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The top quark at LHC: status and prospects

The top quark at LHC: status and prospects. Marina Cobal-Grassmann “Journee ATLAS France” Londe Les Maure, 3-5 May, 2004. Top quark exists and will be produced abundantly! In SM: top- and W-mass constrain Higgs mass Sensitivity through radiative corrections

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The top quark at LHC: status and prospects

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  1. The top quark at LHC: status and prospects Marina Cobal-Grassmann “Journee ATLAS France” Londe Les Maure, 3-5 May, 2004

  2. Top quark exists and will be produced abundantly! In SM: top- and W-mass constrain Higgs mass Sensitivity through radiative corrections Scrutinize SM by precise determination top mass Beyond SM: New Physics? Many heavy particles decay in tt Handle on new physics by detailed properties of top Experiment: Top quark useful to calibrate the detector Beyond Top: Top quarks will be a major source of background for almost every search for physics beyond the SM Motivations for Top Physics studies Summer 2003 result direct EXCLUDED indirect Marina Cobal - Londe Les Maure 2004

  3. LEP+SLD: VCKM (4) UA2+Tevatron: s(1) NuTeV: predictions GF (1) SM APV: mfermions (9) (down to 0.1% level) mbosons (2) eeqq l.e.: What we know.. mH No observable directly related to mH. However the dependence can appear through radiative corrections. tree level quantities changed , r = f [ln(mH/mW), mt2] The uncertainties on mt, mW are the dominating ones in the electroweak fit By making precision measurements (already interesting per se): • one can get information on the missing parameter mH • one can test the validity of the Standard Model Marina Cobal - Londe Les Maure 2004

  4. Top mass: Where we are Marina Cobal - Londe Les Maure 2004

  5. Near future of Mtop Tevatron only (di-lepton events or lepton+jet ) from W decays Status of inputs (preliminary): mt=(178.0  2.7 (stat)  3.3 (syst)) GeV/c2 (latest Tevatron updated combination – RunI data) mt=(175  17 (stat)  8 (syst)) GeV/c2 (CDF di-leptons – RunII data) mt=(178+13-9(stat)  7 (syst)) GeV/c2 (CDF lepton+jets – RunII data) Matter of statistics (also for the main systematics) and optimized use of the available information. Each experiment expects 500 b-tagged tt l+jets events/fb  DMtop ~ 2-3 GeV/c2 for the Tevatron combined (2-4/fb) • mt  2.5 GeV ; mW  30 MeV  mH/mH  35% In 2009 (if upgrade is respected) from Tevatron: DMtop = 1.5 GeV !! Marina Cobal - Londe Les Maure 2004

  6. √s [TeV] Luminosity [cm-2s-1] ∫L [fb-1/y] TeVatron 2 <1032 0.3 LHC (low lum) 14 1033 10 LHC LHC (high lum) 14 1034 100 TeVatron What can we do at LHC? - - Marina Cobal - Londe Les Maure 2004

  7. Cross section determined to NLO precision Total NLO(tt) = 834 ± 100 pb Largest uncertainty from scale variation Compare to other production processes: Top production cross section approximately 100x Tevatron Opposite @ FNAL Top production at LHC ~90% gg~10% qq Low lumi LHC is a top factory! Marina Cobal - Londe Les Maure 2004

  8. In the SM the top decays to W+b All decay channels investigated Using ‘fast parameterized’ detector response Checks with detailed simulations Di-leptons (e/) BR≈4.9%  0.4x106 ev/y No top reconstructed Clean sample Single Lepton (e/) BR=29.6%  2.5x106 ev/y One top reconstructed Clean sample Fully Hadronic BR≈45%  3.5x106 ev/y Two tops reconstructed Huge QCD background Large combinatorial bckgnd Top decay Marina Cobal - Londe Les Maure 2004

  9. Br(ttbbjjl)=30%for electron + muon Golden channel Clean trigger from isolated lepton The reconstruction starts with the W mass: different ways to pair the right jets to form the W jet energies calibrated using mW Important to tag the b-jets: enormously reduces background (physics and combinatorial) clean up the reconstruction Lepton side Hadron side MTop from lepton+jet Typical selection efficiency: ~5-10%: • Isolated lepton PT>20 GeV • ETmiss>20 GeV • 4 jets with ET>40 GeV • >1 b-jet (b40%, uds10-3, c10-2) Background: <2% W/Z+jets, WW/ZZ/WZ Marina Cobal - Londe Les Maure 2004

