Electron Identification Efficiency from Z→ee Maria Fiascaris University of Oxford In collaboration with Tony Weidberg and Lucia di Ciaccio ATLAS UK SM Meeting, Sept 24th 2007

Sept 24th ATLAS UK SM MeetingOverview Electron Identification Efficiency (tight/medium/loose electron) from Data Electron Identification Efficiency (tight/medium/loose electron) from Data Evaluate efficiencies independently from MC Evaluate efficiencies independently from MC Use Tag and Probe method Use Tag and Probe method Comparison with MC to evaluate systematics Comparison with MC to evaluate systematics Differential Efficiencies in Eta and Pt Differential Efficiencies in Eta and Pt Global Efficiencies Global Efficiencies Results from Signal Only (Z →ee) Results from Signal Only (Z →ee)

Sept 24th ATLAS UK SM Meeting Sample: Sample:trig1_misal1_csc11_V PhytihiaZee.recon.AOD.c Using custom NTuples produced by Ellie Dobson (Oxford) and Mike Flowerdew (Liverpool) Using custom NTuples produced by Ellie Dobson (Oxford) and Mike Flowerdew (Liverpool) IsEM Flag Definition for Athena version 12 (will change in version 13) IsEM Flag Definition for Athena version 12 (will change in version 13) Tight Electron : isEM & 0x7FF Tight Electron : isEM & 0x7FF (all but TRT) Medium Electron: isEM & 0x30F Medium Electron: isEM & 0x30F (all calo + track quality + cluster-track matching in η and φ) Loose Electron:isEM & 0x7 Loose Electron:isEM & 0x7 (3 out of 4 calo bits) Samples and IsEM Flag Definition DatasetProcess Cross section (pb) Efficiency Eff Cross Section (pb) Number Events Luminosity (pb-1) 5144 Z→ee k290

Sept 24th ATLAS UK SM Meeting Method: Tag and Probe Measure IsEM Efficiencies relative to Electron Offline Reconstruction Efficiency Measure IsEM Efficiencies relative to Electron Offline Reconstruction Efficiency Tag Selection : Tag Selection : First Electron First Electron Offline cut: Pt > 25GeV |η|< 2.4, exclude cracks (1.37<|η|< 1.52) Tight isEM Passes all triggers (e25i): L1+L2+EF (need to match offline electron to trigger object and apply trigger cuts) Second Electron Second Electron Select cluster (from electron container) in opposite hemisphere: ΔφtagEle-clus > ¾ pi Offline cut: Pt > 25GeV |η|< 2.4, include cracks Probe Condition: Probe Condition: Require 2 nd Electron to have Tight/Medium/Loose IsEM Require 2 nd Electron to have Tight/Medium/Loose IsEM Select ee pairs with invariant mass: 70GeV < M ee < 100GeV Select ee pairs with invariant mass: 70GeV < M ee < 100GeV Use only electrons from EGamma algorithm Use only electrons from EGamma algorithm

Sept 24th ATLAS UK SM Meeting IsEM Efficiency from Data Count number of electron pairs that satisfy tag/probe condition (can be more than 1 per event → combinatorial background) Count number of electron pairs that satisfy tag/probe condition (can be more than 1 per event → combinatorial background) ε = N2/N1 N1= all cases passing tag condition N2= all cases where 2 nd electron passes probe condition (tight/medium/loose IsEM) Statistical Error calculated as: Statistical Error calculated as: Where N 2b is the number of cases when both electrons satisfy tag and probe conditions N2 Loose IsEM N1 Combinatorial Background

Sept 24th ATLAS UK SM Meeting Efficiency from Monte Carlo Use Efficiency from MC to study systematics Use Efficiency from MC to study systematics Select two electrons coming from Z: Select two electrons coming from Z: Require one electron with: Require one electron with: Pt > 25GeV |η|<2.4, cracks excluded (1.37<|η|< 1.52) Second Electron Second Electron Pt > 25GeV Pt > 25GeV |η|<2.4, cracks included |η|<2.4, cracks included Match second electron with offline electron (ΔR <0.2) Match second electron with offline electron (ΔR <0.2) → N1 For matched offline electron: For matched offline electron: require isEM to be tight/medium/loose → N2

