Olivier Mattelaer, that solved my question.