  10. Hadronic side W from jet pair with closest invariant mass to MW Require |MW-Mjj|<20 GeV Assign a b-jet to the W to reconstruct Mtop Kinematic fit Using remaining l+b-jet, the leptonic part is reconstructed |mlb -<mjjb>| < 35 GeV Kinematic fit to the tt hypothesis, using MW constraints j2 j1 b-jet t Lepton + jet: reconstruct top • Selection efficiency 5-10% Marina Cobal - Londe Les Maure 2004

  11. Method works: Linear with input Mtop Largely independent on Top PT Biggest uncertainties: Jet energy calibration FSR: ‘out of cone’ give large variations in mass B-fragmentation Verified with detailed detector simulation and realistic calibration Top mass systematics Challenge: determine the mass of the top around 1 GeV accuracy in one year of LHC Marina Cobal - Londe Les Maure 2004

  12. Mtop Alternative mass determination • Select high PT back-to-back top events: • Hemisphere separation (bckgnd reduction, much less combinatorial) • Higher probability for jet overlapping • Use the events where both W’s decay leptonically (Br~5%) • Much cleaner environment • Less information available from two ’s • Use events where both W’s decay hadronically (Br~45%) • Difficult ‘jet’ environment • Select PT>200 GeV Various methods all have different systematics Marina Cobal - Londe Les Maure 2004

  13. Calibration demands: Ultimately jet energy scale calibrated within 1% Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by mW At startup jet-energy scale known to lesser precision ±10% MTop MTop Scale light-jet energy Scale b-jet energy Jet scale calibration Uncertainty On b-jet scale:Hadronic 1%  Mt = 0.7 GeV 5%  Mt = 3.5 GeV 10%  Mt = 7.0 GeV Uncertainty on light jet scale:Hadronic 1%  Mt < 0.7 GeV 10%  Mt = 3 GeV Marina Cobal - Londe Les Maure 2004

  14. Continuous jet algorithm Reduce dependence on MC Reduce jet scale uncertainty Repeat analysis for many cone sizes R Sum all determined top mass:robust estimator top-mass Determining Mtop from (tt)? huge statistics, totally different systematics But: Theory uncertainty on the pdfs kills the idea 10% th. uncertainty  mt  4 GeV Constraining the pdf would be very precious… (up to a few % might not be a dream !!!) Alternative methods • Luminosity uncertainty then plays the game (5%?) Luminosity uncertainty then plays the game (5%?) Marina Cobal - Londe Les Maure 2004

  15. Use exclusive b-decays with high mass products (J/) Higher correlation with Mtop Clean reconstruction (background free) BR(ttqqb+J/)  5 10-5  ~ 30%  103 ev./100 fb-1(need high lumi) Top mass from J/ MlJ/ Different systematics (almost no sensitivity to FSR) Uncertainty on the b-quark fragmentation function becomes the dominant error M(J/+l) M(J/+l) Pttop Marina Cobal - Londe Les Maure 2004

  16. Many theoretical models include the existence of resonances decaying to top-topbar SM Higgs (but BR smaller with respect to the WW and ZZ decays) MSSM Higgs (H/A, if mH,mA>2mt, BR(H/A→tt)≈1 for tanβ≈1) Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor, “colorons” production, […] xBR required for a discovery σxBR [fb] 30 fb-1 830 fb 300 fb-1 mtt [GeV/c2] 1 TeV Search for resonances • Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt) • Reconstruction efficiency for semileptonic channel: • 20% mtt=400 GeV • 15% mtt=2 TeV 1.6 TeV resonance Mtt Marina Cobal - Londe Les Maure 2004

  17. Couplings and decays • Does the top quark behaves as expected in the SM? • Yukawa coupling to Higgs from ttbarH events • Electric charge • Top spin polarization • CP violation • According to the SM: • Br(t Wb)  99.9%, Br(t  Ws)  0.1%, Br(t  Wd)  0.01% (difficult to measure) • Can probe t W[non-b] by measuring ratio of double b-tag to single b-tag • Statistics more than sufficient to be sensitive to SM expectation for Br(t  W + s/d) • need excellent understanding of b-tagging efficiency/purity Marina Cobal - Londe Les Maure 2004

