1 / 33

Top quark physics at CMS

Top quark physics at CMS. From Tevatron to LHC Reconstruction of the top quark events Precision measurements : m t , s tT , s t , W helicity, spin, ... Discovery potential Standard Model : single-top, couplings, ... Beyond Standard Model : resonances, FCNC, ...

cerise
Download Presentation

Top quark physics at CMS

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. Top quark physics at CMS From Tevatron to LHC Reconstruction of the top quark events Precision measurements : mt, stT, st, W helicity, spin, ... Discovery potential Standard Model : single-top, couplings, ... Beyond Standard Model : resonances, FCNC, ... Calibration sample : jet energy scale, b-tag efficiency, ... Jorgen D’Hondt (Vrije Universiteit Brussel) Roberto Tenchini (Pisa) on behalf of the CMS Collaboration CTEQ workshop, Michigan (US), 14-15 May 2007

  2. From Tevatron/LEP to LHC • Obtaining orders in magnitude in both the integrated luminosity and the energy, we will collect a huge amount of Standard Model benchmarks channels. • ~109 events/10fb-1W (200 per second) • ~108 events/10fb-1Z (50 per second) • ~107 events/10fb-1tt (1 per second) • These can be used as control/calibration samples for searches beyond the Standard Model, but can also be used to scrutinize even further the Standard Model. rates@1033 (10 fb-1 = 1 year of LHC running at low luminosity 1033, hence by end 2009) pile-up becomes an important issue Jorgen D'Hondt (Vrije Universiteit Brussel)

  3. Top Quark Production at the LHC 10 tt pairs per day @ Tevatron  1 tt pair per second @ LHC qq →tt : 85% gg→tt : 87% top pairs t ~87 % p p t • NLO cross-section sNLO = 833 pb  ~8M events/10fb-1 Some references (not a complete list!): (top pairs) N.Nason et al. Nucl.Phys. B303 (1988) 607, S.Catani et al. Nucl.Phys. B478 (1996) 273, M.Beneke et al. hep-ph/0003033, N.Kidonakis and R.Vogt, Phys.Rev. D68 (2003) 114014, W.Bernreuther et al. Nucl.Phys. B690 (2004) 81-137 (single-top) T.Stelzer et al. Phys.Rev. D56 (1997) 5919, M.C.Smith and S.Willenbrock Phys.Rev. D54 (1996) 6696, T.M.Tait Phys.Rev. D61 (2000) 034001 single-top ? single-top @ Tevatron  30 single-tops per minute @ LHC t s-channel associated tW stop & santi-top not equal p p sNLO(total) = 373 pb  ~3.7M events/10fb-1 t-channel X sNLO = 153 pb sNLO = 6.6 pb sNLO = 60 pb  top production sNLO = 90 pb sNLO = 4.1 pb sNLO = 60 pb  anti-top production Jorgen D'Hondt (Vrije Universiteit Brussel)

  4. Topics in top quark physics at the LHC PRODUCTION Cross section Spin-correlations Resonances Xtt Fourth generation t’ New physics (SUSY) Flavour physics (FCNC) DECAY Charged Higgs W helicity Anomalous couplings CKM matrix elements Calibration sample !! PROPERTIES Mass (matter vs. anti-matter) Charge Life-time and width Spin BOTTOM jets b-tagging TOP t≈ 0.4 10-24s W kinematic fit (mW) n missing energy L trigger This data will extend the Tevatron precision reach and allow new possible topics. Jorgen D'Hondt (Vrije Universiteit Brussel)

  5. Current status of Simulation and Reconstruction Still some time before real data… hence all based on Monte Carlo simulation • Main generators used :PYTHIA6.2 or AlpGen (Leading-Order) • does not include many features present in dedicated generators • Next-to-Leading-Order applied where needed (eg. single-top production) • Simulation: studies based on both fast and full simulation • ‘fast’ simulation: generated objects smeared to mimic the detector • ‘full’ simulation: detector responds simulated with GEANT-4 • large Data-Challenge efforts have been made to provide dedicated GEANT-4 simulation (created ~100M simulated events, ~1Mb/event)  pile-up collisions added in low-luminosity settings (2.1033 cm-2s-1) • Reconstruction based on documented advanced tools • Results illustrate what we can learn from CMS data upon arrival • References: talk based on notes/information available on • CMS  http://cms.cern.ch/iCMS(‘full’ simulation results from P-TDR) Jorgen D'Hondt (Vrije Universiteit Brussel)

