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ILC.ParticleFlowAlgorithmr1.1 - 02 Feb 2009 - 16:49 - Main.iglesiastopic end

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Particle Flow Algorithm

The algorithm matches Clusters to Tracks, reconstructs particles and performs particle identification on the basis of the energy fraction deposited in the ECAL. If this fraction is greater than a predefined threshold, the particle is regarded to be a photon or electron depending on whether a track is matched to the cluster. Likewise muons and neutral and charged hadrons are identified.

The algorithm depends on the same geometrical quantities as the TrackwiseClustering processor. In addition it considers the magnetic field.

Wolf Processor

Processor matches Clusters to Tracks, reconstructs particles and performs particle identification on the basis of the fraction of the energy deposited in ECAL. If the fraction of the energy deposited in ECAL greater than predefined threshold, particle is regarded to be EM one (photon or electron). The corresponding threshold is specified with processor parameter FractionEM?. Particle kinematics is defined at point of closest approach to IP.

The following convention is assumed for particle types :

  • 1 = electron/positron
  • 2 = charged hadron or muon
  • 3 = photon
  • 4 = neutral hadron

  • Input: Processor requires Reconstructed Track Collection and Reconstructed Cluster Collection. The names of corresponding collections are specified through Processor Parameters TrackCollection? and ClusterCollection?.

  • Output: Processor produces an output in the form of the ReconstructedParticle? Collection.
Name of this collection is specified with Processor Parameter ParticleCollection?.

Processor needs the following calorimeter geometry parameters : radius of ECAL barrel n-fold symmetry of barrel phi offset of the barrel stave w.r.t. x-axis (0 for TESLA detector) and +/- z coordinate of front face of ECAL endcaps .All these parameters are passed to processor via GEAR XML file Magnetic field (in units of Tesla) must be specified with processor parameter BField

  • Parameters:
    • TrackCollection: name of the Track collection
    • ClusterCollection: name of the Cluster collection
    • ParticleCollection: name of ReconstructedParticle? collection
    • DistanceTrackToCluster: cut on distance between track intersection point with inner boundary of calorimeter and nearest calorimeter hit in a given cluster. If distance between track intersection point and nearest hit in cluster is less then this cut value, then cluster is assigned to track
    • FractionEM: If ratio of the ecal energy to the total energy in cluster is greater that FractionEM?, then particle is considered to be electron/positron or photon.
    • NativeTrackFitter: If 1, then internal track fitter is invoked to define track parameters. If 0, then default track parameters are used to calculate charge particle momentum and track intersecton point with inner boundary of calorimeters.
    • MergeClusters: If 1, then attempt is made to merge additional clusters to track if energy of already associated cluster is still much less than track momentum. The condition to merge additional clusters to track is quantified with the following formula :
         Ptrk - Ecluster > 3.0*HcalResolution*sqrt(Ptrk) 
where Ptrk - track momentum, Ecluster - energy of associated cluster and HcalResolution? is the energy resolution for hadrons (this is assumed to be 0.5 for TESLA calorimeters)
    • LowerMomentum: The above procedure of merging is applied if momentum of track is greater than LowerMomentum?.
    • DistMergeCut: This parameter defines cut on distance of cluster centre-of-gravity to helix for the cluster-to-track merging procedure applied when
         Ptrk - Ecluster > 3.0*HcalResolution*sqrt(Ptrk) 

A more detailed description of the algorithm can be found at

-- Main.iglesias - 02 Feb 2009
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