units in hepmc file

[Sebastian Schaetzel]

Hello,

hepmc files can use different units, like GEV CM or GEV MM or MEV MM, etc.

I was wondering whether Delphes handles these gracefully.

So I generated the same events (Pythia) two times, one time with
GenEvt( HepMC::Units::GEV, HepMC::Units::CM) and the second time with
GenEvt( HepMC::Units::GEV, HepMC::Units::MM), and sent them through Delphes.

The output ROOT files differed in Particle.T, .X, .Y, .Z, reflecting the mm<->cm difference.

So apparently the hepmc units matter. My guess is that the formulas tracking the particles through the magnetic field are sensitive to the energy unit, although I haven't tried it.

What are the correct hepmc units then? From the detector cards it seems that most of the units used internally in Delphes are GeV and cm. Should I use those?

Maybe Delphes could analyse the line in the hepmc file which specifies the units, for example "U GEV MM", and convert the numbers internally?

Thanks,
Sebastian

[Pavel Demin]

Dear Sebastian,

Thank you for the suggestions.

As most of the MC generators use GEV and MM by default, Delphes uses the same units.

Here is a version that read the momentum and position units from the HepMC file:

​http://cp3.irmp.ucl.ac.be/downloads/Delphes-3.0.8.pre2.tar.gz

Could you, please check if this version works for you?

Regards,

Pavel

[Sebastian Schaetzel]

Dear Pavel,

3.0.8.pre2 gives:

Reading pythia.hepmc
ERROR: Header block not found. File is probably corrupted.

The file starts with

HepMC::Version 2.06.09
HepMC::IO_GenEvent-START_EVENT_LISTING
E 0 -1 2.9156162529458029e+03 8.5945706606816857e-02 8.1371040408944259e-03 3001 0 1360 1 2 0 1 1.0000000000000000e+00
N 1 "0"
U GEV CM
C 2.8432991119982389e-03 2.8432991119982389e-03

A gzipped version of the hepmc file can be found here:
​http://www.physi.uni-heidelberg.de/~sschaetz/pythia.hepmc.gz

Thanks,
Sebastian

[Pavel Demin]

Dear Sebastian,

The error message is from the DelphesSTDHEP program. You should use DelphesHepMC to read HepMC files.

The following command works for me:

curl http://www.physi.uni-heidelberg.de/~sschaetz/pythia.hepmc.gz | gunzip | ./DelphesHepMC examples/delphes_card_CMS.tcl delphes_output.root

Regards,

Pavel

[Sebastian Schaetzel]

Dear Pavel,

Sorry about that, everything works just fine with DelphesHepMC.

And I confirm that Particle.X, .Y, etc. don't depend on the input file units anymore.
I think that's very useful!

Thanks a lot,
Sebastian

[Pavel Demin]

The fixed HepMC file reader is in Delphes 3.0.8 now.