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Advances in Top Quark Analyses at DØ: Run II Cross Section and Mass Measurements

This presentation by Marumi Kado (LBL) discusses the significant findings from the DØ Collaboration regarding top quark production, focusing on the Run II data. The analysis illustrates that the top production cross-section is approximately 30% higher than predictions from Run I, with values estimated between 6.7 to 7.5 pb. Kado elaborates on six analysis channels: lepton+jets (topological and soft muon tag), and dileptons, utilizing sophisticated methods to measure efficiencies and estimate QCD backgrounds. Also, improved measurements of the top mass with Run I data are covered.

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Advances in Top Quark Analyses at DØ: Run II Cross Section and Mass Measurements

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  1. Status of Top Quark Analyses at DØ Marumi Kado Measurement of the top production cross section Run II data Improved measurement of the top mass with Run I data Lawrence Berkeley National Laboratory On behalf of the DØ Collaboration Marumi Kado - LBL

  2. _ First Measurement of the tt Cross Section at s=1.96 TeV Lepton+jets (topological) Lepton+jets (soft muon tag) dileptons • Measurement based six analysis channels: • Cross section at Run II ~30% higher than at Run I • Predictions ranging from 6.7 to 7.5 pb • Data sample from mid-August until mid-January • with luminosities from 30 to 50 pb-1 • Triggers em and mm e+jets, m+jets e+jets/m, m+jets/m Br = 2.5 and 1.2% Br = 14.7% Br = 14.7% Efficient Not very pure Pure Not very efficient Pure and efficient Low branching Use calorimeter and muon system at all levels of the trigger (L1, L2 and L3) Marumi Kado - LBL

  3. Improved legacy 0.5 cone algorithm with JES corrections (see talk by B. Kehoe) Object Identification Reconstruction of Jets and Jet energy scale: Reconstruction of electrons (only central): - Select EM particles (simple cone, shower shape, EM fraction) - Match with track (f, h, and E/p) Reconstruction of muons: Missing Transverse Energy: DØ Run II Preliminary DØ Run II Preliminary 2 EM No track required 2 EM 1 track required • Tracks in Muon system • Tracks in Central Tracker • Minimum Ionisation in the calorimeter ID Used for redundancy only to measure efficiencies From calorimeter with JES corrections (and muon correction) Marumi Kado - LBL

  4. Dimuon Channel 42.6 pb-1 2 isolated muons, MET(Mmm), HT and more than 2 Jets Luminosity Selection criteria: Backgrounds: Instrumental Physical backgrounds Zmm DYmm QCD Heavy Flavor Ztt WWmm from MC Estimated from data DØ Run II Preliminary Zmm (MC) WW (MC) Data Signal (MC) * For s = 7pb Marumi Kado - LBL

  5. em Channel 33.0 pb-1 1 electron, 1 isolated muons, MET, METCAL , HT (e) and more than 2 Jets Luminosity Selection criteria: Backgrounds: Physical backgrounds Instrumental QCD and W+jets Ztt WWmm from MC Estimated from data (Fakes) DØ Run II Preliminary Z tt Fakes Njets * For s = 7pb Njets HT(e) (GeV) Marumi Kado - LBL

  6. em Candidate Event m - Muon Track in the Calorimeter (MTC) Jet 1 e+ Jet 2 Marumi Kado - LBL

  7. Lepton-plus-Jets Analyses 49.5 pb-1 40.0 pb-1 - Preselect a sample enriched in W events - Evaluate QCD multi-jet (as a function of Njets) - Estimate W+4jets assuming Berends scaling - Apply topological selection • Luminosities: e+jets and m+jets • Backgrounds: QCD multi-jets and W multi-jets • Method: • Preselection: • QCD background evaluation (matrix method): 1 EM object or muon, MET, soft muon veto Separate W+tt and QCD with loose (L) and tight (T) lepton characteristics. Efficiencies (LT) for signal eW+tt and background eQCD are measured independently: e+jets: Track match to the EM object m+jets: Muon isolation “Matrix method”  Marumi Kado - LBL

