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Expectations for Top Quark Physics in ATLAS at LHC

Expectations for Top Quark Physics in ATLAS at LHC. Top Mass Single top production Couplings and decays Top spin polarization. Clara Troncon On Behalf of the ATLAS Collaborat ion Università and INFN Milano ICHEP 2004. LHC. TeVatron. ~90% gg. ~10% qq. (Opposite @ FNAL).

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Expectations for Top Quark Physics in ATLAS at LHC

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  1. Expectations for Top Quark Physicsin ATLAS at LHC Top Mass Single top production Couplings and decays Top spin polarization Clara Troncon On Behalf of the ATLAS Collaboration Università and INFN Milano ICHEP 2004

  2. LHC TeVatron ~90% gg ~10% qq (Opposite @ FNAL) Top pair production at LHC Low lumi = 10 fb-1/y LHC top factory ! LHC start up in April 2007 @ L=1033 Clara Troncon - ICHEP 2004

  3. Top quark mass is a fundamental parameter of the EW theory In SM: top- and W-mass constrain Higgs mass Sensitivity through radiative corrections Want precision measurement of top mass to scrutinize SM Large value of m(t) and G (t) and short lifetime 10-24s make top quark unique Decays before hadronization Sensitive window for New Physics Many new heavy particles produce top quarks Detailed properties of top probe SM & beyond Top will be produced abundantly ! And in addition ... 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 Summer 2003 result direct EXCLUDED indirect Motivations for Top Physics studies Clara Troncon - ICHEP 2004

  4. 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 Tau + X BR≈21%  1.6 x106 ev/y No top reconstructed Lepton side Hadron side Top decays In the SM the top decays to W+b All decay channels investigated • Using ‘fast parametrized’detector response: • Checks with detailed GEANT simulations Clara Troncon - ICHEP 2004

  5. Golden channel BR 30% and clean trigger from isolated lepton Important to tag the b-jets: enormously reduces background (physics and combinatorial) Hadronic side W from jet pair with closest invariant mass to MW Require |MW-Mjj|<20 GeV Ligth jet calibrated with Mw constraint Assign a b-jet to the W to reconstruct Mtop Leptonic side Using remaining l+b-jet, the leptonic part is reconstructed |mlb -<mjjb>| < 35 GeV Kinematic fit to the t t hypothesis, using MW constraints Br(ttbbjjl)=30%for electron + muon j2 j1 b-jet t MTop from lepton+jet SN-ATLAS-2004-040 70% top purity - efficiency 1.2 % • Isolated lepton PT>20 GeV • ETmiss>20 GeV • 4 jets with ET>40 GeV DR=0.4 • >1 b-jet (b60%, ruds102, rc101) Background: <2% W/Z+jets, WW/ZZ/WZ Clara Troncon - ICHEP 2004

  6. 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 with~1 GeV accuracy in one year of LHC Clara Troncon - ICHEP 2004

  7. Mtop Alternative mass determination • High PT semi-leptonic back to back events: PT>200 GeV and hemisphere separation • (bckgnd reduction, much less combinatorial) • Higher probability for jet overlapping Sum calo towers over alarge DR>0.8 • Use the events where both W’s decay leptonically (Br~5%) • Much cleaner environment • Less information available from two ’s • S/B=10 • Use the events where both W’s decay hadronically (Br~45%) • Difficult ‘jet’ environment • Select PT>200 GeV, S/B=18 Statistically independent samples Different systematics but similar values < 3 GeV Clara Troncon - ICHEP 2004

  8. 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 luminosity) 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 Clara Troncon - ICHEP 2004

  9. Mtop(GeV) Commissioning Commissioning phase: Assume pessimistic scenario: No b-tagging No jet calibration But: Good lepton id Semi-leptonic events selection • Isolated lepton with PT>20 GeV • Exactly 4 jets with PT>40 GeV • Reconstruct top selecting 3 jets with maximal resulting PT • Kinematic constrained fit assuming MW1=MW2 and MT1=MT2 Signal (MC@NLO,Pythia,Herwig) + background (W+4j-Alpgen) at initial phase of LHC  Top peak visible, minimal selection and reco (3-7 GeV) L = 150 pb-1 (few days low lumi) Sample for b-tagging and jet scale calib. studies Δσstat(t-tbar) ~ 2% (1 week) !!! Clara Troncon - ICHEP 2004

