1 / 22

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

avel
Download Presentation

Jesús Vizán (on behalf of CMS collaboration)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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), τ channels • Mass, spin correlations, rare decays, tt resonnances • 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 CMS • 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 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 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 analyses, 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. Top quark signal • 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): Nout (est)DY = NinDYDATARMCout/in • Use DY MC to predict RMCout/in=NoutDY MC/NinDYMC • 30% systematics: from variations in RMCout/inwrt 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 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% systematicsfrom 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. Other signatures(t-channel) CMS PAS TOP-09-005 @10 TeV • Single top (t-channel) • template-fit method: takes advantage of spin correlations of decay products Cos angle(µ, untagged jet) (top rest frame) Δσ/σ (200 pb-1)=35%(stat) ± 14%(syst) ± 10% (lumi) • Channels with hadronicτ (ttdileptons) • S/B up to ~0.4 (1-prong τ) using sequential cut procedure (@ 14 TeV) CMS PAS TOP-08-004 @14 TeV Jesús Vizán

  15. 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 • Measurement in the main decay modes (dilepton. semilepton) competitive wrtTevatron (~ few GeV) • Need good understanding of systematic uncertainties • Spin Correlations in ttdecays accesible viaanasymmetrymeasurementusingsemileptonic W decays • Semileptonic W decay→ψ=angle(lepton/W(restframe),Wtop(restframe)) distr. • ΔA ~ ± 0.05 (dominated systematyc errors) • Measure R = |Vtb|2 • Contrary to Tevatron case measurement dominated by systematic uncertainties (~10%) (250 pb-1) • Main uncertainty due to b-tagging (10 fb-1 @14 TeV P-TDR2) CMS PAS TOP-09-001 Jesús Vizán

  16. Anomalous top production and rare top decays • Large Yukawa coupling (~1) => Significantpotentialtodiscover new physics (top resonances, Z’,Kaluza-Klein modes, Susy) • FCNC rare decays (t->(Z,γ,g)q) can be investigated • Smallest 5σ observation BR(t→Zq) = 14.9 x10-410 fb-1 @14TeV (PTDR-2) • Smallest 5σ observation BR(t→γq) = 8.4 x10-410 fb-1 @14TeV (PTDR-2) • Resonance Z’→tt→ lνqqbb CMS PAS TOP-09-009 Expected limits on the σZ’ × Br(Z’→tt) at 95% C.L. (100pb-1 @10TeV) 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 energ • 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→µνbjjb final states and identification of hadronic top system • Use of b-tagging CMS PAS TOP-07-004 @14 TeV • 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 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 (robust methods). For 10 TeV collisions: • Δσ/σ (10 pb-1)=15%(stat) ± 10% (syst) ±10%(lumi) (dilepton @10 TeV) • Δσ/σ=23%(stat) ± 20% (syst) ± 10%(lumi) (semi e @10 TeV) • Δσ/σ=20%(stat) ± 25% (syst) ± 10%(lumi) (semi µ @10 TeV) • Other challenging top signatures will be detected: • Δσ/σ (200 pb-1)=35%(stat) ± 14%(syst) ± 10% (lumi) (Single top t-channel @10 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-1 @10 TeV) • Spin correlations ΔA ~ ±0.05 (high lumi: 10 fb-1 @ 14 TeV) • Top events will be used to calibrate detector performance • Mtop and Mw constrains to calibrate residual JES. Uncertainties as low as 1% • Estimate b-tagging efficiency [6-10]% uncertainty (1 fb-1) 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

More Related