Jesús Vizán (on behalf of CMS collaboration)

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Física del quark top en CMS. Jesús Vizán (on behalf of CMS collaboration). Universidad de Oviedo Departamento de Física. Outline. Introduction to Top Quark Physics at LHC First measurements at CMS Top rediscovery and σ in tt dilepton channel Detailed example - PowerPoint PPT Presentation

Transcript of Jesús Vizán (on behalf of CMS collaboration)

  • Julio 2009Jess Vizn

    *Jess Vizn (on behalf of CMS collaboration)Fsica del quark top en CMSUniversidad de OviedoDepartamento de Fsica

    Jess Vizn

  • Outline.Introduction to Top Quark Physics at LHCFirst measurements at CMSTop rediscovery and in tt dilepton channelDetailed exampleTop rediscovery and in tt semileptonic channelOther top-quark properties and signaturesSingle top (t-channel), channelsMass, spin correlations, rare decays, tt resonnancesTop as a detector calibration toolB-taggingJet Energy ScaleSummary

    Julio 2009Jess Vizn

    *

    Jess Vizn

  • What makes top-quark special?

    Top quark mass is a fundamental parameter of the EW theory. In SM, mtop and mW constrain Higgs massLarge mass and short life time makes top unique. It decays before fragmenting observe naked quarkTop quark in searches beyond the SM at CMSA decay product of new particles thanks to higher sMajor background to many searchesDue to distinct experimental signatures and final state topologies, tt events will also constitute one of the main benchmark sample in detector commissioning, useful from the very early data taking periodunderstanding of most physics objects requiredjet energy scale determinationmeasurements of performance of b-taggingJulio 2009Jess Vizn

    *

    Jess Vizn

  • From Tevatron to LHC

    Julio 2009Jess Vizn

    *Discovered at Tevatron (1995)We know much about top already from CDF and D0Mass, spin, QCD coupling, EW coupling, constraints on its mixing helicity in decays.Except for mass, precision for most of the measurements is statistically limitedLHC opens up new era of precision measurements in the Top quark sector: ~8M top pairs & ~2M single top events/year expected at the low luminosity at s=14 TeV

    Jess Vizn

  • In this talk

    Special emphasis on latest results approved by CMS: obtained considering 10 TeV collissions and focused on low luminosityMore detailed description of the top-antitop dilepton cross-section measurement.Example of the kind of objects used for low luminosity analyses, data-driven methods considered, study of systematic uncertainties etcParticipation of Spanish groups (U. Oviedo)Early analysis are being repeated now considering 7 TeV collisions. Not results approved yet but first comparisons and preliminary analysis ready.Julio 2009Jess Vizn

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    Jess Vizn

  • Top Rediscovery at CMSMeasurement of top pair production cross section is one of early physics goals:Test the theoretical predictions at the LHC energy. Top quark signalDuring the commissioning phase, the top quark signal will play an important role in understanding the detector performanceExtensive and robust analyses to extract the top signal. In the beginning, focus on channels with leptonic W decay(s) without using b-tagging information and even missing ETdilepton : simple counting experiment3 independent analysis merged6 institution: 3 USA, 3 Europe (including U. Oviedo); 27people2004 (CMS PTDR-2): 2 institution (U. Oviedo & Aachen)lepton + jets: reconstruct top from 3-jet combination with highest vector sum pT. Further enhance the signal by finding one of the dijet combinations with mass closer to mWJulio 2009Jess Vizn

    *CMS PAS TOP-09-002

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  • Julio 2009Jess Vizn

    *tt dilepton channel: signatureRelatively clean final stateIt represents a small fraction of tt sampleSignatureTwo opposite signed isolated high PT leptons+/- (1/81) Less fakese/ (2/81) Clearest Signale+/e- (1/81) CompletenessEvents with leptonic tau decays also considered as signal(1/45)High Missing ET coming from the two neutrinosTwo b-tagged high ET jets

    Advantages2 charged leptonsGood energy resolutionReduce backgroundsFewer jetsReduce dependence on JESDisadvantages2 neutrinosLoss of informationNo hadronic WCant do in situ calibration of JES

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  • tt dilepton channel: Event SelectionTriggersUsed triggers depends final state : Single Muon trigger (9 GeV)ee: Single Electron (15 GeV)e: OR of previousEfficiency per lepton ~95%Efficiency per dilepton ~99%LeptonsAt least two isol. leptons PT> 20 GeV, || 0.1DY removal ee, : |M-91| < 15 GeVJulio 2009Jess Vizn

    *CMS PAS TOP-09-002Jets2 (SIScone) jets with ET>30GeV, || 30 GeV: ~86%;reject ~70% DYe > 20 GeV: ~93% ;reject ~50% QCD

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  • tt dilepton channel: expected event yield36 (e) + 25 (ee, ) signal eventse cleanest final stateDY main background in ee, 15% stat uncertaintyFake leptons most visible in Njet=0,1DY and fake leptons to be estimated using data-driven methodsJulio 2009Jess Vizn

    *Ldt = 10 pb-1 @ 10 TeV

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  • tt dilepton channel: data-driven methodsEstimate DY contributionEvent count near Z-peak |M-91|
  • Dileptons: SystematicsLepton ID and isolation to be obtained from tag and probeJES: estimated from scaling all jets by 10% up/downTheory: comparison with Pythia and [email protected] backgrounds (tW, part of VV, DY) assigned 50% systLuminosity normalization uncertainty is treated separatelyJulio 2009Jess Vizn

