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Top physics at LHC with tt events

Top physics at LHC with tt events

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Top physics at LHC with tt events

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  1. Top physics at LHC with tt events Fabrice Hubaut(hubaut@in2p3.fr) CPPM/IN2P3–Univ. de la Méditerranée (Marseille, FRANCE) On Behalf of the ATLAS and CMS Collaborations Rencontres de Moriond 2006, QCD session, March 18-25 Top physics at LHC with tt events

  2. Motivations for top quark physics 5 orders of magnitude • Special role in the EW sector and in QCD • Heaviest elementary particle known  Yukawa coupling close to 1.0 • Top and W masses constrain the Higgs mass • Short lifetime (<tQCD): unique window on bare quarks •  A tool for precise SM studies • Special role in various SM extensions through EWSB • New physics might be preferentially coupled to top • Non-standard couplings between top and gauge bosons • New particles can produce / decay to tops •  A sensitive probe to new physics • Special interest even if it is just a «normal» quark •  A major source of background for many searches •  A tool to understand/calibrate the detector, all sub-detectors involved Top physics at LHC with tt events

  3. Top properties scorecard We still know little about the top quark, limited by Tevatron statistics Mass precision <2% Electric charge ⅔-4/3 excluded @ 94% C.L. (preliminary) Spin ½ not really tested – spin correlations Isospin ½ not really tested BR to b quark ~ 100% at 20% level in 3 generations case V – A decay at 20% level FCNC probed at the 10% level Top width ?? First observe single top ! Yukawa coupling ?? • This leaves plenty of room for new physics in top production and decay • Tevatron run II starts to incisely probe the top quark sector • The LHC will open a new opportunity for precision measurements Top physics at LHC with tt events

  4. Top production and decay at LHC Golden channel (early physics, precision meas.) See talk by M. Najafabadi Strong Interaction tt Weak Interaction single top* W* LHC σ ~850 pb 10% qq, 90% gg Tevatron σ ~7 pb 85% qq, 15% gg LHC σ ~300 pb 75%Wg, 20%Wt Tevatron σ ~3 pb 65%Wg, 30%Wt W t W-g fusion *not observed yet ! BR (tWb) ~ 100 % in SM and no top hadronisation Wen, mn Wen, mn, qq Single top final states(LHC, 10 fb-1) tt final states(LHC,10 fb-1) • W-g(0.5 M): l + n+ 2jets • Wt(0.2 M): l + n+ 3jets • W*(0.02 M): l + n+ 2jets • Full hadronic(3.7 M): 6 jets • Semileptonic(2.5 M): l + n+ 4jets • Dileptonic (0.4 M): 2l + 2n+ 2jets Top physics at LHC with tt events

  5. Early studies (<1 fb-1) • Remarkable topology: t and t central and back-to-back in the transverse plane • Easy to trigger and select L=100 pb-1 (1 day @ 1033 cm-2s-1) 3 jets with highest ∑ pT 4 jets pT> 40 GeV NO b-TAG !! Full simulation Signal (MC@NLO) Isolated lepton pT> 20 GeV  trigger W+n jets (Alpgen) + combinatorial pTmiss > 20 GeV Mjjj (GeV) • Observation of clean top sample should be very fast • Initial measurement of cross-section and mass • Feedback on detector performance (JES, b-tagging, …) and on MC description Top physics at LHC with tt events

  6. Precision studies (1-10 fb-1) Purity of reconstructed tt ~ 70% with erec ~ 30% • When performance improve, such as b-tagging (b60%, ruds100, rc10)  non tt background (W+jets, bb, ...) negligible L=10 fb-1 Full reconstruction Selection • Use Wjj to calibrate light jet energy • b with max. pT(jjb) for hadronic top • pTmiss for pTnandMW constraint for pZ • Other b for leptonic top: s ~12 GeV • 1 isolated lepton pT>20 GeV • pTmiss>20 GeV • ≥4 jets (cone DR=0.4) pT>40 GeV • 2 b-tagged jets s~11 GeV combinatorial esel ~ 3%, 80k evts/10 fb-1 S/B~12 (ttt+X) • High statistics with a few fb-1, measurements limited by systematics • Dileptonic channel also interesting  6 equations (ΣpT=0, Mlv= MW, Mlvb= Mt) with 6 unknowns (pn)  Apply this selection-recons. for Χ-section, mass, polarization studies, … Top physics at LHC with tt events