Sept 24th ATLAS UK SM Meeting Differential Efficiency in Eta Data MC → Drops in endcaps for medium/tight due to Track Quality bits or to bit from Calo? (see later)

Sept 24th ATLAS UK SM Meeting Understanding Eta Differential Efficiency: Track Quality Track Quality Bit in isEM flag requires: Num of BLayer Hits ≥ 1 Num of SCT+Pixel Hits ≥ 9 Impact Parameter < 0.1  No η dependence below threshold values: does not explain efficiency drop at the edges of η  Need to look at calorimeter bit that is excluded from Loose Electron isEM Num Pixel+SCT HitsNum BLayer HitsImpact Parameter

Sept 24th ATLAS UK SM Meeting Efficiency of Bit 3 of Calo: (isEM & 0x8) Efficiency of Bit 3 of Calo: (isEM & 0x8) First sampling of the LAr calorimeter, uses fine granularity to apply cuts based on shower shape Efficiency of Bit 3 of Calo: (isEM & 0x8) First sampling of the LAr calorimeter, uses fine granularity to apply cuts based on shower shape Included in tight/medium electron, but excluded from loose electron In Eta: explains the shape for differential efficiency of tight/medium electron (drops in endcaps and central eta)

Sept 24th ATLAS UK SM Meeting Understanding Pt Differential Efficiecy  Can correct for the energy scale between true and offline electron: plot MC results in bins of Pt Offline Electron  Corrects for discrepancy at high Pt  Discrepancy at low Pt remains: Combiantiorial Background? Before EnScale CorrectionAfter EnScale Correction

Sept 24th ATLAS UK SM Meeting Differential Efficiency in Pt Data MC Efficiency systematically underestimated at low Pt (after correction for Energy Scale)

Sept 24th ATLAS UK SM Meeting Global Efficiecy (for Pt>25GeV) IsEM Efficiency (Data) Efficiency (MC) With cracks Tight (no TRT) 78.87± ±0.09 Medium 82.30± ±0.08 Loose 98.27± ±0.03 No cracks Tight (no TRT) 79.73± ±0.09 Medium 82.61± ±0.08 Loose 98.83± ±0.02

Sept 24th ATLAS UK SM Meeting Tight Electron: with/without TRT No TRTWith TRT IsEM Eff (data) Eff (MC) Tight (no TRT) 78.87± ±0.09 Tight (with TRT) 67.66± ±0.10 Global Efficiency drops by more than 10 % with TRT (including crakcs)

Sept 24th ATLAS UK SM MeetingConclusions Differential Efficiency in Eta, Pt and Global Efficiency calculated for Z →ee (signal only) Differential Efficiency in Eta, Pt and Global Efficiency calculated for Z →ee (signal only) Differential Efficiency in Eta: Differential Efficiency in Eta: Agreement data-MC Agreement data-MC Cuts based on 1st Sampling of LAr Calo affect medium/tight electron efficiency Cuts based on 1st Sampling of LAr Calo affect medium/tight electron efficiency Differential Efficiency in Pt: Differential Efficiency in Pt: Need to correct for energy scale Need to correct for energy scale Still discrepancy between data and MC at low Pt (combinatorial background) Still discrepancy between data and MC at low Pt (combinatorial background) Tight Electron with/without TRT bit: Tight Electron with/without TRT bit: TRT bit decreases the efficiency by more than 10% TRT bit decreases the efficiency by more than 10% Further Studies Study QCD background (Dijet samples, Pythia): methodology is ready (Mike Flowerdew and Ellie Dobson) Study QCD background (Dijet samples, Pythia): methodology is ready (Mike Flowerdew and Ellie Dobson) Other studies on offline electron and trigger efficiencies from Z→ee: Other studies on offline electron and trigger efficiencies from Z→ee: Guillaume Kirsch (offline efficiency), Ellie Dobson and Mike Flowerdew (trigger efficiency)