  6. Top Quark Pair Selection • Fully hadronic channel : 3.7Mevnt/10fb-1 • QCD multijet (2→2 parton processes) → apply also topological cuts • at least 6-jets pT>30-40GeV, b-tags≥2 : S/B~1/9, e~2.7% • Lepton + jets channel (lepton = e/m) : 2.5Mevnt/10fb-1 • W+jets→lv+jets, Z+jets→ll+jets, WW→lv+jets, WZ→lv+jets, ZZ→ll+jets • before selection we have S/B ~ 10-5 • pTlepton>20GeV, kinematic fit, pTjet>30GeV, b-tags≥2 : S/B ~ 27, e~6.3% • Di-lepton channel : 0.4Mevnt/10fb-1 • Drell-Yan processes, Z+jets→ll+jets, WW+jets, bb → Z-mass cuts • pT>20GeV, ETmiss>40GeV, pTjets>20GeV, b-tags=2 : S/B~5.5, e~5% • main backgrounds in lepton+jet and di-lepton channel from ttbar other decays Jorgen D'Hondt (Vrije Universiteit Brussel)

  7. Top Quark Pair Production Cross-Section • Shape analysis (identical á la D0 in Phys.Lett. B626 (2005) 45) • combine the topological/kinematic information of several event observables • not as powerful as at the Tevatron (CMS Note 2006/064 & ATLAS Eur.Phys.J.C39(2005)63) • Simple counting experiment  possible due to high S/N after selection • di-lepton : (e/m) 0.9 (stat) ± 11 (syst) ± 3 (lumi) % (10fb-1) (t) 1.3 (stat) ± 16 (syst) ± 3 (lumi) % (10fb-1) • lepton+jet : (e/m) 0.4 (stat) ± 9.2 (syst) ± 3 (lumi) % (10fb-1) • fully hadronic : 3 (stat) ± 20 (syst) ± 3 (lumi) % (10fb-1) CMS Note 2006/064 PYTHIA (tt) normalized Phys.Lett B626 (2005) 45 AlpGen (W+jets) Jorgen D'Hondt (Vrije Universiteit Brussel)

  8. Top Quark Pair Production Cross-Section • Breakdown of total uncertainty (CMS Note 2006/064) • example in the semi-leptonic muon channel • dominated by uncertainty on b-tagging efficiency which is conservative when assuming 2% (rather than 5%)  total uncertainty ~ 7% (10fb-1) • 2-3 GeV uncertainty on mt is feasible via cross section measurement conservative conservative → Tevatron ~2% Jorgen D'Hondt (Vrije Universiteit Brussel)

  9. Properties: Top Quark Mass Most important parameter is the top quark mass (mt), to be estimated with an accuracy of around Dmt ~ 1 GeV/c2. BOTTOM TOP t≈ 0.4 10-24s W n L • fully hadronic channel: background rejection • semi-leptonic channel: kinematic fits • di-leptonic channel: neutrinos Jorgen D'Hondt (Vrije Universiteit Brussel)

  10. ‘golden channel’ CMS Note 2006/066 • Three top quark mass estimators are investigated: • Gaussian fit on reconstructed mass spectrum → mtSimple • convolution with Gaussian parametrized ideogram→ mtParamIdeo • convolution with full scanned ideogram → mtFullIdeo • A likelihood variable is constructed reflecting the probability for signal → Psign • Jet combinations are ordered according to a likelihood variable → Pcomb • A kinematic fit is applied (CMS Note 2006/023) forcing the W boson mass • The Ideogram is convoluted with a theoretical template • Maximum likelihood gives the estimated top quark mass • 4th estimator: mtFullIdeo but IterCone, MidPoint & kT should give same jet direction  can be fixed in kinematic fit Breit-Wigner Monte Carlo parametrized Jorgen D'Hondt (Vrije Universiteit Brussel)

  11. ‘golden channel’ CMS Note 2006/066 Properties of the estimators : spectrum with kinematic fit Jorgen D'Hondt (Vrije Universiteit Brussel)

  12. ‘golden channel’ Main uncertainy arises from heavy quark jet energy scale. CMS Note 2006/066 (5% On-Off) (1.5%) (2%) • Pile-Up: could be reduced to 10% and overlaps with JES, hence 5% taken of On-Off • JES: could reach 1.5% in well understood detector range (barrel) + energy flow • (to be seen as a benchmark for the jet calibration methods) • B-tagging: 2% could be reached according to Tevatron experience •  1 GeV uncertainty reachable with a good detector understanding Jorgen D'Hondt (Vrije Universiteit Brussel)