  18. Rare decays: FCNC • In the SM the FCNC decays are highly suppressed (Br<10-13-10-10) • Any observation would be sign of new physics • Sensitivity according to ATLAS and CMS studies : • t  Zq(CDF Br<0.137, ALEPH Br<17%, OPAL Br<13.7%) • Reconstruct t  Zq  (l+l-)j • Sensitivity to Br(t  Zq) = 1 X 10-4 (100 fb-1) • t  q(CDF Br<0.032) • Sensitivity to Br(t  q) = 1 X 10-4 (100 fb-1) • t  gq • Difficult identification because of the huge QCD bakground • One looks for “like-sign” top production (ie. tt) • Sensitivity to Br(t  gq) = 7 X 10-3 (100 fb-1) Marina Cobal - Londe Les Maure 2004

  19. Radiative top production Radiative top decay Top Charge determination • Can we establish Qtop=2/3? • Currently cannot exclude exotic possibility Qtop=-4/3 • Assign the ‘wrong’ W to the b-quark in top decays • tW-b with Qtop=-4/3 instead of tW+b with Qtop=2/3 ? • Technique: • Hard  radiation from top quarks • Radiative top production, pptt cross section proportional to Q2top • Radiative top decay, tWb • On-mass approach for decaying top: two processes treated independently • Matrix elements havebeen calculated and fed intoPythia MC Marina Cobal - Londe Les Maure 2004

  20. Top Charge determination • Yield of radiative photons allows to distinguish top charge • Determine charge of b-jet andcombine with lepton • Use di-lepton sample • Investigate ‘wrong’ combination b-jet charge and lepton charge • Effective separation b and b-bar possible in first year LHC • Study systematics in progress 10 fb-1 One year low lumi events pT() Marina Cobal - Londe Les Maure 2004

  21. e+/+ + top Top spin correlations • In SM with Mtop175 GeV, (t)  1.4 GeV » QCD • Top decays before hadronization, and so can study the decay of ‘bare quark’ • Substantial ttbar spin correlations predicted in pair production • Can study polarization effects through helicity analysis of daughters • Study with di-lepton events • Correlation between helicity angles + and -for e+/+ and e-/- <CosΘ+ · CosΘ-> <CosΘ+ · CosΘ-> No helicity correlation With helicity correlation Marina Cobal - Londe Les Maure 2004

  22. Also study spin correlations in hadronic decays (single lepton events) Least energetic jet from W decay:  ~ 0.5 Ratio between ‘with’ and ‘without’ correlations Able to observe spin correlations in asymmetry C 30 fb-1 of data: ± 0,035 statistical error ± 0,028 systematic error 10 statistical significance for a non-zero value with 10 fb-1 Top spin correlations 30 fb-1 <CosΘ+ · CosΘ-> Marina Cobal - Londe Les Maure 2004

  23. Direct determination of the tWb vertex (=Vtb) Discriminants: - Jet multiplicity (higher for Wt) More than one b-jet (increase W* signal over W- gluon fusion) 2-jets mass distribution (mjj ~ mW for the Wt signal only) Three production mechanisms: Main Background [xBR(W→ℓ), ℓ=e,μ]: tt σ=833 pb [ 246 pb] Wbbσ=300 pb [ 66.7 pb] Wjjσ=18·103 pb [4·103 pb] Single top production 1) Determination of Vtb 2) Independent mass measurement +16.6 Wg fusion: 245±27 pb S.Willenbrock et al., Phys.Rev.D56, 5919 Wt: 62.2 pb A.Belyaev, E.Boos, Phys.Rev.D63, 034012 W* 10.2±0.7 pb M.Smith et al., Phys.Rev.D54, 6696 -3. 7 Wg [54.2 pb] Wt [17.8 pb] W* [2.2 pb] Marina Cobal - Londe Les Maure 2004

  24. Single top results • Detector performance critical to observe signal • Fake lepton rate • b and fake rate id  • Reconstruction and vetoing of low energy jets • Identification of forward jets • Each of the processes have different systematic errors for Vtb and are sensitive to different new physics • heavy W’  increase in the s-channel W* • FCNC gu  t  increase in the W-gluon fusion channel • Signal unambiguous, after 30 fb-1: • Complementary methods to extract Vtb • With 30 fb-1 of data, Vtb can be determined to %-level or better(experimentally) Marina Cobal - Londe Les Maure 2004

