Particle.IsPU zero for all particles

[nkgarner]

Hello,

I've been working on adding pileup to an event generated by MadGraph and then hadronized by Pythia, and everything seems to be working well except for the Particle branch of my Delphes output.

I used the MinBias.pileup file given by ​http://cp3.irmp.ucl.ac.be/downloads/MinBias.pileup.gz and I was using the examples/delphes_card_ATLAS_PileUp.tcl delphes card with Delphes 3.1.2. When opening the .root file output by Delphes I noticed two thing:

1) While root spit out "Warning in <TClass::TClass>: no dictionary for class GenParticle is available" yet there is no GenParticle branch, although there is a branch called Particle with the leaves for the GenParticle branch. (all other branches are present)

2) When looking at the IsPU leaf, I noticed that all particles had IsPU = 0.

Furthermore, when running the same process through Delphes without pileup using examples/delphes_card_ATLAS.tcl. I noticed that there are the same number of entries in the Particle branch as the pileup run! (the number is different than the number of particles in the .hep file run through Delphes)

Is this a bug or am I missing something? I can provide the delphes output if it would help, it is only 10k events.

Thanks ahead of time,

Niklas

[Pavel Demin]

Dear Niklas,

The warning about missing dictionary for class GenParticle is OK. To make this warning disappear you can load Delphes shared library before opening the .root file:

root
gSystem->Load("libDelphes");
TFile::Open("delphes_output.root");

It's also normal that there is no GenParticle branch. GenParticle is the name of the class used to fill the Particle branch. The names of the branches are defined in the TreeWriter module configuration.

The content of the Particle branch is defined by the following line in the TreeWriter module configuration:

add Branch Delphes/allParticles Particle GenParticle

So, it's normal that the Particle branch contains only particles generated by Pythia and no pile-up particles.

Unfortunately, we can't mix particles generated by Pythia and pile-up particles in the same branch because it'll break all the mothers/daughters indices.

If you need a branch with pile-up particles you can add the following line to the TreeWriter module configuration:

add Branch PileUpMerger/stableParticles PileUpMix GenParticle

This way, the output ROOT tree will contain a new branch called PileUpMix containing all the stable particles generated by Pythia and all the stable pile-up particles.

Regards,

Pavel