Version 3 (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.
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.
the average amount of pile-up events per bunch-crossing. For each hard scattering, N pile-up events will be randomly chosen from the PileUp file, where N is a random number following Poisson statistics with a mean MeanPileUp.
*ZVertexWidth:
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 ZveterxWidth.
*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 OutputArrayRho rho
The method for the area calculation can also be specified (active area, passive area, Voronoi …) via the parameter AreaAlgorithm.
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 and photon isolations.
- 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