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Electron Identification Efficiency from Z →ee

This presentation discusses the electron identification efficiency in Z→ee decays using data and Monte Carlo simulations. It covers the evaluation of efficiencies independently from MC, the use of the Tag and Probe method, comparison with MC to evaluate systematics, and the differential efficiencies in Eta and Pt. The results from signal-only samples and the IsEM flag definition are presented.

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Electron Identification Efficiency from Z →ee

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  1. 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

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

  3. Samples and IsEM Flag Definition • Sample: trig1_misal1_csc11_V1.005144.PhytihiaZee.recon.AOD.c12000601 • Using custom NTuples produced by Ellie Dobson (Oxford) and Mike Flowerdew (Liverpool) • IsEM Flag Definition for Athena version 12 (will change in version 13) • Tight Electron : isEM & 0x7FF (all but TRT) • Medium Electron: isEM & 0x30F (all calo + track quality + cluster-track matching in η and φ) • Loose Electron: isEM & 0x7 (3 out of 4 calo bits) ATLAS UK SM Meeting

  4. Method: Tag and Probe • Measure IsEM Efficiencies relative to Electron Offline Reconstruction Efficiency • Tag Selection: • 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 Select cluster (from electron container) in opposite hemisphere: ΔφtagEle-clus > ¾ pi Offline cut: Pt > 25GeV |η|< 2.4, include cracks • Probe Condition: • Require 2nd Electron to have Tight/Medium/Loose IsEM • Select ee pairs with invariant mass: 70GeV < Mee < 100GeV • Use only electrons from EGamma algorithm ATLAS UK SM Meeting

  5. IsEM Efficiency from Data N1 N2 Loose IsEM 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 2nd electron passes probe condition (tight/medium/loose IsEM) • Statistical Error calculated as: Where N2b is the number of cases when both electrons satisfy tag and probe conditions ATLAS UK SM Meeting

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

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

  8. 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 Hits Num BLayer Hits Impact Parameter ATLAS UK SM Meeting

  9. 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 Included in tight/medium electron, but excluded from loose electronIn Eta: explains the shape for differential efficiency of tight/medium electron (drops in endcaps and central eta) ATLAS UK SM Meeting

  10. 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 Correction After EnScale Correction ATLAS UK SM Meeting

  11. Differential Efficiency in Pt • Data • MC • Efficiency systematically underestimated at low Pt (after correction for Energy Scale) ATLAS UK SM Meeting

  12. Global Efficiecy (for Pt>25GeV) ATLAS UK SM Meeting

  13. Tight Electron: with/without TRT No TRT With TRT Global Efficiency drops by more than 10 % with TRT (including crakcs) ATLAS UK SM Meeting

  14. Conclusions • Differential Efficiency in Eta, Pt and Global Efficiency calculated for Z→ee (signal only) • Differential Efficiency in Eta: • Agreement data-MC • Cuts based on 1st Sampling of LAr Calo affect medium/tight electron efficiency • Differential Efficiency in Pt: • Need to correct for energy scale • Still discrepancy between data and MC at low Pt (combinatorial background) • Tight Electron with/without TRT bit: • 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) • Other studies on offline electron and trigger efficiencies from Z→ee: Guillaume Kirsch (offline efficiency), Ellie Dobson and Mike Flowerdew (trigger efficiency) ATLAS UK SM Meeting

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