Electromagnetic Identification in HF

Contact person: Kevin Klapoetke

HF Electron ID Tool:

A method of using the forward hadron calorimeter (HF) for identifying electromagnetic particles has been developed and tested. The current focus is on identification of electrons, which are believed to be more commonly useful in analyses. There will be little observable differences between an electron and a photon in HF, as there is no tracker to observe charge. In general HF reconstructed electromagnetic candidates are referred to as electrons.

HF electron ID has been added to the standard reco path as of CMSSW_3_1_X

Electron Clusters in HF

HF rec hits are sorted by Et, with an energy cut placed upon them, and clusters are made in a 3x3 tower space around seed cells. Towers within a 5x5 cluster of the seed are excluded from becoming future seeds and information is gathered from them. Shape variables are defined in terms of long and short fiber energies for 5x5 clusters, 3x3 clusters, seed tower, and the core of the cluster (the seed and its highest energy neighbor of at least 50% seed Et). These are formed into standard super clusters with an associated HF EM cluster shape.

Minor clarification: while the super cluster is made out of the 3x3 cluster (ie the energy of the super cluster is defined by the 3x3 cluster) information from the 5x5 cluster is used for EM ID, and thus is included in the shape variable information.

Electron ID Techniques

Electrons are identified in HF by observing cluster shapes withing HF towers. These are compared to the cluster shapes created by hadrons and jets and identification criteria is developed. Current ID techniques are:
  • Isolation cut: a measurement of how broad the cluster is within a 5x5 tower area
  • Longitudinal cut: a measurement of how deep into HF the majority of the energy is located. (see long/short fibers in HF)
  • Transverse cut: a measurement of how broad the shower is in a 3x3 tower area
  • Two dimensional cut: a cut made using a combination of Longitudinal and Transverse cuts that is superior to making the two individually

Energy and position corrections are also added in this step, but only to the final product, not the super cluster (Corrections can be turned off, but is not advisable for standard analyses).

HF Clusters that pass these cuts are stored as RecoEcalCandidates.


HF Electrons in Data

  • recoSuperClusters_hfEMClusters_RECO: The initial 3x3 potential EM cluster in HF in SuperCluster form
  • recoHFEMClusterShapes_hfEMClusters_RECO: The various shapes (5x5, 3x3, seed, Core for long and short fiber readings) of the EM HF Super Cluster
  • recoSuperClustersToOnerecoHFEMClusterShapeAssociation_hfEMClusters_RECO: an association between the previous two objects. Needed so the Shapes can be used to pass the appropriate super cluster on to electron ID.
  • recoRecoEcalCandidates_hfRecoEcalCandidate_RECO: A final HF electron candidate. This has a reference to the superCluster it was made from, and is thus still connected with the HFEM Cluster Shape.

For simply using HF electrons in a basic analysis, likely only the hfRecoEcalCandidate will be needed.

Accessing HF Electrons

  • /CMSSW/DataFormats/EgammaReco/ contains the definition of HFEMClusterShape (and its association with super clusters).

  • CMSSW/RecoEgamma/EgammaHFProducers contains the producers that make HF SuperClusters and the producer that performs the electron ID on the super clusters to make RecoEcalCandidates.
    • CMSSW/RecoEgamma/EgammaHFProducers/python/hfClusterShapes_cfi.py controls the making of the SuperClusters and allows one to set the minimum tower energy to be included in Cluster shape calculations and the minimum Et for a tower to be considered to be a HF superCluster seed.
    • CMSSW/RecoEgamma/EgammaHFProducers/python/hfRecoEcalCandidate_cfi.py controls the electron ID and provides variables to control the ID cuts, as well as an option to turn off energy and position corrections (True = corrections are applied, and is the default)

Loose/Tight Electron Definitions

Within CMSSW/RecoEgamma/EgammaHFProducers/python/hfRecoEcalCandidate_cfi.py tight electrons are defined as the default with
hfRecoEcalCandidate = cms.EDProducer("HFRecoEcalCandidateProducer",
    e9e25Cut = cms.double(0.94),
    hfclusters = cms.InputTag("hfEMClusters"),
    intercept2DCut = cms.double(0.5),
    Correct = cms.bool(True)

The tight cut has an electron efficiency of ~50% while letting only ~5% of background through (determined from EM enriched QCD).

For a looser electron cut the lines

process.hfRecoEcalCandidate.intercept2DCut = 0.32
can be added to the configuration. This looser cut has an electron efficiency of ~85% while letting only ~27% of background through (determined from EM enriched QCD).

--Main.KevinKlapoetke - 30 Jul 2009

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Topic revision: r3 - 2010-04-09 - KevinKlapoetke
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