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Top quark mass For DØ collaboration Regina Demina University of Rochester Wine and Cheese seminar at FNAL, 07/22/05.

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Presentation on theme: "Top quark mass For DØ collaboration Regina Demina University of Rochester Wine and Cheese seminar at FNAL, 07/22/05."— Presentation transcript:

1 Top quark mass For DØ collaboration Regina Demina University of Rochester Wine and Cheese seminar at FNAL, 07/22/05

2 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL2Outline Introduction Top quark mass measurement in Run II –Matrix element method description –In situ jet energy scale calibration on hadronic W-mass –Sample composition –Result –Systematics Tevatron combined top mass Top quark production –Update on cross section in l+jets channel –Search for resonance production

3 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL3 Top Quark Mass: Motivation Fundamental parameter of the Standard Model. Important ingredient for EW precision analyses at the quantum level: which were initially used to indirectly determine m t. After the top quark discovery, use precision measurements of M W and m t to constrain M H. WW t b WW H  M W  m t 2  M W  ln(M H ) CDF&D0 RUNII

4 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL4 Top production At √s=1.96 TeV top is produced in pairs via quark- antiquark annihilation 85% of the time, gluon fusion accounts for 15% of ttbar production

5 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL5 Top Lifetime and Decay Since the top lifetime  top ~ 1/ M 3 top ~ sec  qcd ~  -1 ~ sec BR(t  Wb)  –Both W’s decay via W  l final state: l  l bb - DILEPTON –One W decays via W  l final state: l  qq bb - LEPTON+JETS –Both W’s decay via W  qq final state: qq  qq bb ALL HADRONIC the top quark does not hadronize. It decays as a free quark! Lepton provides a good trigger, all jets are tough

6 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL6 Top ID in “lepton+jets” channel 2 b-jets Lepton: electron or muon Neutrino (from energy imbalance) 2 q’s – transform to jets of particles Note that these two jets come from a decay of a particle with well measured mass – W-boson – built-in thermometer for jet energies

7 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL7 DØ detector Electrons are identified as clusters of energy in EM section of the calorimeter with tracks pointing to them Muons are identified as particles passing through entire detector volume and leaving track stubs in muon chambers. Track in the central tracking system (silicon+SciFi) is matched to track in muon system Jets are reconstructed as clusters of energy in calorimeter using cone algorithm DR<0.5

8 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL8 Top mass using matrix element method in Run I Method developed by DØ (F. Canelli, J. Estrada, G. Gutierrez) in Run I Systematic error dominated by JES 3.3 GeV/c 2 With more statistics it is possible to use additional constraint on JES based on hadronic W mass in top events – in situ calibration Single most precise measurement of top mass in Run I M t =180.1±3.6(stat) ±4.0(syst) GeV/c 2

9 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL9 Matrix element method Goal: measure top quark mass Observables: measured momenta of jets and leptons Question: for an observed set of kinematic variables x what is the most probable top mass Method: start with an observed set of events of given kinematics and find maximum of the likelihood, which provides the best measurement of top quark mass Our sample is a mixture of signal and background

10 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL10 Matrix Element Method Normalization depends on m t Includes acceptance effects probability to observe a set of kinematic variables x for a given top mass Integrate over unknown q 1,q 2, y f(q) is the probability distribution than a parton will have a momentum q d n σ is the differential cross section Contains matrix element squared t t W(x,y) is the probability that a parton level set of variables y will be measured as a set of variables x b q’ q

11 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL11 Transfer functions (parton  jet) Partons (quarks produced as a result of hard collision) realize themselves as jets seen by detectors –Due to strong interaction partons turn into parton jets –Each quark hardonizes into particles (mostly  and K’s) –Energy of these particles is absorbed by calorimeter –Clustered into calorimeter jet using cone algorithm Jet energy is not exactly equal to parton energy –Particles can get out of cone –Some energy due to underlying event (and detector noise) can get added –Detector response has its resolution Transfer functions W(x,y) are used to relate parton energy y to observed jet energy x

12 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL12  Dependence of JES  dependence of JES is derived on  +jet data, but the overall scale is allowed to move to optimize M W