  10. Measure s( t t ) to < 10 % (stat.) with a few days of luminosity !! Could be first indication of new physics at the LHC (?) Many theoretical models include the existence of resonances decaying to t t SM Higgs (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, […] Reconstruct m( t t ) to search for resonances Χ if σΧ, ΓΧ and BR(Χ t t )predicted Study of semileptonic evts found : mass resolution 6% andreconstruction efficiency 20% mtt=400 GeV - 15% mtt=2 TeV xBR required for a discovery σxBR [fb] 30 fb-1 830 fb 300 fb-1 mtt [GeV/c2] 1 TeV Search for Resonances 1.6 TeV resonance Mtt Clara Troncon - ICHEP 2004

  11. 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 at LHC: 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] 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 EW Single Top Quark Production 1) Determination of Vtb 2) Independent mass measurement 3) Opportunity to measure top spin pol. 4) May probe FCNC +16.6 -3. 7 Wg [54.2 pb] Wt [17.8 pb] W* [2.2 pb] Pre -selection:  1 isolated lepton Pt>20 GeV  2 jets Pt>30 GeV  1 b-tagged jet Pt>50 GeV + Njets =2 or 3 Forward jet , pT>50 GeV (for Wg) N-bjet=1 (for Wt) or =2 (for W*) Clara Troncon - ICHEP 2004

  12. EW Single Top Quark Production • Can measure cross-sections for all 3 processes separately • important since each is sensitive to different kinds of possible new physics and diff. Vtb syst. • heavy W’  increase in the s-channel W* • FCNC gu  t  increase in the W-gluon fusion channel • For 30 fb-1, can measure Vtb with stat. error of 0.4% – 2.7% (dep. on process) • For W-gluon fusion, can measure predicted W and top helicity • sensitive to possible V+A contribution at level of few per cent Detector performance critical: fake l, b-tag, forward jets, low E jets • 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) Clara Troncon - ICHEP 2004

  13. Top Quark Couplings and Decays • Does the top quark behaves as expected in the SM? • Branching Ratios • Electric charge • Top spin polarization • Yukawa coupling to Higgs from t t H events Can be measured to <20% from t-tbar H production • ……. • According to the SM, top decays rather “uninteresting” • Br(t W b)  99.9%, Br(t  W s)  0.1%, Br(t  W d)  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 • Many Beyond SM models involve anomalous top couplings • Several possible rare decay modes (eg FCNC) have clear experiment signatures and, if observed at the LHC, would be evidence for new physics • FCNC decays highly suppressed (Br< 10-13-10-10) and 10-3 to 10-5 sensitivity Clara Troncon - ICHEP 2004

  14. Top Quark Couplings and Decays • Does the top quark behave as expected in the SM? • Branching Ratios ACHIEVABLE • Electric charge ACHIEVABLE with 10 fb-1 radiative tt events or t charge reconstruction • Top spin polarization ACHIEVABLE with 10 fb-1 5 statistical significance for a non-zero value in di-lepton and semi-leptonic events • Yukawa coupling to Higgs from t t H events ACHIEVABLE Can be measured to <20% from t-tbar H production • According to the SM, top decays rather “uninteresting” • Br(t W b)  99.9%, Br(t  W s)  0.1%, Br(t  W d)  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 • Many Beyond SM models involve anomalous top couplings • Several possible rare decay modes (eg FCNC) have clear experiment signatures and, if observed at the LHC, would be evidence for new physics • FCNC decays highly suppressed (Br< 10-13-10-10) and 10-3 to 10-5 sensitivity Clara Troncon - ICHEP 2004

  15. Top Quark FCNC Rare Decays • In the SM the FCNC decays are highly suppressed (Br<10-13-10-10) • Several models of Physics Beyond SM can give HUGE enhancements • FCNC can be detected through top decay or single top production • Sensitivity according to ATLAS studies of top decays : t  Zq(CDF Br<0.137, ALEPH Br<17%, OPAL Br<13.7%) • Reconstruct t  Zq  (l+l-)j • Sensitivity to Br(t  Zq) = 1.1 X 10-4 (100 fb-1) t  q(CDF Br<0.032) • Sensitivity to Br(t  q) = 1.0 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) Clara Troncon - ICHEP 2004

