Truth leptons are too soft

Asked by Ewan McCulloch on 2017-08-08

On two separate MG5 installs with pythia8 and Delphes I have encountered a problem where the electrons and muons produced are overwhelmingly soft. No errors occurred when running this process from parton level through to detector level. I Obtained a Delphes root file output and upon inspection I noticed that the leptons were very soft, the majority (around 80% falling into the first bin).

In a p p > t t~ process with unchanged default cards I see a mean pT of 5.8 GeV for electrons, with 83% of all electrons falling into a 0 - 3.3 GeV bin. This is from the truth particle data so Delphes isn't to blame here. Can you think of a reason why MG5 would produce overwhelmingly soft leptons?

Best
Ewan

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Last query:
2017-08-08
Last reply:
2017-08-08

Hi,

Those leptons are generated by Pythia8.
I guess that this is normal since basically any soft photon (and they are so many) are going to split to lepton pair. In top of photon radiation, you will also have a lot of photon emitted during the decay of hadron. I do not know which is the main physical reason for having that many lepton (you should ask PY8 author for that). But
I would definitively expect that. This explains why we use for the analysis only lepton/jet with a minimal (transverse) energy.

Cheers,

Olivier

> On 8 Aug 2017, at 19:14, Ewan McCulloch <email address hidden> wrote:
>
> New question #655471 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/655471
>
> On two separate MG5 installs with pythia8 and Delphes I have encountered a problem where the electrons and muons produced are overwhelmingly soft. No errors occurred when running this process from parton level through to detector level. I Obtained a Delphes root file output and upon inspection I noticed that the leptons were very soft, the majority (around 80% falling into the first bin).
>
> In a p p > t t~ process with unchanged default cards I see a mean pT of 5.8 GeV for electrons, with 83% of all electrons falling into a 0 - 3.3 GeV bin. This is from the truth particle data so Delphes isn't to blame here. Can you think of a reason why MG5 would produce overwhelmingly soft leptons?
>
> Best
> Ewan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Ewan McCulloch (ewanmcc) said : #2

Hi thanks for the speedy response. That is reassuring although I'm still not confident about my build, do you have any recommendations on validating a MG5 + Pythia8 installation?

Many thanks,
Ewan

> do you have any recommendations on
> validating a MG5 + Pythia8 installation?

Not really no. The only recomendation would be to check the MCnet slide of the various lecture associated to the school
And/or to the tutorial.

Cheers,

Olivier
> On 8 Aug 2017, at 20:57, Ewan McCulloch <email address hidden> wrote:
>
> Question #655471 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/655471
>
> Ewan McCulloch posted a new comment:
> Hi thanks for the speedy response. That is reassuring although I'm still
> not confident about my build, do you have any recommendations on
> validating a MG5 + Pythia8 installation?
>
> Many thanks,
> Ewan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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