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Electrons and photons in ATHENAFrédéric DERUE – LPNHE ParisATLAS offline software tutorial

Detectors involvedinner detector : track reconstruction, particle-id in TRTLAr EM calorimeter : cell, clusters + shower shapes for particle-idhadronic calorimeter : for leakage and isolation requirements

Three reconstruction algorithmsstandard egamma algorithm – cluster-based for |η|<2.5

“softe” - track-based for |η|<2.5forward electrons for |η|>2.5 – no inner detector information

Several analysis algorithmsdedicated for electrons and photons : e.g conversion recovery

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Software location

Software informationall packages under Reconstruction/egammamain algorithms in Reconstruction/egamma/egammaRec

python/egammaGetter.py, src/egammaBuilder.cxxpython/softeGetter.py, src/softeBuilder.cxxpython/forwardGetter.py, src/forwardBuilder.cxx

main analysis algorithms in Reconstruction/egamma/egammaAnalysis

PhotonAnalysisAlgsElectronAnalysisAlgs

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egamma algorithmsStandard reconstruction procedure

seeded by clusters reconstructed in LAr EM by a sliding window (SW) algorithm

NB: details of SW reconstruction have changed/improved since CSC notes

cluster-track matching procedureNB: details have changed/improved since CSC notes

cluster-conversion matching procedureDefinition of objects

cluster+track : electron cluster+ no track : non-converted photoncluster+conversion : converted photon

A word on softeuses tracks as input with high preselection cuts to minimize fakesIf track is same as the one used by standard algo then keep standard objectessentially useful for ET<5 GeVapart “this”, objects or identification is the same as for standard a

A word on forward electronsuses topo clusters, no inner detectordedicated identification

A certain overlap (few %) exist between electrons and converted photons

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cluster-size and calibrationCluster size

electrons : 3*7 in barrel, 5*5 in end-capsnon-converted photons : 3*5 in barrel, 5*5 in end-capsconverted photons : 3*7 in barrel, 5*5 in end-caps

Calibrationdifferent corrections are applied for electrons/photonsfor position and energy measurements

There are different calorimeter calibrations, cluster sizes, and identification variables and cuts used for electrons and photons: if the user reassigns the type of the object in analysis, the user needs to properly correct for this. Tools are available to do this.

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four-momentumI4Momentum

base class to keep 4-momentum doxygen documentation http://tinyurl.com/6h2o23

Combinationsince 15.3.0 the energy of the four-momentum is a combination of the calorimeter and tracker measurementsince rel 15.5 for photons energy is only from calo

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egamma object

CaloClustercalorimeter informationavailable for all objects

Link to a TrackParticletracking informationuseful only for electrons

Link to a VxCandidatevertex informationavailable for converted electrons – in the PhotonContainer

egDetailadditional reconstructed informationavailable for all objects

PIDparticle identificationavailable for all objects

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What's on AODThree containers available in StoreGate

ElectronAODContainerPhotonAODContaineregDetailsAODContainer

Links to other containersTrackParticle, CaloCluster, VxCandidateinformation is sufficient for the user to be able to redo some basic PID

Documentationhttps://twiki.cern.ch/twiki/bin/view/AtlasProtected/EgammaAOD

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Cluster informationCaloCluster object

reconstructed E, px, py, pzposition eta, phi

Recalibration of clustershttps://twiki.cern.ch/twiki/bin/view/AtlasProtected/EgammaAODRenderegDetails

object called EMShower contains most of the detailed calorimeter informationdescribe shower shapes define detailed cuts for identification purpose

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Tracking informationPointer to the Trk::Track with which the particle was createdTracks associated to electron can be refitted at AOD levelAlso contains

chargeTrk::FitQualityDefiningParameters()

For electrons this is the Trk::Perigee – useful for vertex fitting

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VxCandidateUsed to create conversionsSingle track and double track conversions possibleContains

position of the vertexVxTrackAtVertex(s) that make the vertex

from which you can obtain the corresponding TrackParticle

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egDetailEMShower

calorimeter informationisolation, leakage

EMTrackMatchtrack to calorimeter matching informatione.g , at each of the sampling layers

EMConvertconversion matchin variableable to obtain a full Trk::VxCandidate

EMBremFitresults from CaloBremFite.g brem radius, improved pT

EMTrackFitable to obtain full Trk::Perigee parameters

EMFourMomErrors

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electron pid - IsEM

Multivariate techniques are available but not discussed here (eg. Log Likelihood, Boosted Decision Trees, H-matrix, Neural Nets, …).These techniques will be commissioned after the cut-based electron id has been validated with early data. Only certified variables will be used.

Default method used in electron identificationA basic series of cuts based on:

shower shape properties in different compartments of the calorimeter combination of ID and Calo

Loose - Calorimeter only information hadronic Leakage, Shower Shape (middle layers only of EM cal)excellent ID efficiency, low BG rejection

Medium - Adds tracking information basic Track quality cutsmore detailed Calorimeter Cutsincreased jet rejection efficiency by a factor 3-4 while reducing ID efficiency by 10%

Tight - Using all information available b-layer hit requirement,TRT High Threshold hit informationstrongest background rejection but lowest ID efficiency ISO - For isolated electrons provides tighter calorimeter cutsTRT - For non-isolated electrons (from heavy quark decay) tighter TRT PID

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photon pid - IsEM

Default method used in photon identificationA basic series of cuts based on:

shower shape properties in different compartments of the calorimeter different cuts for converted and non converted photons

Loose - Calorimeter only information hadronic Leakage, Shower Shape (middle layers only of EM cal)excellent ID efficiency, low BG rejection

Tight - Using all information available use information from the strips in particularstrongest background rejection but lowest ID efficiency ISO - For isolated photons provides tighter calorimeter cuts

Multivariate techniques are available but not discussed here (eg. Log Likelihood, Boosted Decision Trees, H-matrix, Neural Nets, …).These techniques will be commissioned after the cut-based photon id has been validated with early data. Only certified variables will be used.

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More information

• egamma twiki:

• https://twiki.cern.ch/twiki/bin/view/AtlasProtected/ElectronGamma

• egamma CSC notes:

• http://cdsweb.cern.ch/record/1119939

• egamma hypernews

• https://hypernews.cern.ch/HyperNews/Atlas/get/EGammaWG.html