MadGraph5_aMC@NLO

Vetoed events from showering

Asked by Rob Verheyen

Dear MadGraph authors,

I'm trying to study the behaviour of the Pythia final state parton shower on some particular events without any matching or merging. However, when running Pythia from the MadGraph interface, events are getting vetoed which as far as I know, with only the parton shower active, shouldn't happen.

All I do is add to pythia8_card.dat

PartonLevel:ISR = off
HadronLevel:all = off

and generate some vm vm~ > e- e+ events. The pythia log file shows that about half of the events are vetoed, and gives this error for every of the vetoed events:
Error in ProcessContainer::constructProcess: setting mass failed

Is this to be expected or am I doing something stupid?

Thanks!

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

Hi,

I do not think that this has anything to do with the matching/merging.
The veto is related to the treatment of the mass of the final state particle.
Setting the electron massive in mad graph (and to the internal mass of pythia8) might help to solve your problem.
For more information on that issue, I would actually suggest that you contact pythia8 author.

Cheers,

Olivier

> On 21 Mar 2017, at 17:17, Rob Verheyen <email address hidden> wrote:
>
> New question #579007 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/579007
>
> Dear MadGraph authors,
>
> I'm trying to study the behaviour of the Pythia final state parton shower on some particular events without any matching or merging. However, when running Pythia from the MadGraph interface, events are getting vetoed which as far as I know, with only the parton shower active, shouldn't happen.
>
> All I do is add to pythia8_card.dat
>
> PartonLevel:ISR = off
> HadronLevel:all = off
>
> and generate some vm vm~ > e- e+ events. The pythia log file shows that about half of the events are vetoed, and gives this error for every of the vetoed events:
> Error in ProcessContainer::constructProcess: setting mass failed
>
> Is this to be expected or am I doing something stupid?
>
> Thanks!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Rob Verheyen (robverheyen) said : 		#2

Thanks Olivier,

I tried changing mass settings in MadGraph and in Pythia, but neither seems to help. I sent an e-mail to the Pythia authors for advice. If I figure out what's wrong, I'll post it here for others to see.

Cheers,
Rob

Revision history for this message
Rob Verheyen (robverheyen) said : 		#3

Turns out it is a bug within Pythia.

Solution for now:
LesHouches:matchInOut = off

A permanent solution should be available in the next Pythia version.

Cheers
Rob

Revision history for this message
Sho Iwamoto (misho) said : 		#4

Hi Rob,

I have encountered a similar problem regarding the matchInOut routine of Pythia 8.2.26 (latest).
Do you know whether the bug you found(?) is solved in this version, which is published on 26 Apr 2017?

(Maybe I should directly communicate with Pythia8 Developers)

Best,
Sho