e+e- events with ISR and pythia showering

[Matteo]

Dear MG team,

I came across an issue when Pythia8 is showering LHE events produced in MG for e+e- collisions with ISR.

I generated e+e->ZH samples with Madgraph v3.5.4 with ebeam1=ebeam2=120 and with ISR turned on (set lpp1 -3, set lpp2 +3, set pdlabel isronlyll), then run Pythia8 which stops showering events after a certain point.
Pythia complains for LHE events with the energy of one of the beams larger than 120 GeV, which is instead what is set to be the energy in the <init> part of the LHE file. According to a Pythia developer, this could be a round-off error or a boost error, but the error is coming from the LHE file (Madgraph).

For more information on the Pythia error please refer to this git issue opened by the Py8 developer: https://gitlab.com/Pythia8/releases/-/issues/478 .

An LHE file produced with Madgraph with 22 events to reproduce the error can be downloaded from this link: https://web.etp.kit.edu/~mpresill/FCC/test_22events.lhe

Do you have any idea of what is happening and if it's possible to run py8 for showering MG events with ISR?

Thank you in advance!

Cheers,

Matteo

[Olivier Mattelaer (olivier-mattelaer)]

Yeah given that that the error is that
1.2000071365e+02 > 120

So indeed the mismatch is small so it makes sense to investigate boost issue.
And clearly this is a mistake on our side and not pythia8 side. I will take a deeper look.

Cheers,

Olivier

[Matteo (matteo-2)]

Dear Olivier,
thank you for taking care of this. I was wondering if there was any news on this. We would like to profit of MG simulated samples for ECFA/FCC studies with py8 showering, but the timeline is really tight and we would like to understand the reliability/feasibility of these simulations.

thanks again and regards,
Matteo

[Olivier Mattelaer (olivier-mattelaer)]

Hi,

Not yet, with holiday and various other stuff, this has not yet moved forward. This is in my todo list (which is 80 items long for the moment ...). So I do what I can...

Sorry,

Olivier

[Olivier Mattelaer (olivier-mattelaer)]

Hi,

Finally update on this:
https://github.com/mg5amcnlo/mg5amcnlo/commit/165e0bbd4c8c17d01a7fa69e0cc58cea973b240e
better boost for lepton beam, allow to fully ignore beam mass in high… · mg5amcnlo/mg5amcnlo@165e0bb
github.com

except if the CI complains too much about this, this should be part of the 3.6.0 release.

Cheers and thanks for your patience,

Olivier

> On 30 Jul 2024, at 15:19, Olivier Mattelaer <email address hidden> wrote:
>
> Hi,
>
> Not yet, with holiday and various other stuff, this has not yet moved
> forward. This is in my todo list (which is 80 items long for the moment
> ...). So I do what I can...
>
> Sorry,
>
> Olivier
>
> --
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/2073523
>
> Title:
> e+e- events with ISR and pythia showering
>
> Status in MadGraph5_aMC@NLO:
> New
>
> Bug description:
> Dear MG team,
>
> I came across an issue when Pythia8 is showering LHE events produced
> in MG for e+e- collisions with ISR.
>
> I generated e+e->ZH samples with Madgraph v3.5.4 with ebeam1=ebeam2=120 and with ISR turned on (set lpp1 -3, set lpp2 +3, set pdlabel isronlyll), then run Pythia8 which stops showering events after a certain point.
> Pythia complains for LHE events with the energy of one of the beams larger than 120 GeV, which is instead what is set to be the energy in the <init> part of the LHE file. According to a Pythia developer, this could be a round-off error or a boost error, but the error is coming from the LHE file (Madgraph).
>
> For more information on the Pythia error please refer to this git
> issue opened by the Py8 developer:
> https://gitlab.com/Pythia8/releases/-/issues/478 .
>
> An LHE file produced with Madgraph with 22 events to reproduce the
> error can be downloaded from this link:
> https://web.etp.kit.edu/~mpresill/FCC/test_22events.lhe
>
> Do you have any idea of what is happening and if it's possible to run
> py8 for showering MG events with ISR?
>
> Thank you in advance!
>
>
> Cheers,
>
> Matteo
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/mg5amcnlo/+bug/2073523/+subscriptions
>