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Jesús Vizán (on behalf of CMS collaboration)

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

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Jesús Vizán (on behalf of CMS collaboration)

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  1. Física del quark top en CMS Jesús Vizán (on behalf of CMS collaboration) Universidad de Oviedo Departamento de Física Jesús Vizán

  2. Outline. • Introduction to Top Quark Physics at LHC • First measurements at CMS • Top rediscovery and σ in tt dilepton channel • Detailed example • Top rediscovery and σ in tt semileptonic channel • Other top-quark properties and signatures • Single top (t-channel) • Spin correlations • Rare decays • Top as a detector calibration tool • B-tagging • Jet Energy Scale • Summary Jesús Vizán

  3. What makes top-quark special? • Top quark mass is a fundamental parameter of the EW theory. In SM, mtopand mW constrain Higgs mass • Large mass and short life time makes top unique. It decays before fragmenting  observe “naked” quark • Top quark in searches beyond the SM at LHC • A decay product of new particles thanks to higher s • Major background to many searches • 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 and lepton ID tools Jesús Vizán

  4. From Tevatron to LHC • 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 Jesús Vizán

  5. In this talk • Special emphasis put on latest results approved by CMS. Obtained considering 10 TeVcollissions and focused on low luminosity • More detailed description of the top-antitopdilepton cross-section measurement. • Example of the kind of objects used for low luminosity, data-driven methods considered, study of systematic uncertainties etc • Participation 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. Jesús Vizán

  6. Top Rediscovery at CMS • 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 • 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 ET • dilepton : simple counting experiment • 3 independent analysis merged • 6 institution: 3 USA, 3 Europe (including U. Oviedo); 27people • 2004 (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 mW CMS PAS TOP-09-002 Jesús Vizán

  7. ttdilepton channel: signature • Relatively clean final state • It represents a small fraction of tt sample • Signature • Two opposite signed isolated high PT 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 ET coming from the two neutrinos • Two b-tagged high ET jets • Advantages • 2 charged leptons • Good energy resolution • Reduce backgrounds • Fewer jets • Reduce dependence on JES • Disadvantages • 2 neutrinos • Loss of information • No hadronic W • Can’t do in situ calibration of JES Jesús Vizán

  8. ttdilepton channel: Event Selection • Jets • 2 (SIScone) jets with ET>30GeV, |η|<2.4 • Njet = 0,1 cross-check sub-sample • MET • ee, µµ > 30 GeV: ε~86%; reject ~70% DY • eµ > 20 GeV: ε~93%; reject ~50% QCD CMS PAS TOP-09-002 • Triggers • Used triggers depends final state • µµ : Single Muon trigger (9 GeV) • ee: Single Electron (15 GeV) • eµ: OR of previous • Efficiency per lepton ~95% • Efficiency per dilepton~99% • Leptons • At least two isol. leptons PT> 20 GeV, |η|<2.4 • Isolation: Separate tracker and calorimeter cuts • Electrons faking muons: ΔR(e, µ’s) > 0.1 • DY removal ee, µµ: |M-91| < 15 GeV Jesús Vizán

  9. ttdilepton channel: expected event yield Ldt = 10 pb-1 @ 10 TeV • 36 (eµ) + 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 Jesús Vizán

  10. ttdilepton channel: data-driven methods • Estimate DY contribution • Event count near Z-peak |M-91|<15GeV in data (Nin) used to estimate what’s (outside) passing the Z-veto (Nout) • Use DY MC to predict • 30%systematics: from variations in Rout/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 • FR=N(fakes | pass full cuts)/ N(fakes | pass loose ID&iso cuts) • Main variable ratio of fake leptons after full cuts wrt looser 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 • Agreement within 15%, precision limited by MC statistics • 50%systematics from FR, signal leptons not passing full cuts but passing loose cuts in fake-dominated sample, double fakes Jesús Vizán

  11. 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 MC@NLO • Residual backgrounds (tW, part of VV, DY→ττ) assigned 50% syst • Luminosity normalization uncertainty is treated separately Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi) Jesús Vizán

