Spurious high energy jets

[Jamie]

Hi,

I'm analysing some sherpa events with Delphes and I get some events with extremely high energy. Mostly these have high pz but some also lead to high pt values. By high energy, I mean far bigger than the cms of the collisions.

I have also run exactly the same events through Rivet and see no issues. Examining the HepMC file does not help either.

I am running the latest version (3.1.2) and the ATLAS detector card.

Many thanks,
Jamie

[Michele Selvaggi]

Hi Jamie,

can you attach here a small HepMC file for which you obtain this issue?
I might be that there is an issue with the units used in the HepMc format you are using.

Michele

[Jamie]

I don't think this is the issue because using the default setup, the problem is not reproducible, i.e the events with strange momenta happen randomly (and also rarely). In addition, apart from the spurious events, the distributions look ok.

I will try to find an event and random seed combination that reproduces the problem for you.

Many thanks,
Jamie

[Jamie]

Hi Michele,

Ok, I think it's a lost cause trying to do this with a small event sample.

I've uploaded the hepmc and root file after Delphes to:

​https://www.dropbox.com/sh/1gaztb5dy798fy1/AAAk4G6FV6bg_jUl9bkFd-rha

In the root file, Event 19506 has a jet with energy 9104 GeV (but I have seen much higher with bigger samples).

Cheers,
Jamie

[Jamie]

Hi Michele,

After more investigation, I can reproduce this fully with Herwig++ as well. I've simply taken the default t tbar run card and roughly 1 in 10000 events contain a jet with energy > 10TeV (for 13TeV cms). In the first 100000 event run I did, there is a jet with 17TeV.

Many thanks,
Jamie

[Jamie]

I should also add that Rivet fails to find any of these jets.

[Jamie]

Hi Michele,

Sorry for the continued spam but I can also reproduce this with the Madgraph-Pythia chain. I get similar results for t tbar production as with Herwig.

To me, it looks like something goes wrong with the smearing of forward jets.

Cheers,
Jamie

[Jamie]

Hi Michele,

The factor 0.9420² for the forward HCAL resolution seems really high to me. This essentially means that the smearing of the jet is as large as the jet energy itself unless I have mis-interpreted how things work.

Are you sure this is not supposed to be 0.09420²?

Cheers,
Jamie

[Michele Selvaggi]

Hi Jamie,

sorry for the very late reply, most of us were offline in the last 3 weeks.
It seems you are right indeed, the constant energy resolution term cannot be ~ 100%.
The resolution has now been corrected in svn. It will be included in a further release.

Thanks for spotting this!

Meanwhile, can you please check that with 0.09420 you don't get spurious jets anymore?

Michele

[Jamie]

Hi Michele,

No worries at all. If I change this to 0.09420 then the problem goes away.

Many thanks,
Jamie