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W and Z Physics at ATLAS

W and Z Physics at ATLAS. Corrinne Mills Harvard DOE Site Visit 20 September 2010. W and Z at the LHC. 5 months of 7 TeV collisions 5 months of coherent effort by Harvard group on muon-focused analysis

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W and Z Physics at ATLAS

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  1. W and Z Physics at ATLAS Corrinne Mills Harvard DOE Site Visit 20 September 2010

  2. W and Z at the LHC • 5 months of 7 TeV collisions • 5 months of coherent effort by Harvard group on muon-focused analysis • Results presented at PLHC, ICHEP, HCP/SUSY conferences • Work shown here to be submitted for publication lepton charge asymmetry Martinez- Outschoorn muon definition & efficiency Smith data quality event selection Belloni Kagan QCD background Prasad Jeanty cross section calculations Mills Kashif ZevidellaPorta

  3. Muons in ATLAS • Combined muon: matched inner detector (ID) and muon spectrometer (MS) track • Selection: • pT (combined) > 15 GeV • pT (MS) > 10 GeV • |pT(MS) – pT(ID)| < 15 GeV reject decays in flight • |h| < 2.4 (trigger geometry) • Trigger: L1 (hardware) • pT > 6 GeV

  4. Muon Quality Criteria • Leverage knowledge from studies of cosmic ray data • Consistency requirement for combined muon kinematics: |pT(MS) – pT(ID)| < 15 GeV

  5. Selecting the W signal (I) • Refine muon selection: pT > 20 GeV and relative track isolation < 0.2 • SumpT of tracks in cone around muon of DR < 0.4, divided by the muon pT • Reduce backgrounds by requiring ETmiss > 25 GeV muon channel electron channel

  6. Selecting the W signal (II) • Clean up sample with MT > 40 GeV • Transverse mass muon channel electron channel

  7. W Cross Section • Measure cross section times branching ratio BR(W→l n) • Theoretical prediction: • 10.46 ± 0.02 nb • Luminosity uncertainty is 11% S. Prasad thesis: graduation ~ May 2011

  8. W Cross Section in Context

  9. Charge Asymmetry • W+ favored in proton-proton collisions • Sensitive to valence quark PDFs V. Martinez-Outschoorn thesis: graduation ~ May 2011 muon electron

  10. Selecting the Z → mm signal • Oppositely-charged muon candidates • pT > 20 GeV, h range, quality requirements as with W analysis, including track isolation • 66 GeV < Mll < 116 GeV muon muon channel

  11. Z Cross Section • Measure cross section times branching ratio BR(W→l n) • Theoretical prediction: • 0.964 ± 0.039 nb • Luminosity uncertainty is 11% L. Kashif thesis: graduation ~ Dec. 2010

  12. Z Cross Section in Context

  13. More Data in the Pipeline muon channel

  14. Conclusion • Establishing the W and Z samples at ATLAS • Rapidly increasing dataset • Better precision • W/Z properties, differential cross sections • W pT (next talk) • W and Z data at the LHC will illuminate the Standard Model in a new momentum regime • And pave the way to find what may lie beyond it • key to validation of high-pT leptons and ETmiss • Harvard role • Developing baseline muon selection for high-pT muon analysis • Driving W and Z cross section analyses, W lepton charge asymmetry in muon channel • Major contributor to 310 nb-1 paper, to be submitted soon

  15. Backup

  16. Wmn event selection Preselection W selection

  17. Backgrounds to W → mn • Z → mm, W → tn, Z → tt, ttbar: 77.6 ± 5.4 (stat+sys) events • From simulation • QCD: 21.1 ± 9.8 (stat+sys) events • “Matrix Method” • Solve for NQCD using number of candidates with and without isolation req. (Nloose = 1272, Nisol = 1181) • Measure enon-QCD = 0.984 ± 0.01 from Z’s • Measure eQCD in data with 15 < pTm < 20 GeV (get 0.292 ± 0.004) • extrapolate to pTm > 20 GeV by scaling based on simulated dijet events (get 0.227) • Cosmics: 1.7 ± 0.8 event • Consideration of empty and unpaired bunch crossings

  18. QCD BG: Matrix Method (1) • Solve for NQCD in isolated candidate sample • Nisol (1181) and Nloose (1272) are number of W candidates with and without isolation cut • eQCD and enon-QCD are efficiency of isolation cut for QCD and prompt muons • Measure enon-QCD = 0.984 ± 0.01 in tag-and-probe with Z’s • Measure eQCD in QCD-dominated data: candidate events with 15 < pTm < 20 GeV • extrapolate to pTm > 20 GeV by scaling by e(pTm > 20 GeV)/e(15 < pTm < 20 GeV) as measured in the MC (more on next slide)

  19. QCD BG: Matrix Method (2) • Measure eQCD in QCD-dominated data: candidate events with 15 < pTm < 20 GeV (get 0.292 ± 0.004) • extrapolate to pTm > 20 GeV by scaling by e(pTm > 20 GeV)/e(15 < pTm < 20 GeV) as measured in the MC • (0.238 ± 0.005)/(0.307 ± 0.003) = 0.776 ± 0.017 • Uncertainties • systematic from 100% uncertainty on extrapolation • stat. uncert. from enon-QCD also significant • Bottom line 21.1 ± 4.5 (stat) ± 8.7 (sys)

  20. Backgrounds to Z • Predicted total backgrounds: • electron: 1.18 ± 0.11 (stat) ± 0.41 (syst) • muon: 0.25 ± 0.01 (stat) ± 0.04 (syst) • compare to 3 (0) same-sign events in electron (muon) channel • 2.8 same-sign events from Z → ee signal are expected • Magnitude is small (<1% relative to expected signal) • ttbar • Z → tt • W → en/mn • QCD (muon channel) • QCD (electron channel) • Sideband subtraction for loose-loose electron-positron pairs • Apply loose  medium “rejection factor” measured in data from simulation

  21. Electrons in ATLAS • EM calorimeter cluster matched to inner detector (ID) track • ET > 20 GeV, |h| < 2.47 • exclude gap between barrel and endcap 1.37 < |h| < 1.52 • “Loose” selection • shower shape in middle layer of calorimeter • “Medium” selection • add fine-granularity shower shape and track match • “Tight” selection • add E/p, more track quality, high-threshold TRT hits, conversion veto • Trigger: Level 1 (hardware) requires coarse-granularity cluster with |h| < 2.5 ET > 5 GeV

  22. More on Electrons • Trigger: sliding-window algorithm using reduced-granularity clusters Dh x Dj = 0.1 x 0.1 • Offline reconstruction: sliding window of 3x5 cells or 0.075 x 0.125 in hxj • Electron = cluster with ET > 2.5 GeV and matched track with pT > 0.5 GeV • Reconstruction: exact requirements vary with ET and |h|, but three categories: • Loose electrons • Fiducial: |h| < 2.37 and exclude 1.37 < |h| < 1.52 • Shower shape in middle (largest) layer of calorimeter: cluster width in h • Hadronic leakage: ET(innermost later of HCAL) / cluster ET • Medium electrons: loose += • Shower shape in innermost (finely segemented in h) layer of calorimeter • Track match (Dh) • Track quality (pixel, SCT hits and impact parameter) • Tight electrons: medium += • High-threshold hits in transition-radiation tracker (TRT); hit in innermost pixel layer • E/p • http://cdsweb.cern.ch/record/1273197/files/ATLAS-CONF-2010-005.pdf

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