  13. Top quark mass: other methods • From t  l + J/y + X decays : • 100 fb-1 gives after selection ~ 1,000 signal events (S/B > 100) • the large mass of the J/y induces a strong correlation with the top mass • easier to identify (extremely clean sample) • BR(overall in tt) ~ 5.3 x 10-5 hep-ph/9912320 CMS Note 2006/058 ‘full’ simulation First ‘full’ simulation study 1GeV @ 20fb-1 Jorgen D'Hondt (Vrije Universiteit Brussel)

  14. Top quark mass: other methods CMS Note 2006/058 • no jet related systematics • 2 GeV uncertainty after 2 years of LHC running • conservative estimate as most of the systematic uncertainties are related to the theoretical modeling of the events • after a better understanding of these phenomena a 1 GeV uncertainty is feasible THEORETICAL EXPERIMENTAL Jorgen D'Hondt (Vrije Universiteit Brussel)

  15. Top quark mass: combination !? CMS notes [1] ATLAS Eur.Phys.J. C39 (2005) 63-90 [2] LHC Yellow Report on Standard Model Physics [3] Stat.Unc. MeV Syst.Unc. MeV • Single-lepton channel (Full-Analysis) [1,2]100 ~1000 • High pT single-lepton sample (Jet-Analysis) [3] 250 ~1000 • High pT single-lepton sample (Cluster-Analysis) [3] 150 ~1500 • Single-lepton channel (Continuous jet algorithm) [2] 100 ~1000 • Di-lepton channel (Jet-Analysis with mlb) [3] 900 ~1300 • Di-lepton channel (Energy-Analysis) [3] 400 ~2000 • Di-lepton channel (Tri-lepton events) [3] 1000 ~1500 • Di-lepton channel (Full-reconstruction) [1,2] 300 ~1300 • From t  l + J/y + X decays [1,2,3] 1000 <1000 • High pT fully-hadronic channel [1,2] 180 >3500 • combining all those results could lead to a more precise measurement (correlations to be estimated !!) • systematic effects do not necessary overlap between analyses • Expectation : top mass determination better than 1 GeV after understanding the detector Jorgen D'Hondt (Vrije Universiteit Brussel)

  16. Top versus anti-top CMS Note 2006/111 The top quark does not loose its spin properties before it is decaying. Hence in the decay of top quark pairs, angular correlations should be present. Main systematics are related to the jet energy scale and the b-tagging results (CMS, 10fb-1) : good agreement with TopRex input Measuring spin correlations is something different compared to observing spin correlations. The second one could be easier. Jorgen D'Hondt (Vrije Universiteit Brussel)

  17. Single-top production Never observed ? • Each channel sensitive to different signals • heavy W’ → s-channel • FCNC → t-channel • H± → Wt-channel • Also directly related to |Vtb| to percent level • (s-channel preferred, t-channel dominated by PDF scale uncertainties of ~10%) Jorgen D'Hondt (Vrije Universiteit Brussel)

  18. Single-top production: t-channel CMS Note 2006/084 • Production channel with largest cross section • Optimized event selection (sNLO ~ 243pb) • MET > 40 GeV • b-jet: pT>35GeV, |h|<2.5 • light-jet: pT>40GeV, |h|>2.5 (forward) • topological cuts: mrec(top), mT(W) • Expected number of events (10fb-1) • signal = 2389 • tt = 1188 • W+jets = 597 (CompHEP, TopReX, MadGraph) • QCD = negligible (using factorisation of e) • Resulting S/B~1.34 • Estimate of cross section (10fb-1) 0% 3 theory, JES, b-tagging Jorgen D'Hondt (Vrije Universiteit Brussel)

  19. Single-top production: Wt-channel CMS Note 2006/086 • Two production channels (CMS Note submitted) • di-lepton→ pTm/e>20GeV, pTj1>60GeV & pTj2>20GeV (veto others), MET>20GeV • semi-lepton → pTe>30GeV & pTm>20GeV, 1 strong b-tag & 2 non-b-jets pT>35GeV (veto others), MET>40GeV .... jet pairing via likelihood variable • Reject jets with a bad reconstruction quality (fakes)→ likelihood variable • Control regions for background • reduce tt bck uncertainty 60%→negligible • Results (10fb-1) • Ds/s (di-lepton) 8.8 (stat) ± 23.9 (syst) ± 9.9 (MC stat) % • Ds/s (semi-lepton) 7.4 (stat) ± 17.7 (syst) ± 15.2 (MC stat) % • Main uncertainty from jet energy scale and b-tagging (will improve with better detector understanding) Jorgen D'Hondt (Vrije Universiteit Brussel)

