MadGraph5_aMC@NLO

Survey return zero cross section.

Asked by Tim Lu

Hello,
I use Feynrule to construct a new model, this model has a light scalar just like higgs, but the mass is around 400 MeV to 1200 MeV, but when I want to test this scalar decays to muon pair, then the Madgraph5 would show this error :

Survey return zero cross section.
   Typical reasons are the following:
   1) A massive s-channel particle has a width set to zero.
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s.
   3) The cuts are too strong.
   Please check/correct your param_card and/or your run_card.

But if we change the sclar mass to be 5000 MeV, then it decays to muon pair will work.
I don't know how to solve this problem. Could you help me ?

Thanks a lot !

Best,
Tim

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

What is the width of the scalar?

Cheers,

Olivier
On Nov 29, 2013, at 3:31 PM, Tim Lu <email address hidden> wrote:

> New question #240040 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/240040
>
> Hello,
> I use Feynrule to construct a new model, this model has a light scalar just like higgs, but the mass is around 400 MeV to 1200 MeV, but when I want to test this scalar decays to muon pair, then the Madgraph5 would show this error :
>
> Survey return zero cross section.
> Typical reasons are the following:
> 1) A massive s-channel particle has a width set to zero.
> 2) The pdf are zero for at least one of the initial state particles
> or you are using maxjetflavor=4 for initial state b:s.
> 3) The cuts are too strong.
> Please check/correct your param_card and/or your run_card.
>
> But if we change the sclar mass to be 5000 MeV, then it decays to muon pair will work.
> I don't know how to solve this problem. Could you help me ?
>
> Thanks a lot !
>
> Best,
> Tim
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Revision history for this message
Tim Lu (timluyu) said : 		#2

Width around 10^-11 to 10^-9 for scalar mass around 400 MeV to 1200 MeV.
But width will change to 10^-7 when I change scalar mass to be 5000 MeV. (This case is OK !)

Thanks !

Best,
Tim

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

You are probably sensitive to numerical accuracy since the width is so small that it start to not be enough in order to regulate the propagator.

My suggestion is to artificially increase the total width to something 10^-5 and then rescale the crosse-section accordingly.

Cheers,

Olivier
On Nov 29, 2013, at 5:11 PM, Tim Lu <email address hidden> wrote:

> Question #240040 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/240040
>
> Status: Answered => Open
>
> Tim Lu is still having a problem:
> Width around 10^-11 to 10^-9 for scalar mass around 400 MeV to 1200 MeV.
> But width will change to 10^-7 when I change scalar mass to be 5000 MeV. (This case is OK !)
>
> Thanks !
>
> Best,
> Tim
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Revision history for this message
Tim Lu (timluyu) said : 		#4

Thanks Olivier Mattelaer, that solved my question.