  7. Top mass (1) • Measurement method (semileptonic) • Kinematic fit event by event using t and t sides Mjj = Mlv= MW and Mjjb = Mlvb = Mtfit  (Mtfit, c2) by slices of c2  top mass estimator: mt=Mtfit(c2=0) • This selects well reconstructed b-jets (low effect due to final state radiation or leptonic b-decay) • Results (semileptonic) • mt linear with generated top mass • Statistical error with 10 fb-1: ~ 0.1 GeV hep-ex/0403021 Top physics at LHC with tt events

  8. Top mass (2) • Systematic errors on mt(GeV) in semileptonic channel • Systematics from b-jet scale (full simulation): 184 slope=0.7 GeV / % 180 176 Rec. Top mass (GeV) 172 168 0.9 0.95 1. 1.05 1.1 b-jet miscalibration factor • Other methods (invariant 3 jet jjb mass, large pT events, ...) give higher systematics but will allow reliable cross-checks hep-ex/0403021 • A ~1 GeV accuracy on Mt seems achievable with 10 fb-1 at ATLAS/CMS Top physics at LHC with tt events

  9. Top mass (3) • Dileptonic (10 fb-1) Input top mass=175 GeV • Need to reconstruct full tt event to assess the 2 n momenta  6 equations (ΣpT=0, Mlv= MW, Mlvb= Mt) • Assume mt and compute solution probability event by event using MC kinematic distributions • Choose mt with highest mean probability on all events • Systematic uncertainty: ~2 GeV (PDF + b-frag.) mean probability hep-ex/0403021 Mass hypthesis(GeV) • Final states with J/ (100 fb-1) • Correlation between MlJ/ and mt • Low statistics: ~1000 evts/100 fb-1 • No systematics on b-jet scale ! • Systematic uncertainty: ~1 GeV (b-frag.) Charge identification MlJ/y hep-ph/9912320 Top physics at LHC with tt events

  10. W polarization in top decay (1) • Test the top decay (in fully reconstructed tt) with W polarization ... Right-handed W+ (FR) Longitudinal W+ (F0) Left-handed W+ (FL) NLO 0.695 0.304 0.001 Sensitive to EWSB Test of V-A structure • ...measured through angular distribution of charged lepton in W rest frame 1/N dN/dcos n b W+ • Angle between: • lepton in W rest frame and • W in top rest frame t  1/2 1/2 1 spin l+ cos Top physics at LHC with tt events

  11. W polarization in top decay (2) 10 fb-1 Semilep. 1/N dN/dcos (Mt=175 GeV) Combined results of semilep+dilep 2 parameter fit with F0+FL+FR=1 hep-ex/0508061 cos • Systematics dominated by b-jet scale, top mass and final state radiation (FSR) • With 10 fb-1, can measure F0 with a~2% accuracy and FR with a precision ~1% • Tevatron expectations (2 fb-1): δF0stat/F0~12%andδFRstat/FR~3% Top physics at LHC with tt events

  12. W polarization in top decay (3) • From W polarization, deduce sensitivity to tWb anomalous couplings model independent approach, i.e. effective Lagrangian ) and 4 couplings (in SM LO F0 • 2s limit (statsyst) on = 0.04 • 3 times better than indirect limits (B-factories, LEP) • Less sensitive to and already severely constrained by B-factories ±1s Anomalous coupling Top physics at LHC with tt events

  13. tt spin correlation • Test the top production … t and t are not polarized in tt pairs, but their spins are correlated PL B374 (1996)169 Mtt<550 GeV =0.33 A=0.42 LHC s(a.u.) =-0.24 AD=-0.29 Tevatron top spin ≠ 1/2, anomalous couplings, tH+b Mass of tt system, Mtt (GeV) • … by measuring angular distribution of daughter particles in top rest frame Semilep. + dilep. (10 fb-1) • Syst. dominated by b-JES, top mass and FSR • ~4% precision on spin correlation parameters • Tevatron expectations (2 fb-1):dAstat/A~40% hep-ex/0508061 Top physics at LHC with tt events

