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Kinematics of Top Decays in the Dilepton and the Lepton + Jets channels: Probing the Top Mass

University of Athens - Physics Department Section of Nuclear and Particle Physics. Kinematics of Top Decays in the Dilepton and the Lepton + Jets channels: Probing the Top Mass. Nikos Giokaris. OUTLINE. Introduction Top Quark Production and Decay Physics Motivation

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Kinematics of Top Decays in the Dilepton and the Lepton + Jets channels: Probing the Top Mass

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  1. University of Athens - Physics Department Section of Nuclear and Particle Physics Kinematics of Top Decays in the Dilepton and the Lepton + Jets channels: Probing the Top Mass Nikos Giokaris

  2. OUTLINE • Introduction • Top Quark Production and Decay • Physics Motivation • Sensitivity to the Top Mass at TEVATRON and LHC • Results from PYTHIA/HERWIG simulation study • Conclusions and Future Work

  3. Introduction on top quark • Top quark was predicted by the SM as the I3=+1/2 member of a weak SU(2) isodoublet that also contains the b quark • It was discovered by both CDF end DØ at the Fermilab Tevatron in 1995

  4. Top Production and Decay At high energy collisions at Tevatron and for Mtop>100 GeV/c2 fusion to a gluon is the main production mechanism.

  5. Dilepton BR=11.2% Lepton + jets BR=44.4% All Hadronic BR=44.4% Decay Modes

  6. tW+b jet l+vl W- jet l- What about the Dilepton Signal? • Expect to observe: • two leptons with high PT • large missing ET from the two v’s • two or more jets

  7. CDF/D0 2 fb-1goal Why study the to Dilepton channel • another measurement of the top quark mass with smallest systematic error • better ‘localization’ of SM Higgs mass • it can provide many checks on the SM

  8. The Top Mass New World Average (2004) (includingRunI results) Mt = 178.0 ± 4.3 (±2.7st±3.3sys)GeV/c2 hep-ex/0404010 CDF Run II : • Moderate (CDF RUN II) or very high (LHC) statistics of top production are expected soon • The statistical error will decrease • The systematic error will dominate

  9. The Top Mass • Stat: 2.7 GeV/c2 • Syst: 3.3 GeV/c2 • 2.6 GeV/c2 JES • 1.6 GeV/c2 signal • 0.88 GeV/c2background • 0.83 GeV/c2 UN/MI • 0.35 GeV/c2 fit • 0.12 MC Mt = 178.0 ± 4.3 GeV (±2.7st±3.3sys)

  10. Sensitivity to the Top Mass PROBLEMThe main source of the systematic error is the jet energy scale. PROPOSALUse a variable(s) that does not depend on the jet energy. EXAMINE LEPTON sensitivity to the top mass in the decay channels where leptons are electrons or muons

  11. Analysis Outline • Generation of events for several top masses (130 -230 GeV/c2) for: • CDF energy, 2 TeV (HERWIG) • LHC energy, 14 TeV (Pythia) • Selection of dilepton events, in parton level, where leptons are electrons or muons Requirements on leptons: • PT > 20 GeV/c • |η| <1.1 • Study of the mean value of the leptons’ PT for the samples generated with the above top masses

  12. Leptons’ PT events generated with HERWIG

  13. Distributions of Leptons’ PT events generated with HERWIG Mtop = 170 GeV/c2 Mtop = 180 GeV/c2

  14. Leptons’ PT vs top mass events generated with HERWIG PT sensitive to the top quark mass Fit to a straight line gives slope: 0.16

  15. Other kinematic variables vs top mass events generated with HERWIG

  16. Leptons’ PT events generated with Pythia

  17. Distributions of Leptons’ PT events generated with Pythia Mtop = 170 GeV/c2 Mtop = 180 GeV/c2

  18. Leptons’ PT vs top mass events generated with Pythia PT sensitive to the top quark mass Fit to a straight line gives slope: 0.21

  19. Other kinematic variables vs top mass events generated with Pythia Again, the numbers for 

  20. Leptons’ PT events generated with HERWIG, after simulation

  21. Leptons’ PT vs top mass events generated with HERWIG, after simulation PT sensitive to the top quark mass Fit to a straight line gives slope: 0.15

  22. Other kinematic variables vs top mass events generated with HERWIG, after simulation Again, the numbers in parton level 

  23. Estimation of the statistical error of the top mass (TEVATRON) Expected statistical error in the top mass, as extracted from the Pt spectrum of the two leptons in the dilepton channel, as a function of Luminosity L Top mass is linear dependent to the <PT>  <PT>=λmtop+κ For TEVATRON

  24. Estimation of the statistical error of the top mass (LHC) Top mass is linear dependent to the <PT>  <PT>=λmtop+κ For LHC

  25. Estimation of the systematic error of the top mass Top mass is linear dependent to the <PT>  <PT>=λmtop+κ • Contributions to systematic error:uncertainties in • the fit parameters due to finite MC statistics & omission of non linear terms • the measurement of leptons’ PT • the measurement of jets’ Pt (MET also) • the MC event generator • the knowledge of background

  26. Summary • Top mass analysis, using only lepton PT information in the ttbarDilepton channel, looks promising • The method is applicable both in Tevatron and LHC experiments • The systematic error of the top mass is expected to decrease considerably by this method • The statistical error will also be very small at LHC

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