  25. CP violation in top events (K. Martens, University of Toronto ) Top spin polarization in di-lepton events (V. Simak et al., Prague) Top spin polarization in single lepton events (E. Monnier, P. Pralavorio, F. Hubaut, CPPM) Single top studies (M. Barisonzi, NIKHEF) Optimization of kinematic reconstruction in the single lepton channel (V. Kostioukhine, University of Genova) Commissioning studies (S. Bentvelsen, NIKHEF) New MC validation (S. Bentvelsen, E. Monnier, P. Pralavorio) Full simulation studies of detector effects (A. Etienvre, J. Schwindling, JP Meyer, Saclay) Full simulation studies of b-tagging (S. Moed, University of Geneva) Top mass and calibration studies (D. Pallin, F. Binet, Clermont-Ferrand) Ttbar resonances (E. Cogneras, Clermont-Ferrand) Undergoing analyses Marina Cobal - Londe Les Maure 2004

  26. What is left before the LHC starts? • Cover topics still open: cross section, couplings, exotic, resonances, • Define a strategy for validation of the MC input models (e.g: UE modeling and subtraction, jet fragmentation properties, jet energy profiles, b-fragmentation functions..) see M. Mangano talk at IFAE 2004 • Explore the effects of changing detector parameters in evaluating the top mass. • Perform commissioning studies with top events • Contribute to simulation validation • … Marina Cobal - Londe Les Maure 2004

  27. Determination MTop in initial phase Use ‘Golden plated’ lepton+jet Selection: Isolated lepton with PT>20 GeV Exactly 4 jets (R=0.4) with PT>40 GeV Reconstruction: Select 3 jets with maximal resulting PT Signal can be improved by kinematic constrained fit Assuming MW1=MW2 and MT1=MT2 Commissioning the detectors • Calibrating detector in comissioning phase • Assume pessimistic scenario: • -) No b-tagging • -) No jet calibration • -) But: Good lepton identification No background included Marina Cobal - Londe Les Maure 2004

  28. Signal plus background at initial phase of LHC Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig) Commissioning the detectors ALPGEN: W+4 extra light jets Jet: PT>10, ||<2.5, R>0.4 No lepton cuts Effective : ~2400 pb L = 150 pb-1 (2/3 days low lumi) With extreme simple selection and reconstruction the top-peak should be visible at LHC measure top mass (to 5-7 GeV) give feedback on detector performance Marina Cobal - Londe Les Maure 2004

  29. Top in DC2 Tier test • The 10M tier1 events in light of top: • Generation/simulation of 106 top events, inclusive decays, using MC@NLO • Using Herwig for MC + UE • Simulation with full geometry • Simulation 500K top events with displaced ID • Same truth generated top events as above • 1 cm displacement of ID – check tracking performance • Simulation of 106 W+jet events MC@NLO • For W+2 jet background • Simulation of 250K W+4jet events with AlpGen • pT>15 GeV approximately Marina Cobal - Londe Les Maure 2004

  30. What is still missing? • Top production is ‘over-weighted’ in 10M sample • Unrealistic to ask for more in this sample • One of priorities in ‘post-production’: • Regenerate half of the top MC@NLO sample • Now using Jimmy UE (much more activity), after tuning • Still on the wish-list: • Top events with spin correlations • TopRex available, perhaps AcerMC? • Single top events • also TopRex • Dedicated samples single- and dilepton top events • Top events with PYTHIA (cross check with DC1) Marina Cobal - Londe Les Maure 2004

  31. Conclusions • LHC is top factory • (tt)~830 pb-1 • 107 events in first year • Precise determination of Mtop is waiting… • Challenge to get Mtop ~ 1 GeV • Confirmation that top-quark is SM particle • Measure Vtb, charge, CP, spin, decays • Top quarks for commissioning the detectors • Top peak should be visible with eyes closed • Today’s signal, tomorrow’s background • Top quarks as main background for many new physics channels Marina Cobal - Londe Les Maure 2004

  32. Rare SM top decays • Direct measurement of Vts, Vtdvia decays tsW, tdW • Decay tbWZ is near threshold (mt~MW+ MZ+mb)  BRcut(t bWZ)  610-7 (cut on m(ee) is 0.8 MW) • Decay tcWW suppressed by GIM factorBR(t cWW) ~ 110-13 • If Higgs boson is light: tbWH • FCNC decays: tcg, tc, tcZ(BR: 510-11 , 510-13 , 1.310-13 ) • Semi-exclusive t-decays tbM (final state 1 hadron recoiling against a jet: BR(t b)  410-8, BR(t bDs)  210-7) Marina Cobal - Londe Les Maure 2004

