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B-tagging , leptons and missing energy in ATLAS after first data

Explore the latest findings on top quark pair interactions in ATLAS after the initial data collection, focusing on b-tagging and lepton analysis. Gain insights on jets, tracking devices, and electron/muon identifications at different collision energies. Visit the ATLAS results portal for public data.

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B-tagging , leptons and missing energy in ATLAS after first data

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  1. B-tagging, leptons and missing energy in ATLAS afterfirst data Ivo van Vulpen (Nikhef) on behalf of the ATLAS collaboration

  2. Studying top quarks pairs – the building blocks Jets are discussed in a separate talk 2/40

  3. ATLAS data-sets 2009: √s = 900 GeV 2010: √s = 7 TeV Integratedluminosity Lumi = 12 μb-1 (stable beams) 538,000 collision candidates Main focus of this talk is on this data-set Public ATLAS results: https://twiki.cern.ch/twiki/bin/view/Atlas/AtlasResults 3/40

  4. Upcoming ATLAS results You are here

  5. B-tagging

  6. B-tagging in top analyses B-tagging in top analyses • Reduce W+jets & QCD bckg. • Reduce jet-combinatorics Complications: - JESb-jet difficult (vital for Mtop) - multi-jet environment 6/40

  7. Inner detector Transition Radiation Tracker barrel: ≥ 30 straws ATLAS inner detector SemiConductor Tracker barrel: 4 layers (x2) 3 systems in 2 T solenoid |η|< 2.5 Pixel detector barrel: 3 layers 7/40

  8. Tracking performance Minimum bias events at √s=900 GeV Events with: - Minimum bias trigger, primary vertex ≥ 3 tracks - Tracks with:PT > 0.5 GeV, Npixel ≥ 1, NSCT ≥ 6, |d0|<1.5 mm, |z0 sinθ|<1.5 mm - Simulation: reweighted beamspot and corrections for disabled modules in data Track reconstruction efficiency SCT - number of hits Monte Carlo Excellent agreement:simulation reproduces track properties 8/40

  9. Alignment tracking detectors Tracks with: PT > 2 GeV, Nsilicon ≥ 6, |d0|<10 mm SCT –x residuals Radius inATLAS [mm] 514 443 MC: 38 μm 371 299 Data: 43 μm barrel residual Pixel –x residuals MC: 23 μm Data: 28 μm 122.5 barrel 88.5 50.5 residual 9/40

  10. Vertexing Pile-upevent Primaryvertices ~2.5 cm Luminous region: σX=45 μm, σY=70 μm Vertex resolution ~75 μm 10/40

  11. Secondary vertices Ks invariant mass Λ invariant mass 11/40

  12. d0 significance: d0/σd0 Impact parametersignificance jet secondary vertex Flight length significance: L/σL primary vertex d0 B-tagging algorithms: - jet tracks (combined) incompatibility with originating from primary vertex - soft lepton tagging - secondary vertices & their properties 12/40

  13. ‘Typical’ b-jet candidate √s=7 TeV primary vertex secondary vertex B-jet details: - PT = 19 GeV (EM scale) - 4 b-tag quality tracks Prob(PV compatibility) 9∙10-6 - Secondary vertex: o 50 sigma in 3d away from PV o Mass sec. vertex = 3.9 GeV 13/40

  14. electrons

  15. Electrons in top analyses Electrons in top analyses isolated & high-PT • dominant trigger stream • reduce QCD multi-jet bckg. • handle on leptonic W-boson Complications: - extrapolation from Ze+e-to top multi-jet environment - fake and non-prompt electrons o e/jet ~ 10-5 at PT=40 GeV o identify b/c-decays: isolation 15/40

  16. Electromagnetic calorimeter& particle identification Transition Radiation Tracker e/π separation Electromagnetic calorimeter Liquid Argon / Lead |η|< 3.2 20-30 X0 16/40

  17. EM calorimeter trigger- level 1 - L1 efficiency w.r.t. offline (~3 GeV) L1 rates: normalised to L=1027 cm-2s-1 Nominal settings / rates ATLAS detector paper 17/40

  18. EM & e/γ trigger- level 2 and event filter - L2 rate versus ET Showershapes at L2 7 TeV 900 GeV Note: - Already at Level 2 fairly good agreement on e/γ specifics - Highest level (Event Filter) trigger offline - more refined 18/40

  19. Electrons in 900 GeV data 879 electron candidates:  electrons from photon conversions and fake electrons (hadrons) Transverse energy Eta γ/π0 – separation bulk energyleakage-estimate 19/40

  20. Conversion radius Photon conversions - Clean electron sample - Good material description in MC SCT layer 1 beampipe Pixel layers Eta of candidate 20/40

