MadGraph5_aMC@NLO

Bug #1466119
Comment #5

Comment 5 for bug 1466119

Revision history for this message

Olivier Mattelaer (olivier-mattelaer) wrote on 2015-06-18: Re: [Bug 1466119] errors in ep colliders when running Pythia6

Hi,

If you provide to me a patch to pythia-pgs,
I will be more than happy to apply it.

Cheers,

Olivier

On 17 Jun 2015, at 17:23, tang yilei <email address hidden> wrote:

> Although you decide not to fix it, I still would like to instruct that
> it is fairly easy to fix the problem. You can modify your Pythia-pgs
> package. In fact, when I read this, I found that long times ago, when
> you wrote the ME2....f file, in the subroutine UPINIT, you were aware of
> the fact that e+e- colliders are different from ep colliders so you
> modified MSTP(51) when ee colliders are detected. You might just add one
> more IF...ENDIF in order to modify MSTP(11) when ep cases are
> encpuntered. I think this is a simple modification, although to
> pythia–pgs rather than madgraph itself.
>
> --
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1466119
>
> Title:
> errors in ep colliders when running Pythia6
>
> Status in MadGraph5_aMC@NLO Generator:
> Won't Fix
>
> Bug description:
> This bug is related to my previous question entitled the same.
> I have been aware of the reason. That is because Pythia6 set MSTP(11)=1 as a default. According to Pythia6's mannual http://cepa.fnal.gov/psm/simulation/mcgen/lund/pythia_manual/pythia6.4/lutp0613man2.pdf, Page194, MSTP(11)=1 means that "electron carries only a fraction of beam energy in agreement with next-to-leading electron parton-distribution function, thereby including the effects of initial-state bremsstrahlum". However, when doing so, it seems that pythia6 is trying to shower the electrons and then there are then intolerable rounding errors in the programs flow, e.g., 10^-16 != 0 so that pythia reported an error. Therefore, it is convenience to set MSTP(11)=0 in the pythia-card in order to avoid this problem, or to make some further modifications of the MadGraph or Pythia6.
>
> Totally speaking, I suggest you add an extra line MSTP(11)=0 as
> default in pythia-card, or modify MSTP(11)=0 when doing e-p collisions
> just after the initializing process in pythia.f
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/mg5amcnlo/+bug/1466119/+subscriptions

Hi,

If you provide to me a patch to pythia-pgs, 
I will be more than happy to apply it.

Cheers,

Olivier

On 17 Jun 2015, at 17:23, tang yilei <tangyilei10@itp.ac.cn> wrote:

> Although you decide not to fix it, I still would like to instruct that
> it is fairly easy to fix the problem. You can modify your Pythia-pgs
> package. In fact, when I read this, I found that long times ago, when
> you wrote the ME2....f file, in the subroutine UPINIT, you were aware of
> the fact that e+e- colliders are different from ep colliders so you
> modified MSTP(51) when ee colliders are detected. You might just add one
> more IF...ENDIF in order to modify MSTP(11) when ep cases are
> encpuntered. I think this is a simple modification, although to
> pythia–pgs rather than madgraph itself.
> 
> -- 
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1466119
> 
> Title:
>  errors in ep colliders when running Pythia6
> 
> Status in MadGraph5_aMC@NLO Generator:
>  Won't Fix
> 
> Bug description:
>  This bug is related to my previous question entitled the same.
>  I have been aware of the reason. That is because Pythia6 set MSTP(11)=1 as a default. According to Pythia6's mannual http://cepa.fnal.gov/psm/simulation/mcgen/lund/pythia_manual/pythia6.4/lutp0613man2.pdf, Page194, MSTP(11)=1 means that "electron carries only a fraction of beam energy in agreement with next-to-leading electron parton-distribution function, thereby including the effects of initial-state bremsstrahlum". However, when doing so, it seems that pythia6 is trying to shower the electrons and then there are then intolerable rounding errors in the programs flow, e.g., 10^-16 != 0 so that pythia reported an error. Therefore, it is convenience to set MSTP(11)=0 in the pythia-card in order to avoid this problem, or to make some further modifications of the MadGraph or Pythia6.
> 
>  Totally speaking, I suggest you add an extra line MSTP(11)=0 as
>  default in pythia-card, or modify MSTP(11)=0 when doing e-p collisions
>  just after the initializing process in pythia.f
> 
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/mg5amcnlo/+bug/1466119/+subscriptions