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Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel.

Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel. Fany Dudziak On behalf of the ATLAS collaboration. - Outline. The Higgs -> ZZ* -> 4 electrons channel Electron reconstruction and identification in ATLAS

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Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel.

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  1. Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel. Fany Dudziak On behalf of the ATLAS collaboration Rencontres Moriond EW 2010 - Young Scientist Forum

  2. - Outline • The Higgs -> ZZ* -> 4 electrons channel • Electron reconstruction and identification in ATLAS • Improvements since ATLAS expected performance (Dec 2008) (“Expected Performance of the ATLAS Experiment” , Dec 2008, arXiv:09010512; CERN-OPEN 2008-020 ) • Impact on the Higgs to 4 electrons analysis Rencontres Moriond EW 2010 - Young Scientist Forum

  3. - The H -> ZZ* -> 4 electrons analysis Analysis skeleton • 4 leptons with Pt > 7 GeV, at least 2 with Pt > 20 GeV et |η| < 2.5 • Lepton quality cuts : 4 e medium • Isolation cuts : (jets,b,t) • Impact parameter cut (b) Higgs Decays Branching Ratios l+ l- l+ l- Z |(mZ)mes - (mZ)| < 15 GeV Z(*) Direct search limit ~114 GeV mH Clean electron signal => Golden channel for masses above 130 GeV Resonance?? Rencontres Moriond EW 2010 - Young Scientist Forum

  4. - The Higgs to 4 electrons analysis 150GeV 130 GeV 14TeV, 30 fb-1 180GeV 300GeV 600GeV 400GeV • Backgrounds : • irreducible : ZZ->4e, • reducible : Zbb, tt, Z+jets • => Their importance varies with the Higgs mass Rencontres Moriond EW 2010 - Young Scientist Forum

  5. trackers calorimeter - The electron reconstruction in ATLAS detector • Electron = TRACK+ ENERGY in CALORIMETER • => calo = LArg Accordion calorimeter • 3 longitudinal layers with different transverse segmentation, and a presampler layer in the front . • => tracker = pixels + SCT (silicium strips) + TRT (transition radiation tracker) • Sliding window algorithm to reconstruct a cluster in the electromagnetic calorimeter. • Search for a reconstructed track which points to the cluster • -> If so, calibrate the cluster as an electron in an adequate size. Otherwise, calibrate the cluster as a photon. • Various selection criteria are used for electron identification and background discrimination • => 3 levels of identification : Loose, Medium and Tight Rencontres Moriond EW 2010 - Young Scientist Forum

  6. - Electron identification in the latest ATLAS publication (ATLAS expected performance book dec 2008) • Loose : • Acceptance, hadronic leakage and constraints on 2nd sampling of the calorimeter variables • Medium : Cuts on 1st sampling of the calorimeter Quality of the track (number of hits in the different layers of the trackers) • Tight • TRT hits , E/p & tighter cuts on track match • => Optional calo and track isolation Efficiency ATLAS expected Performance book Dec 2008 Impact on H->4e analysis for M=130GeV (ATLAS expected Performance book Dec 2008) Rencontres Moriond EW 2010 - Young Scientist Forum

  7. Pt>20GeV optimized - Improvements of the electron reconstruction • Better track choice for match -> priority to Si tracks • Ambiguities between conversions and electrons are now reconstructed as both photons & electrons • Identification criteria optimized to harmonize trigger and offline selection and to increase signal background separation, starting from 5 GeV • New background-discriminating variables • Gain of +12% for medium • Slightly improved jet rejection power Rencontres Moriond EW 2010 - Young Scientist Forum

  8. - Summary and perspectives • The Higgs to 4 electron channel is a very promising channel to discover the Higgs boson for masses above 130 GeV thanks to its clean signal of 4 electrons. • Since the Atlas expected performance book a lot of work has been done to optimize : • - the electron reconstruction • - the electron identification efficiency •  The efficiency gain is 12% (absolute) for medium criteria, with slightly improved jet rejection. • The optimization on the electron identification leads to an expected improvement of the Higgs to 4 electrons recontruction efficiency of ~(relative gain)^4 • Waiting for first data to study electron efficiency and expected backgrounds to H->4e Rencontres Moriond EW 2010 - Young Scientist Forum

  9. BACK-UP SLIDES Rencontres Moriond EW 2010 - Young Scientist Forum

  10. Lead absorbers “accordion” shaped, in a liquid argon bath at 88.5K Cells are graved in the readout electrodes. Detector radiation hard, fully hermetic in phi, good energy resolution, “easy” and stable calibration. Covers the full azimuth and -3.2< η < 3.2 following beams axis. 173K channels Segmented longitudinally in four layers - PRESAMPLER: to measure the energy loss before the calo - FRONT: High granularity to disentangle π0s vs single photons - MIDDLE: Contains the majority of an electromagnetic shower energy - BACK : offers rejection power against hadronic showers. - Lar Calorimeter Middle Strips Back Longitudinal segmentation offers direction measurement for photons TEST BEAM 2004 Resolution E(data)/E(mc) vs E(beam) Rencontres Moriond EW 2010 - Young Scientist Forum

  11. - ATLAS trackers The Inner Detector is composed by 3 sub-systems: Pixels : 1 removable barrel layer 2 barrel layers 4 end-cap disks on each side => 0.8x108 channels Silicon Tracker (SCT) 4 barrel layers 9 end-cap wheels on each side => 6x108 channels Transition Radiation Tracker (TRT) Axial barrel straws Radial end-cap straws ~36 straws by track => 4x105 channels => electron identification capability  Total coverage |η| < 2.5 Rencontres Moriond EW 2010 - Young Scientist Forum

  12. - Perspective for Higgs searches at 7 TeV • The only mass region where we have sensitivity with low luminosity is around 160 GeV (H->WW mode favourized in this region). • A Higgs discovery with 1 fb-1 is unlikely for any Higgs mass at 7 TeV (~2010+2011 @ 7TeV) • => a minimum of 3.3 fb-1 is needed for optimistic uncertainties • If data are well understood a Higgs can be excluded with 1 fb-1 in the mass range • 140 < MH < 185 GeV at 95% CL • H -> 4 leptons channel @ 7 TeV : • No H->4 leptons signal expected but : • Little irreducible background, can be estimated from Drell-Yann • Plenty of events to measure rejection of reducible background • Study of electron’s reconstruction with data Rencontres Moriond EW 2010 - Young Scientist Forum

  13. - Identification cuts Rencontres Moriond EW 2010 - Young Scientist Forum

  14. - Electron efficiencies in the latest ATLAS publication (ATLAS expected performance book dec 2008) Rencontres Moriond EW 2010 - Young Scientist Forum

  15. - Electrons in 900 GeV data • Monte Carlo predictions for electron candidates: • ~33% from conversions • ~1% of prompt electrons from b and c • ~3% of backgrounds electrons (Dalitz…) • fakes from hadrons Rencontres Moriond EW 2010 - Young Scientist Forum

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