  21. Electron classifications and data Loose ECAL: shower shapes Track: loose track Expectations, √s=10 TeV (MC) Efficiency(Z e+e-) Jet rejection (x 103) Medium ECAL: strip layer info Track: tighter quality + matching Loose 94.3 % ~ 1 Medium: 90.0 % ~ 7 Tight: 71.5 % ~ 140 Tight Track: B-layer hit + tight match TRT: high-threshold hits 21/40

  22. Strip layer info Calorimeter information electrons Loose ECAL: shower shapes Track: loose track Fraction of energy in strip layer Medium ECAL: strip layer info Track: tighter quality + matching Strip layer info Tight Track: B-layer hit + tight match TRT: high-threshold hits hadrons Fraction of energy in strip layer 22/40

  23. Number of pixel hits Tight electrons Loose ECAL: shower shapes Track: loose track Medium ECAL: strip layer info Track: tighter quality + matching Fraction TRT high-threshold hits Tight Track: B-layer hit + tight match TRT: high-threshold hits 23/40

  24. TransitionRadiationTracker (e/πseparation) TRT: high-thresholdprobability Note: also tested using high-energy muons in the cosmic runs 24/40

  25. Ze+e- candidate at √s=7 TeV ET (e-) = 45 GeV ET (e+) = 40 GeV Invariant mass = 89 GeV 25/40

  26. Muons

  27. Muons in top analyses Muons in top analyses isolated & high-PT • dominant trigger stream • reduce QCD multi-jet bckg. • handle on leptonic W-boson Complications: - extrapolation from Zμ+μ-to top multi-jet environment - fake and non-prompt muons mainly from b-decays:  identify using isolation 27/40

  28. Muon Spectrometer Barrel/endcap toroid B=0.5/1.0 T Coverage:|η| < 2.7 Muon trigger and momentum resolution < 10% up to 1 TeV standalone or combined with inner detector information 28/40

  29. Cosmic runs Transverse momentumresolution Millions and millions of cosmics Transverse momentum resolution Ongoing effort on alignmentand calibration Resolution close to nominal 29/40

  30. L2 eff. (standalone) w.r.t. offline L2 eff. (combined) w.r.t. offline Muon trigger |η|< 1.05 PT track: EF versus offline Level 2 plots from events with: - L1 muon trigger - offline muon (PT > 2 GeV, P > 4 GeV) - Nsilicon≥ 6, match L1 ROI in dR<0.5 30/40

  31. transverse plane Toroids are off 31/40

  32. Muons in √s=900 GeV data phi Monte Carlo expectation: Mainly π/K decays (~25% b/c decays) Momentum eta 32/40

  33. Muons in √s=7 TeV data J/Ψpeak in di-muonmass (opposite charge with p>3 GeV) Mean = 3.06 ± 0.02 GeV Resolution = 0.08 ± 0.02 GeV 33/40

  34. Muons in W- and Z-boson candidates in 7 TeV data Wμν candidate Zμμ candidate M(μ+μ-) = 87 GeV 34/40

  35. Missing transverse energy

  36. ET-miss in top analyses ET-miss in top analyses • reduce QCD multi-jetbckg. ET>20 GeV: ε(tt) ~ 90% QCD rejec. ~ 10 • handleon leptonic W-boson - tails important fornewphysics Complications: - multi-jettopology (2 b-jets) - high-pTmuons 36/40

  37. Calorimeter response Ecell: EM Endcap ET-miss= Calorimeter (+cryo+muons) Electromagnetic calorimeter Ecell: Tile Calorimeter Hadronic calorimeter 37/40

  38. Missing transverse energy and resolution Ex-miss Minimum bias events at 900 GeV and 2.36 TeV (Ex,Ey)-miss resolution Ey-miss RMS ~ 1.8 GeV 38/40

  39. Missing transverse energy: 7 TeV data Ex-miss Collision data at √s = 7 TeV Missing transverse energy Ey-miss Note: - tails under control - cells from topo-clusters (EM-scale) 39/40

  40. Summary: - ATLAS collecting data at 7 TeV - Good performance on reconstructing building-blocks for a top analysis. shown here: leptons, ET-miss, b-tag • Much more data & results available (at PLHC conference … next week) Reconstruct many top quarks in 2010

  41. Backup slides

  42. Alignmenttracking detectors Tracks with: PT > 2 GeV, Nsilicon ≥ 6, |d0|<10 mm Pixel –x residuals SCT –x residuals MC: 23 μm MC: 38 μm Barrel Data: 28 μm Data: 43 μm Pixel –x residuals SCT –x residuals MC: 20 μm MC: 40 μm Endcap Data: 23 μm Data: 45 μm

  43. B-tag distributions in √s=7 TeV data Flight length significance: L/σL Jet probabillity

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