Julio 2009Jesús Vizán1 Jesús Vizán (on behalf of CMS collaboration) Física del quark top en CMS 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 o Detailed example  Top rediscovery and σ in tt semileptonic channel  Other top-quark properties and signatures  Single top (t-channel), τ channels  Mass, spin correlations, rare decays, tt resonnances  Top as a detector calibration tool  B-tagging  Jet Energy Scale  Summary Julio 2009Jesús Vizán2

What makes top-quark special? In SM, m top and m W constrain Higgs mass  Top quark mass is a fundamental parameter of the EW theory. In SM, m top and m W constrain Higgs mass “naked” quark  Large mass and short life time makes top unique. It decays before fragmenting  observe “naked” quark searches beyond the SM  Top quark in searches beyond the SM at CMS  A decay product of new particles thanks to higher  s  Major background to many searches benchmark sample in detector commissioning  Due 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 period  understanding of most physics objects required  jet energy scale determination  measurements of performance of b-tagging Julio 2009Jesús Vizán3

From Tevatron to LHC Julio 2009Jesús Vizán4  Discovered at Tevatron (1995)  We know much about top already from CDF and D0  Mass, spin, QCD coupling, EW coupling, constraints on its mixing helicity in decays.  Except for mass, precision for most of the measurements is statistically limited  LHC 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

In this talk 10 TeV collissions and focused on low luminosity  Special emphasis on latest results approved by CMS: obtained considering 10 TeV collissions and focused on low luminosity  More 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 etc  Participation of Spanish groups (U. Oviedo) 7 TeV collisions  Early analysis are being repeated now considering 7 TeV collisions. Not results approved yet but first comparisons and preliminary analysis ready. Julio 2009Jesús Vizán5

Top Rediscovery at CMS top pair production cross section is one of early physics goals  Measurement of top pair production cross section is one of early physics goals:  Test the theoretical predictions at the LHC energy.  During the commissioning phase, the top quark signal will play an important role in understanding the detector performance leptonic W decay(s) without using b-tagging information and even missing E T  Extensive 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 E T  lepton + jets: reconstruct top from 3-jet combination with highest vector sum p T. Further enhance the signal by finding one of the dijet combinations with mass closer to m W  dilepton : simple counting experiment o 3 independent analysis merged o 6 institution: 3 USA, 3 Europe (including U. Oviedo); 27people o 2006 (CMS PTDR-2): 2 institution (U. Oviedo & Aachen) Julio 2009Jesús Vizán6 CMS PAS TOP

Julio 2009Jesús Vizán7 tt dilepton channel: signature  Relatively clean final state  It represents a small fraction of tt sample  Signature  Two opposite signed isolated high P T leptons  µ + /µ - (1/81) Less fakes  e/µ (2/81) Clearest Signal  e + /e - (1/81) Completeness  Events with leptonic tau decays also considered as signal(1/45)  High Missing E T coming from the two neutrinos  Two b-tagged high E T jets  Advantages  2 charged leptons oGood energy resolution oReduce backgrounds  Fewer jets oReduce dependence on JES  Disadvantages  2 neutrinos oLoss of information  No hadronic W oCan’t do in situ calibration of JES

tt dilepton channel: Event Selection  Triggers  Used triggers depends final state o µµ : Single Muon trigger (9 GeV) o ee: Single Electron (15 GeV) o eµ: OR of previous ~95%  Efficiency per lepton ~95% ~99%  Efficiency per dilepton ~99%  Leptons P T > 20 GeV, |η| 20 GeV, |η|<2.4 Separate tracker and calorimeter cuts  Isolation: Separate tracker and calorimeter cuts  Electrons faking muons: ΔR(e, µ’s) > 0.1 |M-91| < 15 GeV  DY removal ee, µµ: |M-91| < 15 GeV Julio 2009 Jesús Vizán 8 CMS PAS TOP  Jets E T >30GeV, |η| 30GeV, |η|<2.4  Njet = 0,1 cross-check sub-sample  MET > 30 GeVε~86%~70% DY  ee, µµ > 30 GeV : ε~86% ;reject ~70% DY > 20 GeVε~93% ~50% QCD  eµ > 20 GeV : ε~93% ;reject ~50% QCD

