Version 12 (modified by 12 years ago) ( diff ) | ,
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Pile-up implementation in Delphes
Multiple particle interactions per bunch-crossing are now implemented in Delphes. The procedure is divided in two main parts:
- mixing pile-up events with the main interaction
- pile-up subtraction with the fast jet area method
Mixing pile-up
The mixing procedure is done via the PileUpMerger
module. The user can specify 4 parameters: PileUpFile
, MeanPileUp
, ZVertexWidth
, ZVertexResolution
.
PileUpFile
:
the event sample containing pile-up events in binary format. This format allows for faster random event access compared to root trees. Basic information about the event is contained (particle 4-momenta, vertex position, and Particle ID)
This sample has to be generated in advance with an event generator (typically Pythia6/8 or HERWIG) and then converted into binary format.
MeanPileUp
:
the average amount of pile-up events per bunch-crossing. For each hard scattering, N pile-up events will be randomly chosen from the PileUpFile
, where N is a random number following Poisson statistics with a mean MeanPileUp
.
ZVertexSpread
:
Pile-up events randomly populate the z-axis. The position of each pile-up event is generated from a gaussian distribution with a standard deviation ZVertexSpread.
ZVertexResolution
For |z|< ZVertexResolution the hard interaction vertex cannot be distinguished from pile-up vertices. For such pile-up events both charged and neutrals are then merged in the event (no charged particle subtraction). For |z|> ZVertexResolution the hard interaction vertex can be distinguished from pile-up vertices. For such pile-up events only neutrals are merged in the event (total charged particle subtraction), which assumes perfect vertex resolution and efficiency.
Pile-up contamination
The density (in GeV/A) of pile-up contamination (rho) per event is computed within the FastJetFinder
module if the following parameters are defined:
set ComputeRho true set RhoOutputArray rho
The method for the area calculation can also be specified (active area, passive area, Voronoi …) via the parameter AreaAlgorithm
. By default this parameter is set to 0 (no area calculation)
Pile-up subtraction
Since charged particle have already been subtracted to some extent, pile-up contamination only affects the jet energy resolution and the lepton/photon isolation.
- Jet pile-up subtraction is done via the
JetPileUpSubtractor
module that takes as input the jet collection and rho:
set JetInputArray FastJetFinder/jets set RhoInputArray rho
- Isolation subtraction is done inside the
Isolation
module itself just by adding the line in the delphes card:
set RhoInputArray rho
Running Delphes with Pile-Up
Convert your minimum bias sample into Delphes ROOT format:
./DelphesSTDHEP examples/root_converter.tcl MinBias.root MinBias.hep ./DelphesHepMC examples/root_converter.tcl MinBias.root MinBias.hepmc
Convert from Delphes ROOT format to binary
./root2pileup MinBias.pileup MinBias.root
Run Delphes on your sample X with pile-up:
./DelphesSTDHEP examples/delphes_card_CMS_PileUp.tcl X_PileUp.root X.hep