discarding particles

[gdurieux]

Dear Delphes team,

I'm producing events with a custom model having BSM particles of PDG code set to 998 and -998. They are both neutral and stable.

Pythia6 doesn't seem to have problems with that, but Delphes seems to discard particle -998 (while it keeps 998).

Do you have an explanation for this?

Thanks a lot,
Gauthier

P.S.: I'm using:
commit 486c4c370ad65df39cd6950d6b5b3a60f5f8ee19
Date: Thu Apr 23 17:45:42 2015 +0200

and enclose my one-event lhe file, hep file, pythia log, delphes card, root file.

[gdurieux]

Dear Delphes team,

Additionally, for such new neutral and stable particles, my understanding is that the default CMS card should be adapted

• by specifying they deposit no energy in the calorimeters, in order to get correct "eflow" jets (with "add EnergyFraction {998} {0.0 0.0}" in the "Calorimeter" module)

• by listing them as neutrinos in the "NeutrinoFilter" module, to get correct "GenJets" (with commands like "add PdgCode {998}". Then, shouldn't neutralinos be listed there too?)

Is that correct?
Can you think of any other change that would be required, in such a case, to get physically sensible results?

Again, thanks a great lot!

Gauthier

[Michele Selvaggi]

Hello Gauthier,

many apologies for answering so late.
Your input file seems strange, both in the LHE file and in the pythia log your BSM particles seems to have 4-momentum = 0.
I am not sure this is the real cause of the problem, but it seems that the particle with negative pid has its pid set to 0 already in the STDHEP file, and Delphes does nothing outside reading that value. If this is not too complicated, could try to cross check by producing a HepMC format instead (it should be easy to do that with MC@NLO)?

Also, to answer your second question, you are correct in both your bullets. The second bullet is needed only if you really care about GenJets.

Michele

[gdurieux]

Hi Michele,

Thanks a lot for your answer.

As you figured out by looking at the STDHEP file (which I didn't know how to do), this is indeed not an issue of Delphes.

A first look at the Pythia log showed it is not an issue of Pythia neither. Although looking more carefully, I noticed the warning "(LUTRAN 5:) particle ID translation is zero".

So it is actually an issue of STDHEP and HepMC in which lutran.f converts the Pythia particle ID to PDG ones. There, the lines

C...check for illegal antiparticles
        if(i1.eq.i2 .and. id.lt.0) lutran=0

judges that an ID with equal second figure ('i1' =9 for me) and third figure ('i2' =9 too for me) describes a particle that can have no antiparticle and converts its ID to 0...

Mystery solved!

A comparable issue was actually raised in #262.

Cheers,
Gauthier

[Shankha Banerjee]

Hi,

I am facing a similar problem. I am having a particle with PID -1000015 (the stau) and its antiparticle. So, in the pythia log file I can see both the particle and the anti-particle and also in the showered lhe file.

However, when I try to use Delphes, it just sees the particle (PID -1000015) and not the one with PID = 1000015. Even though when I form my root file, the GenParticle shows both the PIDs.

Also in my shower file I did not get a lutran warning.

Can you please suggest where the problem is ?

This is a continuation of the ticket #855.

I could include this particle (stau) as a muon by changing the module ParticlePropagator.C. But somehow it does not identify its antiparticle.

I will be extremely grateful if you can help me in this regard.

Regards,

Shankha.