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Top Physics in ATLAS

Top Physics in ATLAS. M. Cobal, University of Udine Bologna, 12 Feb 2007. “t-quarks are produced and decay as free particles”. t had = L QCD -1 >> t decay. NO top hadrons. What do we know about the top quark?. The top quark completes the three family structure of the SM

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Top Physics in ATLAS

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  1. Top Physics in ATLAS M. Cobal, University of Udine Bologna, 12 Feb 2007

  2. “t-quarks are produced and decay as free particles” thad = LQCD-1 >> tdecay NO top hadrons What do we know about the top quark? • The top quark completes the three family structure of the SM • Its massive • Spin=1/2, • Charge=+2/3, • Isospin=+1/2 • tbW • Large G=1.42GeV (mb,MW,as,EW corr.) • Short lifetime dm/m <2% Not directly -4/3 excluded @ 94%C.L.(D0) Not directly ~100%, FCNC: probed at the 10% level ct<52.5mm @95%C.L.(CDF) The TEVATRON is probing better than ever the top sector… The LHC will allow precision measurements of Top Quark Physics

  3. The LHC tunnel at CERN 27 km diameter 14 TeV pp collisions

  4. Top quark production at the LHC Production: σtt(LHC) ~ 830 ± 100 pb  1 tt-event per second Cross section LHC = 100 x TevatronBackground LHC = 10 x Tevatron 90% 10% t Final states: t 1) Fully-hadronic (4/9) 6 jets 2) Semi-leptonic (4/9): 1l + 1ν + 4 jets 3) Fully-leptonic (1/9): 2l + 2ν + 2 jets t  Wb ~ 1W qq ~ 2/3W lν ~ 1/3 Golden channel (l=e,μ) 2.5 million events/year

  5. Em Calo Pb-liquid argon s/E ~ 10%/E ~1% uniform • Hadronic Calo Fe-scint Tile (10 l) s/E ~ 50%/E  0.03 • Muon Detectors • Magnets s/pT < 10 % at 1 TeV Toroide+Solenoid (4 magnets) in inner cavity Calorimeters outside field • Tracker Si pixels + strips TRD alows particle ID Solenoid B=2T s/pT ~ 5x10-4 pT  0.01 ATLAS Experiment

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

  7. Process #events 10 fb-1 2 3 4 Top quarks and search for new physics First year at the LHC: A new detectorANDa new energy regime Understand ATLASusing cosmics 1 2 Understand SM+ATLAS in simpletopologies 3 Understand SM+ATLASin complex topologies 4 Look for new physicsin ATLAS at 14 TeV

  8. 2008 should look something like… Hardware commissioning to 7 TeV Machine Checkout 1 month Commissioning with beam 2 months Pilot Physics 1 month Reach 1031 Provisional Running at 75 ns L~ 1032 cm-2s-1 ~ 3 months of running +some optimism ~ 1 fb-1

  9. How many events at the beginning ? Assumed selection efficiency: W l, Z ll : 20% tt  l+X :1.5% (no b-tag, inside mass bin) + lots of minimum-bias and jets (107 events in 2 weeks of data taking if 20% of trigger bandwidth allocated) 10 pb-1 1 month at 1030 and < 2 weeks at 1031,=50% 1 fb-1 Similar statistics to D0/CDF 100 pb-1 few days at 1032 , =50%

  10. Which detector performance on day one ? Based on detector construction quality, test-beam results, cosmics, simulation Expected performance day 1Physics samples to improve ECAL uniformity ~ 1% Minimum-bias, Z ee e/ scale ~ 2 % Z  ee HCAL uniformity ~ 3 % Single pions, QCD jets Jet scale < 10% Z ( ll) +1j, W  jj in tt events Tracking alignment 20(100)-200 m in R? Generic tracks, isolated m , Z mm Ultimate statistical precision achievable after few weeks of operation. Then face systematics…. E.g. : tracker alignment : 100 mm (1 month)  20mm (4 months)  5 mm (1 year) ?