  20. Top quarks for calibration: Jet Energy Scale • Rescale each jet with relative energy shift DC (rescaling |p| to keep jetmass invariant) • Remake/refit the obtained W mass spectrum → mW(DC) from fit • Solve the simple equation mW(DC|data) = MWPDG → best estimate for DC • Compare this measured shift with the true shift from Monte Carlo information (for well matched jet-parton couples (DR<0.2) one can determine the average DC) Result : DCmeas = -14.96 ± 0.26 % (DCtrue = -14.53 %) with 5.41fb-1 CMS Note 2006/025 mW L= 0.33fb-1 true shift DCtrue used space-angle match (~1% effects with DR) measured shift DCmeas DC(%) Erec/Egen scaling to 1fb-1 incl. electron channel : 0.4% Pile-up (On/Off) = 3% Jorgen D'Hondt (Vrije Universiteit Brussel)

  21. Top quarks for calibration: b-tag efficiency Optimize the jet pairing efficiency via mass constraints in kinematic fits. Select a very pure b-jet sample on which the b-tag algorithms can be applied and the efficiency estimated. CMS Note 2006/013 The b-tagging efficiency can be measured to 4% in barrel and 5% in endcaps. Crucial for a correct estimation of the selection efficiency of Higgs decays to bb. Jorgen D'Hondt (Vrije Universiteit Brussel)

  22. Ultimate test of the Standard Model Comparing the direct and indirect values of mH To give mt and mW equal weight : DmW = 0.7 10-2Dmt Goal of LHC experiments : Dmt < 1 GeV DmW < 15 MeV  DmH/mH < 25 % After a discovery one can use EW measurements to differentiate between SM or MSSM Higgs bosons W boson mass references: CMS Note 2006/061 ATLAS ATL-PHYS-PUB-2006-007 Jorgen D'Hondt (Vrije Universiteit Brussel)

  23. Conclusions and outlook • Potential of top quark physics with CMS has been demonstrated • CMS published a Physics-TDR vol.1 on detector & reconstruction tools • and simulation results from CMS (Physics-TDR vol.2) • based on dedicated reconstruction & analysis tools (incl. trigger) • applied in realistic state-of-the-art settings of the detector • talk is based on hundreds of pages of public (or almost public) notes • From Tevatron to LHC • cross sections of signal and background are favourable • for many topics LHC has the potential to go significantly beyond Tevatron • Outlook... prepare for data taking • top quarks are crucial for calibration and commissioning at start-up and beyond • jet calibration and b-tagging performance can be derived from data via top quarks • over the next year prepare to extract top quark out of this first data • thanks to all CMS collaborators who contributed to these Top Quark studies Jorgen D'Hondt (Vrije Universiteit Brussel)

  24. Extra’s • Some extra informative slides : • lepton reconstruction: resolution • jet reconstruction • b-tagging performance • missing transverse energy • kinematic fit performance • trigger • dependency of cross section on top quark mass • some distributions of event selection observables • semi-leptonic: other top mass estimators • di-lepton events: other top mass estimators • top mass and W boson mass • breakdown uncertainty W polarization • more possibilities beyond the Standard Model Jorgen D'Hondt (Vrije Universiteit Brussel)

  25. Event reconstruction: lepton reconstruction • Good resolution is needed for triggering (cfr. turn-on curve) • good performance of tracking, calorimeter and muon systems • muon pT resolution ~1.5% (CMS P-TDR vol.1) • electron pT resolution ~1% (CMS P-TDR vol.1) • Resolution checked with tt events • lepton+jet (CMS Note 2006/024) • Lepton isolation (several lepton candidates per event) • combine several observables per lepton candidate • likelihood ratio variable • correct identification of tlnb lepton in 99% of the selected events • effect of magnetic field s(j) ~ |h| lepton+jet (tt) CMS Note 2006/024 Jorgen D'Hondt (Vrije Universiteit Brussel)

  26. Event reconstruction: jets • Jet clustering :mostly Iterative Cone R=0.5 (optimization eg. Les Houches ’05) • calorimeter towers above E & ET thresholds (added to online zero suppression) • JES calibration :via method which will be applied on data (to the % level) • g+jet and Z+jet events (CMS Note 2006/042, ATLAS P-TDR): based on pT balans • Wjj events (CMS Note 2006/025, ATLAS P-TDR): from well known W boson mass • Pile-up and underlying event treatments • (¤) reject jets not attached to primary vertex (CMS Note submitted) • underlying event measurements (CMS Note 2006/067) CMS Note 2006/066 CMS P-TDR (vol.1) lepton+jet (tt) #jets per event (¤) after primary vertex constraint Jorgen D'Hondt (Vrije Universiteit Brussel)

