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 Nikos Giokaris

 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 OUTLINE

Introduction on top quark  Top quark was predicted by the SM as the I 3 =+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

Top Production and Decay At high energy collisions at Tevatron and for M top >100 GeV/c 2 fusion to a gluon is the main production mechanism.

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

What about the Dilepton Signal? Expect to observe: two leptons with high P T large missing E T from the two v’s two or more jets t  W + b jet l + v l  W - jet l -

Why study the to Dilepton channel CDF/D0 2 fb -1 goal  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

The Top Mass New World Average (2004) (including RunI results) Mt = ± 4.3 (±2.7st±3.3sys) GeV/c 2 hep-ex/ 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 CDF Run II :

The Top Mass Mt = ± 4.3 GeV (±2.7st±3.3sys) Stat: 2.7 GeV/c 2 Syst: 3.3 GeV/c GeV/c 2 JES 1.6 GeV/c 2 signal 0.88 GeV/c 2 background 0.83 GeV/c 2 UN/MI 0.35 GeV/c 2 fit 0.12 MC

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

Analysis Outline  Generation of events for several top masses ( GeV/c 2) 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:  P T > 20 GeV/c  |η| <1.1  Study of the mean value of the leptons’ P T for the samples generated with the above top masses

Leptons’ P T events generated with HERWIG Top Quark Mass (GeV/c 2 ) Lepton (GeV/c) standard deviation (GeV/c)

Mtop = 170 GeV/c 2 Distributions of Leptons’ P T events generated with HERWIG Mtop = 180 GeV/c 2

Leptons’ P T vs top mass events generated with HERWIG P T sensitive to the top quark mass Fit to a straight line gives slope: 0.16

Other kinematic variables vs top mass events generated with HERWIG Kinematic VariableSlope PTPT ± P0.1739± Leading P T ± Leading P0.2523± Sum of P T ’s0.316±0.008 Sum of P’s0.3523±0.0092

Top Quark Mass (GeV/c 2 ) Lepton (GeV/c) standard deviation (GeV/c) Leptons’ P T events generated with Pythia

Distributions of Leptons’ P T events generated with Pythia Mtop = 170 GeV/c 2 Mtop = 180 GeV/c 2

P T sensitive to the top quark mass Fit to a straight line gives slope: 0.21 Leptons’ P T vs top mass events generated with Pythia

Kinematic VariableSlope PTPT ± P0.242±0.005 Leading P T ± Leading P0.3526± Sum of P T ’s0.381±0.008 Sum of P’s0.451± Other kinematic variables vs top mass events generated with Pythia Kinematic Variable Slope PTPT ± P0.1739± Leading P T ± Leading P0.2523± Sum of P T ’s0.316±0.008 Sum of P’s0.3523± Again, the numbers for 

Leptons’ P T events generated with HERWIG, after simulation Top Quark Mass (GeV/c 2 ) Lepton (GeV/c) standard deviation (GeV/c)

Leptons’ P T vs top mass events generated with HERWIG, after simulation P T sensitive to the top quark mass Fit to a straight line gives slope: 0.15

Other kinematic variables vs top mass events generated with HERWIG, after simulation Kinematic VariableSlope PTPT ± P0.1667± Leading P T ± Leading P0.2309± Sum of P T ’s0.3091± Sum of P’s0.3317± Kinematic Variable Slope PTPT ± P0.1739± Leading P T ± Leading P0.2523± Sum of P T ’s0.316±0.008 Sum of P’s0.3523± Again, the numbers in parton level 

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 Integrated Luminosity Expected by Expected number of dilepton events (δM top ) stat (pb -1) (GeV/c 2 )(%) 193Feb Sep Dec Dec Dec For TEVATRON Top mass is linear dependent to the  = λ m top +κ

Estimation of the statistical error of the top mass (LHC) For LHC Integrated Luminosity Expected by Expected number of dilepton events (δM top ) stat (pb -1) (GeV/c 2 )(%) 1000 End of 1 st year of operation ,000 End of 2 nd year of operation 80, Top mass is linear dependent to the  = λ m top +κ

Estimation of the systematic error of the top mass Top mass is linear dependent to the  = λ m top +κ Contributions to systematic error: uncertainties in the fit parameters due to finite MC statistics & omission of non linear terms the measurement of leptons’ P T the measurement of jets’ Pt (MET also) the MC event generator the knowledge of background Uncertainty(δ ) syst δκδλ(δM top ) stat (Gev/c) (GeV/c 2 )(%) Leptons’s P T 0.05 Linear fit Jet energy Monte Carlo Background Total

Summary  Top mass analysis, using only lepton P T 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