  11. Top physics with b-tag information Top physics ‘easy’ at the LHC: S/B=O(100) Top signal Selection: Lepton Missing ET 4 (high-PT)-jets(2 b-jets) signal efficiency few %  very small SM background Number of Events W+jets background Top mass (GeV) • ‘Standard’ Top physics at the LHC: - b-tag is important in selection - Most measurements limited by systematic uncertainties • ‘Early’ top physics at the LHC: - Cross-section measurement (~ 20%) - Decay properties

  12. Missing ET > 20 GeV 1 lepton PT > 20 GeV 4 jets(R=0.4) PT > 40 GeV Top physics without b-tag information Still 1500 events/day • Robust selection cuts: Selection efficiency = 5.3% W CANDIDATE • Assign jets to W-boson and top-quark: TOP CANDIDATE 1) Hadronic top: Three jets with highest vector-sum pT as the decay products of the top 2) W boson: Two jets in hadronic top with highest momentum. in reconstructed jjj C.M. frame.

  13. 3-jet invariant mass 3-jet invariant mass electron+muon estimate for L=100 pb-1 Top-signal Events / 4.15 GeV Events / 4.15 GeV ATLAS preliminary Top-combinatoricsand W+jets background Mjjj (GeV) Mjjj (GeV) Results for a ‘no-b-tag’ analysis: 100 pb-1 100 pb-1 is a few days of nominal low-luminosity LHC operation We can easily see top peak without b-tag requirement Cut on MW

  14. Miscalibrated detector or escaping ‘new’ particle Events Perfect detector What can you do with early tops?  Calibrate light jet energy scale - impose PDG value of the W mass (precision < 1%)  Estimate/calibration b-tagging e - From data (precision ~ 5%) - Study b-tag (performance) in complex events  Study lepton trigger  Calibrate missing transverse energy - use W mass constraint in the event - range 50 GeV < p T < 200 GeV  Estimate (accuracy ~20%) of mt and tt. Use W boson mass to enhance purity Missing ET (GeV)

  15. Systematic Errors: Top mass reconstruction Selected 87000 signal events for L=10fb-1 (S/B~78) In-situ jet energy calibration (W→jj) Mass estimator via fit on spectrum ATLAS Eur.Phys.J C39 (2005) 63 s =10.6GeV Comb. (e=3.5%) Although errors are dominated by systematics It seems possible to determine mt @ 1GeV level (with L=10fb-1)

  16. Vtb Vtb Vtb Vtb Wt-channel t-channel s~ 250 pb s~70pb s~ 10 pb W* (s-channel) Single top @ LHC Electroweak top production Three different Processes (never observed yet) Powerfull Probe of Vtb ( dVtb/Vtb~few% @ LHC ) Probe New Physics Differently: ex. FCNC affects more t-channel ex. W´ affects more s-channel [ PRD63 (2001) 014018]

  17. FCNC kZtc=1 t-channel 4th generation,|Vts|=0.55, |Vtb|=0.835 (extreme values allowed w/o the CKM unitarity assumption) SM Top-flavor MZ’=1 TeV sen2f=0.05 Top-pion Mp±=450 GeV tR-cR mixing ~ 20% s-channel Single top and new physics T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018

  18. Cross Sections

  19. Theoretical errors at the LHC (Z.Sullivan, Phys.Rev. D70 (2004) 114012) Less than at Tevatron, since the x-region for the gluon PDFs is better known. Should be similat to the t-channel and to gg→tt