  14. Direct search for new particles 1.6 TeV resonance xBR required for a discovery 830 fb σxBR [fb] combinatorics + tt continuum 30 fb-1 300 fb-1 1 TeV mtt (GeV) • In top production • Example of resonances decaying to tt, as predicted by various models • Generic analysis for a resonance X with σΧ, ΓΧ and BR(Χtt) • In top decay • Example of tH+b with subsequent H+tn (2<tanβ<40) • Search for excess of t-events or deficit of dilepton events • H+ discovery for MH+<160 GeV with 30 fb-1 J.Phys.G28 (2002) 2443 Top physics at LHC with tt events

  15. Flavor Changing Neutral Currents • Standard Model FCNC are highly suppressed (BR < 10-13-10-10) • Some models beyond SM can give HUGE enhancements (BR up to 10-3) • FCNC could be detected directly through top decay (tt, single top) or anomalous single top production • Any observation would be sign of new physics • ATLAS/CMS 5s sensitivity / 95% CL to FCNC branching ratio in tt events: Reconstruct tZq (l+l-)j Huge QCD background  improve current limits by ~102-103in 1 year: starts to probe models Top physics at LHC with tt events

  16. Conclusions • LHC will be a top factory: 107 events already with 10 fb-1 • First steps towards precision measurements driven by systematics • Challenge to gettop mass~1 GeV SMMHconstrained to <30% • Test top production and decay e.g. by measuring W polarization~1-2% and top spin correlation~4%  anomalous tWb/gtt couplings, tH+b, FCNC, … • New era of precision measurements in top sector in 3 years from now • Powerful probes in the search for new physics • Prior to precision measurements, a huge effort is needed (2007-2008) • Complete study using full simulations and NLO generators • Understand the detectors and control systematics • Early top signals will help !! Top physics at LHC with tt events

  17. Conclusions Rendez-vous in Moriond 2008 for first top events at LHC Top physics at LHC with tt events

  18. SPARES Top physics at LHC with tt events

  19. LHC statistics • LHC: pp collisions at √s=14 TeV every 25 ns in 2007 • 2 phases: 1033cm-2s-1 (initial, 2008-2009), 1034cm-2s-1 (design, >2009) • High statistics at low luminosity • Hard cuts to select clean events • Few pile-up events • SM parameter measurements will be dominated by systematic errors • From Monte Carlo (MC): ISR/FSR, PDF, ... • From detector and machine Top physics at LHC with tt events

  20. Utilizing tt events Light jet energy scale (aim: 1%) Extrapolation from testbeam data (1998-2004): 5-10% Improve with in situ calibration (Z+jet, Wjj in tt events) In situ calibration with tt events A clean Wjj sample (up to 80%) can be extracted Shift of W mass peak related to absolute energy scale extract absolute jet energy scale (Ejet) from data W j2 j1 b-jet t before after  2-3% reachable on absolute scale with 300 pb-1 only Top physics at LHC with tt events

  21. Utilizing tt events b-tagging studies: simple demonstration An enriched (>80%) sample of b-jets can be extracted Cut on m(Whad) and m(tophad) masses Look at b-jet probability for 4th jet (must be b-jet if all assignments are correct) W CANDIDATE TOP CANDIDATE b-jet probability b-jet probability B-JET CANDIDATE ttbar (signal) ‘always b jet if all jet assignments are OK’ b enrichment expected W+jets (background) ‘random jet’no b enhancement expected  check/calibrate b-tagging performance with data Top physics at LHC with tt events

  22. b-tagging b-tagging algorithms: a weight is given to each jet combining signed impact parameters (2D+1D) and secondary vertex reconstruction (mass, number of vertices, …) b-jets Light jets 2D 2D+1D 3D+SVX eb=60% R=230 Jet weight Top physics at LHC with tt events