13 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL13 All jets are corrected by standard DØ Jet energy scale (p T,  ) Overall JES is a free parameter in the fit – it is constrained in situ by mass of W decaying hadronically JES enters into transfer functions: JES in Matrix Element

14 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL14 Normalization e+jetsμ+jets

15 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL15 Signal Integration Set of observables – momenta of jets and leptons: x Integrate over unknown –Kinematic variables of initial (q 1,q 2 ) and final state partons (y: 6 x3 p) = 20 variables –Integral contains 15 (14)  -functions for e(  )+jets total energy-momentum conservation: 4 angles are considered to be measured perfectly: 2x4 jet +2 lepton Electron momentum is also considered perfectly measured, not true for muon momentum: 1(0) –5(6) dimensional integration is carried out by Vegas –The correspondence between parton level variables and jets is established by transfer functions W(x,y) derived on MC for light jets (from hadronic W decay) for b-jets with b-hadron decaying semi-muonically for other b-jets Approximations –LO matrix element –qq  tt process only (no gluon fusion – 15%)

16 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL16 Background integration W+jets is the dominant background process Kinematics of W+jets is used as a representation for overall background (admixture of multijet background is a source of systematic uncertainty) –Contribution of a large number of diagrams makes analytical calculation prohibitively complex –Use Vecbos Evaluate ME wjjjj in N points selected according to the transfer functions over phase space P bkg - average over points

17 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL17 Sample composition Lepton+jets sample –Isolated e (P T >20GeV/c, |  |<1.1) –Isolated  (P T >20GeV/c, |  |<2.0) –Missing E T >20 GeV –Exactly four jets P T >20GeV/c, |  |<2.5 (jet energies corrected to particle level) Use “low-bias” discriminant to fit sample composition –Used for ensemble testing and normalization of the background probability. –Final fraction of ttbar events is fit together with mass e+jets  +jets # of events7080 Signal fraction45±12%29±10%

18 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL18 Calibration on Full MC lepton+jets

19 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL19 calibrated expected: 36.4% DØ RunII Preliminary M t =169.5±4.4 GeV/c 2 JES=1.034±0.034

20 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL20 Systematics summary Source of uncertaintyEffect on top mass (GeV/c 2 ) B-jet energy scale Signal modeling (gluons rad)0.34 Background modeling0.32 Signal fraction QCD contribution0.67 MC calibration0.38 trigger0.08 PDF’s0.07 Total

21 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL21 B-jet energy scale ● Relative data/MC b/light jet energy scale ratio fragmentation: GeV/c 2  different amounts of  0, different  + momentum spectrum  fragmentation uncertainties lead to uncertainty in b/light JES ratio compare MC samples with different fragmentation models: Peterson fragmentation with e b = Bowler fragmentation with r t =0.69 calorimeter response: GeV/c 2 uncertainties in the h/e response ratio + charged hadron energy fraction of b jets > that of light jets  corresponding uncertainty in the b/light JES ratio Difference in p T spectrum of b-jets and jets from W-decay: 0.7 GeV/c 2

22 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL22 Gluon radiation The effect is reduced by –Requiring four and only four jets in the final state –High P T cut on jets Yet in ~20% of the events there is at least one jet that is not matched (DR(parton-jet)<0.5) to top decay products –These events are interpreted as background by ME method We study this systematic by examining ALPGEN ttj sample and varying its relative fraction between 0 and 30% (verified on our data by examining the fraction of events with the 5 th jet) Final effect on top mass 0.34 GeV/c 2

23 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL23 ● W+jets modeling: GeV/c 2 study effect of a different factorization scale for W+jets events ( 2 instead of m W 2 + S p T,j 2 ) ● PDF uncertainty: GeV/c 2 CTEQ6M provides systematic variations of the PDFs reweight ensembles to compare CTEQ6M with its systematic variations (by default the measurement uses CTEQ5L throughout: use a LO matrix element, and for consistency with simulation) Signal/Background Modeling ● QCD background: GeV/c 2 Rederive calibration including QCD events from data (lepton anti-isolation) (note: sample statistics limited) can be reduced in the future