tt dilepton channel: expected event yield  36 (e µ) + 25 (ee, µµ) signal events  eµ cleanest final state  DY main background in ee, µµ  15% stat uncertainty  Fake leptons most visible in Njet=0,1  DY and fake leptons to be estimated using data-driven methods Julio 2009 Jesús Vizán 9  Ldt = TeV

tt dilepton channel: data-driven methods  Estimate DY contribution N out (est) DY = N in DY DATA R MC out/in  Event count near Z-peak |M-91|<15GeV in data (N in ) used to estimate what’s (outside) passing the Z-veto (N out ): N out (est) DY = N in DY DATA R MC out/in R MC out/in =N out DY MC /N in DYMC  Use DY MC to predict R MC out/in =N out DY MC /N in DYMC  30% systematics  30% systematics: from variations in R MC out/in wrt MET, generators, conditions  Fake leptons  Use fake-dominated events with loose leptons failing full cuts  Main variable ratio of fake leptons after full cuts wrt looser cuts o FR= N(fakes | pass full cuts)/ N(fakes | pass loose ID&iso cuts)  Main test: use FR from QCD and apply to Wjets events (with MC truth match to real lepton) and compare to observed count in Wjets o Agreement within 15%, precision limited by MC statistics  50% systematics  50% systematics from FR, signal leptons not passing full cuts but passing loose cuts in fake-dominated sample, double fakes Julio 2009 Jesús Vizán 10

Dileptons: Systematics  Lepton ID and isolation to be obtained from tag and probe  JES: estimated from scaling all jets by 10% up/down  Theory: comparison with Pythia and  Residual backgrounds (tW, part of VV, DY→ττ) assigned 50% syst  Luminosity normalization uncertainty is treated separately Julio 2009 Jesús Vizán 11 Δσ/σ (10 pb -1 )=15%(stat) ± 10% (syst) ±10%(lumi)

tt semileptonic (e+jets)  1 isolated electron  1 isolated electron: p T  30 GeV, |  |  2.5  reject events containing  ’s  4 jets   4 jets with p T  30 GeV, |  |<2.4  Loose electron veto to reduce Z+jets  tightening to barrel-region of |  |<1.442 to reduce fake electrons from QCD  No b-tagging or MET  No b-tagging or MET cut Julio 2009Jesús Vizán12  Ldt = TeV CMS PAS TOP signal 172  1 Bgd 108  10.3 W+Jets 57  2 Z+Jets 12  1 QCD 31  10 Single Top 8  0  To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3 M3: invariant mass of 3-jet combination giving highest vector sum of jet pT’s Δσ/σ=23%(stat) ± 20% (syst) ±10%(lumi)

tt semileptonic ( µ +jets) 1 isolated   select exactly 1 isolated  : p T  20 GeV, |  |  2.1  veto events with  1  to reduce contamination from ttbar , Z+jets and diboson events  reject events with an isolated electron with p T >30 GeV  4 jets   4 jets with p T  30 GeV, |  |<2.4 No b-tagging and cut  No b-tagging and cut on MET Julio 2009Jesús Vizán13  Ldt = TeV CMS PAS TOP  To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3 M3: invariant mass of 3-jet combination giving highest vector sum of jet pT’s Δσ/σ=20%(stat) ± 25% (syst) ±10%(lumi) signal320 Bgd171 W+Jets140 Z+Jets10 QCD7 Single Top14

Other signatures(t-channel)  Single top (t-channel)  template-fit method: takes advantage of spin correlations of decay products Julio 2009Jesús Vizán14 Cos angle( µ, untagged jet) (top rest frame) CMS PAS TeV  Channels with hadronic τ (tt dileptons)  S/B up to ~0.4  S/B up to ~0.4 (1-prong τ ) using sequential cut procedure 14 TeV) CMS PAS TeV Δσ/σ (200 pb -1 )=35%(stat) ± 14%(syst) ± 10% (lumi)

Measure top-quark properties  Large mass, large width → unique to top quark properties: tests of the V-A structure of top decays; top spin; |Vtb|; couplings  Top Quark Mass o Measurement in the main decay modes (dilepton. semilepton) competitive wrt Tevatron (~ few GeV) o Need good understanding of systematic uncertainties  Spin Correlations in tt decays accesible via an asymmetry measurement using semileptonic W decays ψ=angle(lepton/W (restframe),Wtop (restframe) ) distr. o Semileptonic W decay → ψ=angle(lepton/W (restframe),Wtop (restframe) ) distr. o ΔA ~ ± 0.05 o ΔA ~ ± 0.05 (dominated systematyc errors)  Measure R = |Vtb| 2 ~10% o Contrary to Tevatron case measurement dominated by systematic uncertainties (~10%) (250 pb -1 ) o Main uncertainty due to b-tagging Julio 2009Jesús Vizán15 (10 fb TeV P-TDR2) CMS PAS TOP