  12. ttsemileptonic (e+jets) CMS PAS TOP-09-004 • To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3 • 1 isolated electron: pT30 GeV, ||2.5 • reject events containing ’s • 4 jets with pT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 cut M3: invariant mass of 3-jet combination giving highest vector sum of jet pT’s Ldt = 20 pb-1 @ 10 TeV Δσ/σ=23%(stat) ± 20% (syst) ±10%(lumi) Jesús Vizán

  13. ttsemileptonic (µ+jets) CMS PAS TOP-09-003 • To estimate background, employ template fit method which relies on a discriminating variable that has different shape in signal and background: M3 • select exactly 1 isolated : pT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 pT>30 GeV • 4 jets with pT30 GeV, ||<2.4 • No b-tagging and cut on MET M3: invariant mass of 3-jet combination giving highest vector sum of jet pT’s Ldt = 20 pb-1 @ 10 TeV Δσ/σ=20%(stat) ± 25% (syst) ±10%(lumi) Jesús Vizán

  14. Single top (t-channel) CMS PAS TOP-09-005 • σ(t)~300pb~1/3 σ(tt) important back • template-fit method is proposed, that takes advantage of the spin correlations of the decay products in signal events Cosine of the angle between charged muon and untagged jet, in the reconstructed top rest frame after the full event selection Δσ/σ (200 pb-1)=35%(stat) ± 14% (syst) ± 10% (lumi) Jesús Vizán

  15. Top properties • Large mass, large width => unique to top quark properties:tests of the V-A structure of top decays; top spin; |Vtb|; charge; couplings; rare decays • SemileptonicW decay -> distribution of ψ= angle(lepton/W-restframe, Wtop-restframe) • Top spin correlations in tt decays: accessible via an asymmetry measurement: Precision ~O(20%) (10 fb-1) Jesús Vizán

  16. 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 • Resonance Z’→ tt→ lνqqbb @ 10 TeV Expected limits on the σZ’ × Br(Z’→tt) at 95% C.L. for an integrated luminosity of 100pb-1 with and wihtout systematic uncertainties Jesús Vizán

  17. 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 εb for energetic jet • From an enriched sample (topological/kinematicselection) , εb= (Ftag- εb(1-Pb))/Pb , Ftag= measured fraction of jet tagged, Pb= b purity • Get εb versus ET and ηof the jet Δεb /εb (1 fb-1)= 6 (10)% for barrel (endcap) • Main systematic ISR/FSR, event selection and purity Jesús Vizán

  18. Top as a calibration tool: JES • Selection of tt-> lνbjjbfinal states and identification of hadronictop system {jjb} • Use of the B-tagging • Rescale each jet with relative shifts {ΔE(light-jet),ΔE(b-jet)}, remake/refit W had. Mass and had. top (bW) mass spectra, solve the equation M(top,W;{ΔE})=M(top,W)PDG-> best estimate of {ΔE} • ~1%on b-JES and light-JES with 100pb-1 Jesús Vizán

  19. Summary • 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. For 10 TeV collisions: • Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi) (dilepton) • Δσ/σ=23%(stat) ± 20% (syst) ± 10%(lumi) (semi e) • Δσ/σ=20%(stat) ± 25% (syst) ± 10%(lumi) (semi µ) • CMS will improve many measurements of top-quark properties • Limits on the σZ’ × Br(Z’→ tt) at 95% C.L <15 pb in the range [0,75,2] TeV Jesús Vizán

  20. BACK-UP SLIDES Jesús Vizán

  21. Top Quark Production • In high energy proton colliders top quark is mainly produced in tt pairs: • Single top • At LHC • gg ~ 90% • qq ~ 10% • At Tevatron • gg ~ 15% • qq ~ 85% Wt-channel s-channel t-channel • Important increment for top-pair  at LHC • Also for t-channel and W+t channel (not yet evidence at Tevatron) Jesús Vizán

  22. Top Quark Physiscs: Decay • Final state • Energetic Jets • b-jets • Leptons • Missing ET • 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 semileptonicchannel Jesús Vizán

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