  33. Use the events where both W’s decay leptonically (Br~5%) Much cleaner environment Less information available due to two neutrino’s Sophisticated procedure for fitting the whole event, i.e. all kinematical info taken into account (cf D0/CDF) Compute mean probability as function of top mass hypothesis Maximal probability corresponds to top mass Top mass from di-leptons 80000 events (tt) = 20 % S/B = 10 • Selection: • 2 isolated opposite sign leptons • Pt>35 and Pt>25 GeV • 2 b-tagged jets • ETmiss>40 GeV Mean probability Marina Cobal - Londe Les Maure 2004 mass

  34. Use events where both W’s decay hadronically (Br~45%) Difficult ‘jet’ environment (QCD, Pt>100) ~ 1.73 mb (signal) ~ 370 pb Perform kinematic fit on whole event b-jet to W assignment for combination that minimize top mass difference Increase S/B: Require pT(tops)>200 GeV Top mass from hadronic decay • Selection • 6 jets (R=0.4), Pt>40 GeV • 2 b-tagged jets • Note: Event shape variables like HT, A, S, C, etc not effective at LHC (contrast to Tevatron) 3300 events selected: (tt) = 0.63 % (QCD)= 2·10-5 % S/B = 18 Marina Cobal - Londe Les Maure 2004

  35. j2 j1 b-jet Mtop t High Pt sample • The high pT selected sample deserves independent analysis: • Hemisphere separation (bckgnd reduction, much less combinatorial) • Higher probability for jet overlapping • Use all clusters in a large cone R=[0.8-1.2] around the reconstructed top- direction • Less prone to QCD, FSR, calibration • UE can be subtracted Mtop Statistics seems OK and syst. under control R Marina Cobal - Londe Les Maure 2004

  36. Calibration demands: Ultimately jet energy scale calibrated within 1% Uncertainty on b-jet scale dominates Mtop: light jet scale constrained by mW At startup jet-energy scale known to lesser precision ±10% MTop MTop Scale light-jet energy Scale b-jet energy Jet scale calibration Uncertainty on light jet scale:Hadronic 1%  Mt < 0.7 GeV 10%  Mt = 3 GeV Uncertainty On b-jet scale:Hadronic 1%  Mt = 0.7 GeV 5%  Mt = 3.5 GeV 10%  Mt = 7.0 GeV

  37. Rare decays: topWbZ Interesting: branching ratio depends strongly on Mtop • Since Mtop~MW+Mb+MZ • With present error mt  5 GeV, BR varies over a factor  3 • B-jet too soft to be efficiently identified   “semi-inclusive” study for a WZ near threshold, with Z  l+l-and W ->jj • Requiring 3 leptons reduces the Z+jets background • Sensitivity to Br(t  WbZ)  10-3 for 1 year at low lumi. • Even at high L can’t reach Sm predictions ( 10-7 -10-6) G. Mahlon hep-ph/9810485 G(tWbZ)/G(tWb) M(top) (GeV) Marina Cobal - Londe Les Maure 2004

  38. Signal Signal ttH ttH tt tt topHq • Various approaches studied • Previously: ttbarHq Wb(b-bbar)j(lb) for m(H) = 115 GeV • Sensitivity to Br(t  Hq) = 4.5 X 10-3(100 fb-1) • New results for: • t tbarHq WbWW*q Wb(l lj) (lb) • ≥ 3 isolated lepton with pT(lep) > 30 GeV • pTmiss > 45 GeV • ≥ 2 jets with pT(j) > 30 GeV, incl. ≥ 1 jet con b-tag • Kinematical cuts making use of angular correlations • Sensitive to Br(t  Hq) = 2.4 X 10-3 for m(H) = 160 GeV (100 fb-1) Marina Cobal - Londe Les Maure 2004

  39. Non-SM Decays of Top • 4thfermion family Constraints on Vtqrelaxed: • Supersymmetry (MSSM) • Observed bosons and fermions would have superpartners  2-body decays into squarks and gauginos (t  H+ b ) • Big impact on 1 loop FCNC • two Higgs doublets • H LEP limit 77.4 GeV (LEP WG 2000) • Decay t  H+ b can compete with t  W+ b • 5 states (h0,H0,A0,H+,H-) survive after giving W & Z masses • H couples to heaviest fermions  detection through breakdown of e / m / t universality in tt production Marina Cobal - Londe Les Maure 2004

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