  16. Radiative top production Radiative top decay Top Charge determination ATL-PHYS-2003-035 • 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 Clara Troncon - ICHEP 2004

  17. Top Charge Determination ATL-PHYS-2003-035 • 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 • Yield of radiative photons allows to distinguish top charge 10 fb-1 One year low lumi events pT() Clara Troncon - ICHEP 2004

  18. e+/+ + top Top Spin Correlations ATL-PHYS-2002-024 ATL-PHYS-2003-012 • In SM with Mtop175 GeV, (t)  1.4 GeV » QCD • Top decays before hadronization, and so can study the decay of ‘bare quark’ • Decay products keep spin info • Substantial ttbar spin correlations predicted in pair production • Can study polarization effects through helicity analysis of daughters • Study with di-lepton and semi-leptonic events • Spin analyser: Leptonic: leptonHadronic: (W, b) or least energetic jet (lej) • Interesting angles: Θ1 (Θ2) : angle between chosen spin axis and spin analyzer direction in the t(t) rest frame. Spin axis is t(t) direction in the parton c.m.s. (helicity basis) φ : angle betweenspin analyzersdirection in the t(t) rest frame <CosΘ+ · CosΘ-> <CosΘ+ · CosΘ-> No helicity correlation With helicity correlation Clara Troncon - ICHEP 2004

  19. Also study spin correlations in semi- leptonic events Least energetic jet from W decay:  ~ 0.5 Results for S + B: 80500 S, S/B=15 with 10 fb-1 C(lej) = 0.21  0.015  0.04 = ~ 5 σ from 0 with 10 fb-1 Semi-leptonic analysis probe SM at 5 after 1 year at low lumi. Dileptonic analysis complementary and similar power. Top spin correlations ATL-PHYS-2002-024 ATL-PHYS-2003-012 TopReX 4.05(SM): LO spin correlation simulation Pythia 6.221(NC): hadronisation, fragmentation and decays with CTEQ5L structure function, ISR-FSR AlpGen: used for W+jets background Tauola+Photos 2.6: t decay and radiative corrections Atlfast 2.60: ATLAS fast simulation and reconstruction 30 fb-1 Clara Troncon - ICHEP 2004

  20. Conclusions LHC is top factory ( t t )~830 pb-1 + EW( t )~250 pb-1 107 events in first year • ATLAS will be able to provide many precise measurements of top quark properties • Precise determination of Mtop to 1 GeV seems achievable in 1 year of LHC • Crucial for EW physics, precision tests, constraints on Higgs sector, sensitivity to new physics • Confirmation that top-quark is SM particle and/or search for deviation from SM from: • Measure Vtb, charge, spin, decays ACHIEVABLE • First new physics in ATLAS may come from top quark analysis • Early top signals will also be critical to detector commissioning • Top peak should be visible with eyes closed and used for jet E cal, Et, b-tag,high PT e/m Lots of work remaining to be ready to fully exploit the top physics potential of the LHC (MC tuning, full sim., effect of real detector on studies - dead channels …) Clara Troncon - ICHEP 2004

  21. BACKUP SLIDES

  22. Top mass systematics Challenge: determine the mass of the top with~1 GeV accuracy in one year of LHC Clara Troncon - ICHEP 2004

  23. 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 mean prob. Mtop 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 Clara Troncon - ICHEP 2004 mass

  24. 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 Clara Troncon - ICHEP 2004

  25. 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 Clara Troncon - ICHEP 2004

  26. 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 Clara Troncon - ICHEP 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 Clara Troncon - ICHEP 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 Clara Troncon - ICHEP 2004

  29. 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) Clara Troncon - ICHEP 2004

  30. Rare decays: topWbZ G. Mahlon hep-ph/9810485 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(tWbZ)/G(tWb) M(top) (GeV) Clara Troncon - ICHEP 2004

  31. 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) Clara Troncon - ICHEP 2004

  32. 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 Clara Troncon - ICHEP 2004

  33. 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%?) Clara Troncon - ICHEP 2004

  34. 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 Clara Troncon - ICHEP 2004

  35. Top mass: Where we are In 2009 (if upgrade is respected) from Tevatron: DMtop = 1.5 GeV !! Clara Troncon - ICHEP 2004

  36. 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 !! Clara Troncon - ICHEP 2004

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