  23. Dileptonic channel Purity of reconstructed tt ~ 65% with erec ~ 80% • Clean channel, easy to trigger on • 2 neutrinos in final state full reconstruction however possible Full reconstruction Selection • Assume top mass is known • 6 equations (ΣpT=0, Mlv= MW, Mlvb= Mt) with 6 unknowns (pn) • If >= 1 solution (98%),solution’s probability based on MC kinematic distributions • 2 isolated leptons with opposite charge, pT>20 GeV • pTmiss>40 GeV • 2 b-tagged jets pT>20 GeV esel ~ 6%, 20k evts/10 fb-1 S/B~6 (ttt+X) • High statistics with a few fb-1, measurements limited by systematics • Complementary to semileptonic channel Top physics at LHC with tt events

  24. W polarization: full simulation Good agreement Full sim / Fast sim on W and top kinematics • compute a unique function (from Fast sim.) to correct for cuts and rec. effects • apply it on Fast and Full sim. samples Preliminary TopReX – Fast sim. TopReX – Full sim. MC@NLO – Full sim. 10 fb-1 0.7 fb-1 0.5 fb-1 1/N dN/dcos F0=0.70 ± 0.03 FL=0.29 ± 0.02 FR=0.01 ± 0.02 F0=0.699 ± 0.005 FL=0.299 ± 0.003 FR=0.002 ± 0.003 F0=0.69 ± 0.03 FL=0.30 ± 0.02 FR=0.01 ± 0.02 cos cos cos • Very good agreement Full sim / Fast sim Top physics at LHC with tt events

  25. tt spin correlation In top rest frame, polarisation (S) is measured with angular distributions of daughter: Degree to which its direction is correlated with top spin (spin analyzing power) angle between daughter and top spin axis s * lej = least energetic jet in top rest frame • Measurement of tt spin correlation (NP B690 (2004) 81) angle btwnspin analyzersdirection in the t(t) rest frame Top physics at LHC with tt events

  26. Top charge • Qtop=-4/3(tW-b instead of tW+b)? • Method 1: Measurement ofradiative top production and/or decay • s(ppttg) is proportional toQtop2 • After selection+reconstruction (10 fb-1) s (Q=-4/3) > s (Q=2/3) • Method 2: Measurement of daughter particle charge • Associate b-lepton pair from the same top • Compute the charge of b on a statistical basis: • Separate the 2 Qtop hypothesis needs less data than Method 1 (~1 fb-1) • Tevatron (Method 2): • D0 (360 pb-1) excludes Q=-4/3 @ 94% CL (10/2005, not yet published) Top physics at LHC with tt events

  27. Yukawa coupling • gt = √2 Mt/ v ~ 1 : intriguing !! • Most difficult top quark property to measure! • Measurement from associated Higgs production ttH ( bb, WW) • σα gt2·Br(Hbb, WW) • Need separate measurements of Higgs decay branching ratios • Statistical uncertainty on gt ~20% for MH<200 GeV with 30 fb-1 Systematics have to be carefully determined Top physics at LHC with tt events

  28. ATLAS/CMS ATLAS CMS Air-core toroids + solenoid in inner cavity Calorimeters outside field 4 magnets Solenoid Calorimeters inside field 1 magnet MAGNET (S) Si pixels + strips No particle identification B=4T s/pT ~ 1.5x10-4 pT  0.005 Si pixels+ strips TRD  particle identification B=2T s/pT ~ 5x10-4 pT  0.01 TRACKER Pb-liquid argon s/E ~ 10%/E uniform longitudinal segmentation PbWO4 crystals s/E ~ 2-5%/E no longitudinal segmentation EM CALO Fe-scint. + Cu-liquid argon (10 l) s/E ~ 50%/E  0.03 Brass-scint. (> 5.8 l +catcher) s/E ~ 100%/E  0.05 HAD CALO Air s/pT < 10 % at 1 TeV standalone; larger acceptance Fe s/pT ~ 5% at 1 TeV combining with tracker MUON Top physics at LHC with tt events

  29. LHC planning 2007 <L>=3 1030 2008 <L>=5 1032 L=1 1033 L=2 1033 2009 2010 <L>=5 1033 2011 L=1 1034 Top physics at LHC with tt events