    */ (10 pb-1)=15%(stat) 10% (syst) 10%(lumi)

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  • tt semileptonic (e+jets)1 isolated electron: pT30 GeV, ||2.5 reject events containing s 4 jets with pT30 GeV, ||
  • tt semileptonic (+jets)select exactly 1 isolated : pT20 GeV, ||2.1 veto events with 1 to reduce contamination from ttbar, Z+jets and diboson events reject events with an isolated electron with pT>30 GeV 4 jets with pT30 GeV, ||
  • Other signatures(t-channel)Single top (t-channel)template-fit method: takes advantage of spin correlations of decay products

    Julio 2009Jess Vizn

    *Cos angle(, untagged jet) (top rest frame)CMS PAS TOP-09-005 @10 TeVChannels with hadronic (tt dileptons)S/B up to ~0.4 (1-prong ) using sequential cut procedure (@ 14 TeV)

    CMS PAS TOP-08-004 @14 TeV/ (200 pb-1)=35%(stat) 14%(syst) 10% (lumi)

    Jess Vizn

  • Measure top-quark propertiesLarge mass, large width unique to top quark properties: tests of the V-A structure of top decays; top spin; |Vtb|; couplingsTop Quark MassMeasurement in the main decay modes (dilepton. semilepton) competitive wrt Tevatron (~ few GeV)Need good understanding of systematic uncertainties Spin Correlations in tt decays accesible via an asymmetry measurement using semileptonic W decaysSemileptonic W decay =angle(lepton/W(restframe),Wtop(restframe)) distr.A ~ 0.05 (dominated systematyc errors)Measure R = |Vtb|2Contrary to Tevatron case measurement dominated by systematic uncertainties (~10%) (250 pb-1)Main uncertainty due to b-taggingJulio 2009Jess Vizn

    *(10 fb-1 @14 TeV P-TDR2)CMS PAS TOP-09-001

    Jess Vizn

  • Anomalous top production and rare top decaysLarge Yukawa coupling (~1) => Significant potential to discover new physics (top resonances, Z,Kaluza-Klein modes, Susy)FCNC rare decays (t->(Z,,g)q) can be investigatedSmallest 5 observation BR(tZq) = 14.9 x10-4 10 fb-1 @14TeV (PTDR-2)Smallest 5 observation BR(tq) = 8.4 x10-4 10 fb-1 @14TeV (PTDR-2)Resonance Z tt lqqbb

    Julio 2009Jess Vizn

    *Expected limits on the Z Br(Z tt) at 95% C.L. (100pb-1 @10TeV)CMS PAS TOP-09-009

    Jess Vizn

  • Top as a calibration tool: b-taggingtt events used to isolate a highly enriched b-jet sampleExploit it to calibrate jet algorithm and extract b-tagging effficiency b for energFrom an enriched sample (topological/kinematicselection) , b= (Ftag- b(1-Pb))/Pb , Ftag= measured fraction of jet tagged, Pb= b purityGet b versus ET and of the jet

    Julio 2009Jess Vizn

    *b /b (1 fb-1)= 6 (10)% for barrel (endcap)Main systematic ISR/FSR, event selection and purity

    Jess Vizn

  • Top as a calibration tool: JESSelection of tt bjjb final states and identification of hadronic top systemUse of b-taggingJulio 2009Jess Vizn

    *Obtain residual JES corrections for light and heavy quarks using world average values for Mtop and MW by means of least square kinematic fit.~1% on b-JES and light-JES with 100pb-1

    CMS PAS TOP-07-004 @14 TeV

    Jess Vizn

  • SummaryTop events are essential in CMS. Important role to test the standard model, search for new physics and calibrate detector performanceTop events rediscovery already possible at pretty low luminosity (robust methods). For 10 TeV collisions:/ (10 pb-1)=15%(stat) 10% (syst) 10%(lumi) (dilepton @10 TeV)/=23%(stat) 20% (syst) 10%(lumi) (semi e @10 TeV)/=20%(stat) 25% (syst) 10%(lumi) (semi @10 TeV)Other challenging top signatures will be detected:/ (200 pb-1)=35%(stat) 14%(syst) 10% (lumi) (Single top t-channel @10 TeV)Precision measurements of top-quark propertiesLimits on the Z Br(Z tt) at 95% CL
  • Julio 2009Jess Vizn

    *BACK-UP SLIDES

    Jess Vizn

  • Julio 2009Jess Vizn

    *Top Quark ProductionSingle topIn high energy proton colliders top quark is mainly produced in tt pairs:At LHC gg ~ 90%qq ~ 10%At Tevatrongg ~ 15%qq ~ 85%Important increment for top-pair at LHCAlso for t-channel and W+t channel (not yet evidence at Tevatron)Wt-channels-channelt-channel

    ProcessTevatronLHCtt pair7 pb833 pbt-channel1.98 pb246 pbs-channel0.88 pb11 pbWt channel0.25 pb66 pb

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  • Julio 2009Jess Vizn

    *Top Quark Physiscs: DecayFinal stateEnergetic Jetsb-jetsLeptonsMissing ETAll subdetectors in playVital tool to validate detector performanceSM: Almost 100% to Wbtt pair classification depending on Ws decayFully hadronicSemileptonicDilepton channel

    tt semileptonic channel

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    *Niks

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