  20. 1 lepton, pT>25GeV/c High Missing ET 2 jets (at least 1 b-jet) Common feature: Single top production (ATL-COM-PHYS-2006-002) Separate Channels by (Nj,Nb) in final state: L=30fb-1 Stat: 7000 events (S/B=3) Syst: dominated by Eb-jet and Lum. Error Back: tt, Wbb and W+jets t-channel: (Nj=2,Nb=1) ( ds/s<1.5%) Stat: 4700 events, e~1% (S/B=15%) Wt-channel: ( ds/s ~ 4%) (Nj=3,Nb=1) Stat: 1200 events for tb (S/B=10%) Syst: Eb-jet, Lum. Error, back X-section Back_t-channel, tt s-channel: (Nj=2,Nb=2) (ATL-PHYS-PUB-2006-014) ( ds/s ~7-8%)

  21. Beyond the SM • non-SM production (Xtt)  resonances in the tt system  MSSM production  unique missing ET signatures from • non-SM decay (tXb, Xq)  charged Higgs  change in the top BR, can be investigated via direct evidence or via deviations of R(ℓℓ/ℓ)=BR(Wℓ) from 2/9 (H+,cs).  FCNC t decays: tZq tq tgq  highly suppressed in SM, less in MSSM, enhanced in some sector of SEWSB and in theories with new exotic fermions • non-SM loop correction  precise measurement of the cross-section  ttNLO-ttLO/ ttLO <10% (SUSY EW), <4% (SUSY QCD) typical values, might be much bigger for certain regions of the parameter space • associated production of Higgs  ttH

  22. Z’, ZH, G(1),SUSY, ? 500 GeV 600 GeV 400 GeV New physics: Resonances in Mtt • ts< 10-23 s  no ttbar bound states within the SM • Many models include the existence of resonances decaying to ttbar SM Higgs , MSSM Higgs, Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor • Resonances in Mtt • Structure in Mtt Gaemers, Hoogeveen (1984) Resonanceat 1600 GeV ATLAS # events Cross section (a.u.) Mtt (GeV) - Interference from MSSM Higgses H,A tt (can be up to 6-7% effect) Mtt (GeV)

  23. Resonances in a tt system Resolution m(tt) Study the detector sensitivity in an inclusive way: Resonanceat 1600 GeV  Study of a resonance Χ once known σΧ, ΓΧ and BR(Χ→tt)  Assume detector resolution > ΓΧ  Excellent experimental resolution in mass, ranging from 3% to 6% ! Reconstruction efficiency for the semileptonic channel:  20% mtt=400 GeV  15% mtt=2 TeV Δσ/σ ~ 6 % mtt (GeV) xBR required for a discovery fast-sim 5  Shown sensitivity up to a few TeV 1 TeV

  24. Although t and t are produced unpolarized their spins are correlated SM Mtt<550 GeV Error (±stat ±syst) q flq A 0.42 0.014 0.023 l+,n AD -0.29 0.008 0.010 t t 1 dN 1 ( 1 – ADaXaX´cosf ) = N dcosf 2 Top spin correlation s(tLtL) + s(tRtR) - s(tLtR) - s(tRtL) A= s(tLtL) + s(tRtR) + s(tLtR) + s(tRtL) SM: New Physics affects A Other angular distributions: aX=spin analysing power of X SM: (Eur.Phys.J.C44S2 2005 13-33) • Semileptonic + Dileptonic • Syst (Eb-jet,mtop,FSR) • ~4% precision

  25. SM Error (±stat ±syst) (Mt=175 GeV) Semilep. + Dileptonic F0 0.703  0.004  0.015 FL 0.297  0.003  0.024 FR 0.000 (mb=0)  0.003  0.012 Probing the Wtb vertex A) Test the tbW decay vertex Measure W polarization (F0, FL, FR) through lepton angular distribution in W cm system: L=10fb-1 (1/G)dG/dcos(ql*) • Syst ( Eb-jet,mtop,FSR ) • dF0/F0 ~ 2% ; dFR ~ 0.01 (Eur.Phys.J.C44S2 2005 13-33)

