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OverlayTiming

OverlayTiming is a Gaudi algorithm that overlays background events on top of signal events, typically used to simulate pile-up or beam-induced backgrounds. Background events are read from one or more sets of input files independently of the main event loop.

It uses UniqueIDGenSvc to seed the internal random number generator.

Collections

  • MCParticleCollection from signal and background are merged into a single output collection. Background particles are flagged with isOverlay = true.
  • SimTrackerHit collections are cropped to a configurable time window and overlaid. Background hits are flagged with isOverlay = true.
  • SimCalorimeterHit collections are overlaid by merging hits that share the same cell ID. Their CaloHitContribution entries are filtered by the time window.

Properties

Property Default Description
BackgroundFileNames [] List of groups of background input files, one group per overlay stream
NumberBackground [] Number of background events to overlay per stream (fixed or Poisson mean)
Poisson_random_NOverlay [] If true, draw the number of events from a Poisson distribution with mean NumberBackground
NBunchtrain 1 Number of bunch crossings in the bunch train
PhysicsBX 1 Position of the physics event within the bunch train
RandomBx false If true, place the physics event at a random bunch crossing (overrides PhysicsBX)
Delta_t 0.5 Time between consecutive bunch crossings (ns)
TimeWindows {} Map from collection name to [t_min, t_max] (ns) defining the acceptance window. Required for every SimTrackerHit and SimCalorimeterHit collection.
BackgroundMCParticleCollectionName "MCParticle" Name of the MCParticle collection in the background files
AllowReusingBackgroundFiles false If true, wrap around the background file when events are exhausted
CopyCellIDMetadata false Copy cell ID encoding metadata from input to output collections
StartBackgroundEventIndex -1 Index of the background event to start from (-1 means start from the beginning)

Usage example

The following example reads signal events from signal.root and overlays background events from two separate background files onto the MCParticles, VertexBarrelCollection, and HCalRingCollection collections. Two independent overlay streams are configured: one with a fixed number of overlaid events and one drawing from a Poisson distribution.

from Gaudi.Configuration import INFO
from Configurables import EventDataSvc, OverlayTiming, UniqueIDGenSvc
from k4FWCore import ApplicationMgr, IOSvc

uid_svc = UniqueIDGenSvc("UniqueIDGenSvc")

iosvc = IOSvc("IOSvc")
iosvc.Input = "signal.root"
iosvc.Output = "signal_with_background.root"

overlay = OverlayTiming("OverlayTiming")
overlay.MCParticles = "MCParticles"
overlay.SimTrackerHits = ["VertexBarrelCollection"]
overlay.SimCalorimeterHits = ["HCalRingCollection"]
overlay.OutputMCParticles = "OverlayMCParticles"
overlay.OutputSimTrackerHits = ["OverlayVertexBarrelCollection"]
overlay.OutputSimCalorimeterHits = ["OverlayHCalRingCollection"]
overlay.OutputCaloHitContributions = ["OverlayCaloHitContributions"]
overlay.BackgroundMCParticleCollectionName = "MCParticles"
# Two independent streams of background events
overlay.BackgroundFileNames = [
    ["background_beam1.root", "background_beam2.root"],
    ["background_gamma.root"],
]
overlay.NumberBackground = [10, 1]
overlay.Poisson_random_NOverlay = [True, False]
overlay.NBunchtrain = 5
overlay.PhysicsBX = 1
overlay.Delta_t = 0.5  # ns
overlay.TimeWindows = {
    "VertexBarrelCollection": [-0.5, 5.0],
    "HCalRingCollection": [-5.0, 20.0],
}

ApplicationMgr(
    TopAlg=[overlay],
    EvtSel="NONE",
    EvtMax=100,
    ExtSvc=[EventDataSvc("EventDataSvc"), uid_svc],
    OutputLevel=INFO,
)