  8. e W+tt vs Njets DØ Run II Preliminary … are obtained from benchmark signal samplesn of Z ee or mm Non trivial dependence of eW+tt w.r.t. Njets (especially in the m+jets case)… • Signal probabilities… • Background nature • Background Probabilities…  Correction taken from MC m+jets e+jets QCD Background due to leading p0 or compton QCD events and Fake track or g conversion QCD Background essentially due to Heavy Flavor semi-leptonic decays … are obtained from benchmark QCD samples with low MET Dependence of the eQCD w.r.t. MET and Njet … DØ Run II Preliminary DØ Run II Preliminary DØ Run II Preliminary e+jets e+jets e+jets Marumi Kado - LBL

  9. DØ Run II Preliminary DØ Run II Preliminary W W+tt • Estimation of the QCD background: QCD W+tt W+tt DØ Run II Preliminary DØ Run II Preliminary W W+tt DØ Run II Preliminary DØ Run II Preliminary QCD W+tt W+tt Marumi Kado - LBL

  10. DØ Run II Preliminary DØ Run II Preliminary • Berends scaling: Estimation of the W background for Njets 4: Apply topological cuts: a = 0.1640.02 a = 0.1450.02 38 (m+jets) 24.2 11.9 11.9 12.5 22 (e+jets) Apalanarity and HT * For s = 7pb Marumi Kado - LBL

  11. m+jets Candidate Event m - Jet 5 Jet 1 MTC Jet 4 Jet 2 Jet 3 Marumi Kado - LBL

  12. Soft Muon Tag Analyses (Loose/Tight sample) - Use the same preselection as l+jets - Require at least 3 jets - Apply mild topological cuts 75/23 (m+jets)  459/27 (e+jets) DØ Run II Preliminary DØ Run II Preliminary Selection before Soft Muon Tag When SMT is applied: e+jets m+jets 1/0 (m+jets) 0.20.2 (m)  9/2 (e+jets) 0.20.1 (e) QCD background. from matrix method W bkg. from Tag rate functions: DØ Run II Preliminary 0.40.1 (m) 0.00.1 (e) * For s = 7pb Marumi Kado - LBL

  13. Cross Section Measurement • Combining the observation of all channels an excess of 3s is observed, compatible with a signal expectation at the 35% CL • The combined cross section is: Marumi Kado - LBL

  14. Status of the Top Mass Measurement in the Lepton+Jets Channels at Run I Likelihood method using most available information • Uses DØ Run I statistics (125 pb-1) & selection  91 events • Signal and background probability (simple realistic model): Additional cuts for this analysis: 4 Jets exclusively: 71 events Pb.: 22 events (pure sample) x  reconstructed objects in the event (leptons, jets) a Parameter to estimate Folding object resolutions Transfer Functions Matrix element Acceptance PDF’s Background (VECBOS) Signal (No ISR or FSR) Pb Independent of a Marumi Kado - LBL

  15. Likelihood definition: • Estimate the signal and background contributions and mtop: Signal c1=12 mtop W Bkg c2=10 (5.6 GeV from PRD 58 052001,1998) Large improvement on the statistical uncertainty (~2.4 stats) mW Expect a substantial improvement in the JES systematic Marumi Kado - LBL

  16. Conclusions and Outlook • D0 measurement of the tt cross section at s=1.96 TeV carried out at Tevatron Run II: • Short term perspective:Dielectron channel and Lifetime tagging analyses • Medium term perspective:Complete analyses of mass and cross sections • Status of the new top Run I mass measurement: • Short term perspective:Improve JES systematic uncertainty,W helicity (Run I) • Medium term perspective:Top Properties (W helicity,spin correlations) and single top with Run II data • Long term perspective:top mass 2fb-1  2.7 GeV • 15fb-1 1.3GeV Marumi Kado - LBL

  17. m+jets Candidate Event SV m - Jet 2 IP Jet 2 MTC Jet 1 Double tag event IP Jet 1 SV Marumi Kado - LBL

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