24 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL24 ● Signal fraction : GeV/c 2 Fitted top mass depends slightly on true signal fraction (if signal fraction is smaller than expected): => Vary signal fraction within uncertainties from topological likelihood fit - Note: f top fit yields identical result with factor √2 smaller uncertainties Signal fraction Cross check on data: cut on log10(pbkg)<-13 Ftop=31%  46±6% Mtop=170.2±4.1 GeV/c 2

25 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL25 Systematics summary Source of uncertaintyEffect on top mass (GeV/c 2 ) B-jet energy scale Signal modeling (gluons rad)0.34 Background modeling0.32 Signal fraction QCD contribution0.67 MC calibration0.38 trigger0.08 PDF’s0.07 Total

26 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL26 Result and cross checks Run II top quark mass based on lepton+jets sample: M t =169.5 ±4.4(stat+JES) (syst) GeV/c 2 JES contribution to (stat+JES) 3.3 GeV/c 2 Break down by lepton flavor –M t (e+jets)=168.8 ±6.0(stat+JES) GeV/c 2 –M t (  +jets)=172.3 ±9.6(stat+JES)GeV/c 2 Cross check W-mass

27 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL27 Summary of DØ M t measurements Statistical uncertainties are partially correlated for all l+jets Run II results DØ Run II preliminary

28 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL28 Projection for uncertainty on top quark mass Assumptions: only lepton+jets channel considered statistical uncertainty normalized at L=318 pb -1 to performance of current analyses. dominant JES systematic is handled ONLY via in-situ calibration making use of MW in ttbar events. remaining systematic uncertainties: include b-JES, signal and background modeling, etc (fully correlated between experiments) Normalized to 1.7 GeV at L=318 pb-1. Since most of these systematic uncertainties are of theoretical nature, assume that we can use the large data sets to constrain some of the model parameters and ultimately reduce it to 1 GeV after 8 fb -1.

29 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL29 Combination of Tevatron results JES is treated as a part of systematic uncertainty, taken out of stat error

30 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL30Combination M t =172.7±2.9 GeV/c 2 Stat uncertainty: 1.7GeV/c 2 Syst uncertainty: 2.4GeV/c 2 hep-ex/ Top quark Yukawa coupling to Higgs boson g t =M t √2/vev=0.993±0.017

31 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL31 What does it do to Higgs? M H = GeV/c 2 M H <186 GeV/c M H,GeV/c 2 M t,GeV/c 2 M W,GeV/c 2 68% CL

32 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL32

33 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL33 ttbar cross section in l+jets with b-tag Isolated lepton –p T >20 GeV/c, |  e |<1.1, |   |<2.0 Missing E T >20GeV Four or more jets –p T >15 GeV/c, |   = (stat+syst)±0.5(lumi) pb DØ RunII Preliminary, 363pb -1 ≥4j, 1t≥4j, 2t Expect bkg21.8±3.01.9±0.5 Observe8821

34 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL34 Cross section summary DØ RunII Preliminary Submitted for publication Updates

35 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL35 ttbar resonances in l+jets with b-tag Check ttbar invariant mass for possible resonance production DØ RunII Preliminary, 363pb -1 Events are kinematically constrained –m T =175GeV/c 2 –Leptonic and hadronic W masses  NNLO  tt)=6.77±0.42

36 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL36 ttbar resonances in l+jets with b-tag Limit M(Z’)>680 GeV/c 2 with  =1.2%M Z’ at 95%CL *R. Harris, C. Hill, S. Parke hep-ph/ DØ RunII Preliminary, 363pb -1 * Run I limit 560 GeV/c 2 Run II limit 680 GeV/c 2

37 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL37Conclusion First DØ RunII top mass measurement in l+jets channel to surpass Run I precision –M t =169.5 ±4.4(stat+JES) (syst) GeV/c 2 Developed method for in situ jet energy scale calibration using hadronic W-mass constraint Combined Tevatron top mass measurement reaches a precision of 1.7% ttbar production cross sections updated for l+jets channel Invariant mass of ttbar system probed for resonance production, exclusion limit for M(Z’)>680 GeV/c 2 at 95%CL

38 Backup slides

39 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL39 Parton Level Tests Text

40 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL40 L+jets sample composition

41 07/22/05Regina Demina, Joint Theoretical and Experimental Seminar at FNAL41 Kinematics in l+jets sample DØ RunII Preliminary, 363pb -1


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