Anomalous top production and rare top decays  Large Yukawa coupling (~1) => Significant potential to discover new physics (top resonances, Z’,Kaluza-Klein modes, Susy)  FCNC rare decays (t->(Z,γ,g)q) can be investigated 10 fb (PTDR-2)  Smallest 5σ observation BR(t →Zq ) = 14.9 x fb (PTDR-2) 10 fb (PTDR-2)  Smallest 5σ observation BR(t →γq ) = 8.4 x fb (PTDR-2)  Resonance Z’ → tt → l νqqbb Julio 2009Jesús Vizán16 Expected limits on the σ Z’ × Br(Z’ → tt) at 95% C.L. (100pb CMS PAS TOP

Top as a calibration tool: b-tagging  tt events used to isolate a highly enriched b-jet sample  Exploit it to calibrate jet algorithm and extract b-tagging effficiency ε b for energetic jets ε b =[F tag - ε 0 (1-P b )]/P b  From an enriched sample (topological/kinematicselection), ε b =[F tag - ε 0 (1-P b )]/P b, F tag = measured fraction of jet tagged; P b = b-purity and ε 0 =mistag rate from simulation  Get ε b versus E T and η of the jet Julio 2009Jesús Vizán17 Δε b /ε b (1 fb -1 )= 6 (10)% for barrel (endcap)  Main systematic ISR/FSR, event selection and purity

Top as a calibration tool: JES  Selection of tt → µν bjjb final states and identification of hadronic top system  Use of b-tagging Julio 2009Jesús Vizán18 residual JES corrections for light and heavy quarks  Obtain residual JES corrections for light and heavy quarks using world average values for M top and M W by means of least square kinematic fit. 1%  ~1% on b-JES and light-JES with 100 pb -1 CMS PAS TeV

Summary test the standard model, search for new physics and calibrate detector performance  Top events are essential in CMS. Important role to test the standard model, search for new physics and calibrate detector performance  Top events rediscovery already possible at pretty low luminosity (robust methods). For 10 TeV collisions:  Δσ/σ (10 pb -1 )=15%(stat) ± 10% (syst) ±10%(lumi) TeV)  Δσ/σ=23%(stat) ± 20% (syst) ± 10%(lumi) (semi TeV)  Δσ/σ=20%(stat) ± 25% (syst) ± 10%(lumi) (semi TeV)  Other challenging top signatures will be detected:  Δσ/σ (200 pb-1)=35%(stat) ± 14%(syst) ± 10% (lumi) (Single top TeV)  Precision measurements of top-quark properties  Limits on the σZ’ × Br(Z’→ tt) at 95% CL<15 pb in the range [0.75,2] TeV (100 pb TeV)  Spin correlations ΔA ~ ±0.05 (high lumi: 10 fb 14 TeV)  Top events will be used to calibrate detector performance  M top and M w constrains to calibrate residual JES. Uncertainties as low as 1%  Estimate b-tagging efficiency [6-10]% uncertainty (1 fb -1 ) Julio 2009Jesús Vizán19

Julio 2009Jesús Vizán20 BACK-UP SLIDES

Julio 2009Jesús Vizán21 Top Quark Production  Single top ProcessTevatronLHC tt pair7 pb833 pb t-channel1.98 pb246 pb s-channel0.88 pb11 pb Wt channel0.25 pb66 pb  In high energy proton colliders top quark is mainly produced in tt pairs:  At LHC ogg ~ 90% oqq ~ 10%  At Tevatron ogg ~ 15% oqq ~ 85%  Important increment for top-pair  at LHC  Also for t-channel and W+t channel (not yet evidence at Tevatron) Wt-channels-channel t-channel

Julio 2009Jesús Vizán22 Top Quark Physiscs: Decay  Final state  Energetic Jets  b-jets  Leptons  Missing E T  All subdetectors in play  Vital tool to validate detector performance  SM: Almost 100% to Wb  tt pair classification depending on W’s decay  Fully hadronic  Semileptonic  Dilepton channel tt semileptonic channel