  27. Event reconstruction: b-tagging • Combined secondary vertex b-tagging algorithm (CMS Note 2006/014) • based on the combination of several topological and kinematical observables  distribution of discriminator in lepton+jet events lepton+jet (tt) 10% 2% c <1% CMS Note 2006/064 g uds • General performance plot • QCD jets 50<ET<80 GeV in barrel |h|<1.4 60% CMS P-TDR (vol.1) Jorgen D'Hondt (Vrije Universiteit Brussel)

  28. Event reconstruction: missing ET • Missing transverse energy challenging due to multiple pile-up collisions • also magnet field will influence resolution (photons versus pions) • but very good hermiticity and granularity • Missing ET performs well at the LHC but rather hard cuts are needed CMS P-TDR (vol.1) CMS Note 2006/035 inclusive tt before jet correction QCD multi-jet after jet correction 40 GeV single-top & tt Resolution of 20 GeV for ETmiss ~ 40GeV Jorgen D'Hondt (Vrije Universiteit Brussel)

  29. Event reconstruction: kinematic fit Using kinematic fit techniques (applying Least-Square methods with Lagrange multipliers) mass constraints can be enforced to the reconstructed event topology. in the t→Wb decay constrain the W boson mass to its precisely measured value CMS Note 2006/023 CMS Note 2006/023 ~70% purity To obtain the same precision without the fit, one needs 5 times more data (the bias wrt 175GeV is reduced) Jorgen D'Hondt (Vrije Universiteit Brussel)

  30. Triggering • CMS L1 & HLT triggers applied on full simulation • CMS Collaboration ‘DAQ and High-Level Trigger’ TDR • fully hadronic: inclusive jet trigger e~17% (QCD reduced to 23 Hz, S/B~1/300)  b-jet stream improvement of 15% on efficiency • semi-leptonic: single-lepton triggers (pTe>26GeV, |h|<2.4 and pTm>19GeV, |h|<2.1) e(m)~62% and e(e)~48% • di-leptonic: also di-lepton triggers (pTe> 12/12 GeV and pTm> 7/7 GeV)  e(mm)~88% and e(ee)~77% • single-top: .OR. of single-lepton triggers and e+jet (pTe>19GeV and pTjet>45GeV) • e(W→lv) in s-channel: 24% 25% 7% 38% • e(W→lv) in t-channel: 25% 25% 7% 43% single-m single-e e+jet combined • Thresholds and rates to be updated in Physics-TDR vol.2 → changes in algorithms and improved understanding of the background → single-lepton are the dominant streams (rate(e)~23.5Hz and rate(m)~25.8Hz) (numbers for L = 2.1033 cm-2s-1) Jorgen D'Hondt (Vrije Universiteit Brussel)

  31. Top Quark Pair Production Cross-Section • Sensitive to top mass : Ds/s ~ 5 Dmt/mt  5% on s gives 2 GeV on mt systematics dominated by the uncertainty on the luminosity ATLAS ATL-PHYS-PUB-2005-024 Cross-section sensitive to renormalisation and factorisation scale, and to the choice of PDF (Parton Density Function) Jorgen D'Hondt (Vrije Universiteit Brussel)

  32. Top Quark Pair Selection: distributions CMS Note 2006/077 Di-leptonic channel: Mass window around the Z boson mass rejects basically all Z+jet events CMS Note 2006/077 Fully hadronic channel: Neural Network output combining the information of several topological obervables (scaled to effective cross section) Jorgen D'Hondt (Vrije Universiteit Brussel)

  33. Flavour Physics : like-sign top quark pairs • Di-lepton top quark pairs have a clear topology • 2 b-jet and 2 isolated leptons with a different charge, selected with a large S/N • exploit the performance of the lepton isolation criteria (CMS Note 2006/024) • Motivation for this search • FCNC (in SM suppressed, Z’ bosons in Topcolor assisted Technicolor (TC2)) • from top- and techni-pion in TC2 models • in MSSM from for example gluino pairs • Variable to be measured is R = N++,- - / N+- • ratio of events with a pair of leptons with same and different electric charge • most of the systematics cancel in the ratio CMS Note 2006/065 at 30fb-1 a pptt cross section of 1pb becomes visible as a 5s effect on the ratio R Jorgen D'Hondt (Vrije Universiteit Brussel)

More Related