  26. A- AFB A+ cos(ql*) Probing the Wtb vertex B) Anomalous Couplings in the tbW decay (PRD67 (2003) 014009, mb≠0) Angular Asymmetries: AFB, A+ and A- AFB [t=0] A± [t= (22/3-1)] ± SM(LO):

  27. SM(LO): rL=0.423 rR=0.0005 (mb≠0) 1s Results: Probing the Wtb vertex L=10fb-1 B) Anomalous Couplings in the tbW decay

  28. Top quark FCNC decay • GIM suppressed in the SM • Higher BR in some SM extensions (2-Higgs doublet, SUSY, exotic fermions) • 3 channels studied:

  29. Results • BR 5s sensitivity • Expected 95% CL limits on BR (no signal) • Dominant systematics: MT and etag < 20%

  30. Present and future limits Topological likelihood for three channels Resulting 95% CL limits t → qZ SM bck signal t → qg t → qg  With 10 fb-1 already 2 orders of magnitude better than LEP/HERA

  31. Conclusions 1) Top quarks are produced by the millions at the LHC: Almost no background: measure top quark properties 2) Top quarks are THE calibration signal for complex topologies:  Most complex SM candle at the LHC  Vital inputs for detector operation and SUSY background 3) Top quarks pair-like events … window to new physics: FCNC, SUSY, MSSM Higgses, Resonances, … DAY-2 top physics: - Single top production - Top charge, spin(-correlations), mass

  32. Backup

  33. CMS # events Combined b-tagging discriminator b-jet identification efficiency • B-jet identification efficiency:Important in cross-section determination and many new physics searches (like H, ttH) • A clean sample of b-jets from top events 2 out of 4 jets in event are b-jets (a-priori) Use W boson mass to enhance purity B-jet sample from top quark pairs: - Calibrate b-tagging efficiency from data (~ 5%) Dominant systematic uncertainty: ISR/FSR jets - Study b-tag (performance) in complex events Note: Can also use di-lepton events

  34. light jet energy scale • Light jet energy scale calibration (target ~1%) Invariant mass of jets should add upto well known W mass (80.4 GeV) Purity = 83%Nevt ~ 2400 (1 fb-1) Rescale jet energies:Eparton = (1+ ) Ejet, with =(PT,η) σ(Mjj)~ 8 GeV Pro: - Complex topology, hadronic W - Large statisticsCon: - Only light quark jets - Limited PT-range (50-200 GeV) # events MW (PDG) = 80.425 GeV Precision: < 1% for 0.5 fb-1Alternative: PT-balance in Z/γ+jet (6% b-jets) Mjj (GeV)

  35. PT cut = 40 GeV All jj combinations Only 2 light jets Only 2 light jets + 150 < mjjb < 200 mjj (GeV) t  W  jj to calibrate the light JES • Standard tt  lnb jjb selection cuts • Improve W  jj purity by requiring: • 2 light jets only • 150 < mjjb < 200 GeV  Purity ~ 83 %, ~ 1200 W selected for 500 pb-1 Etienvre, Schwindling Number of jj for 491 pb-1: (% purity as fraction of cases with 2 jets at DR < 0.25 from 2 W quarks)

  36. MW(had) MW = 78.1±0.8 GeV Events / 5.1 GeV S/B = 0.5 Jet energy scale (no b-tag analysis) Determine Light-Jet energy scale • (1) Abundant source of W decays into light jets • Invariant mass of jets should add up to well known W mass (80.4 GeV) • W-boson decays to light jets only  Light jet energy scale calibration (target precision 1%) t t Translate jet 4-vectors to parton 4-vectors

  37. Search strategies for H±tb • Resolving 3 b-jets: inclusive mode • LO production through gb tH± • Large background from tt+jets • High combinatorics • Resolving 4 b-jets: exclusive mode • LO production through gg tH±b • Smaller background (from ttbb and ttjj+ 2 mistags) • Even higher combinatorics • Both processes simulated with Pythia; same cross section if calculated at all orders • gbtH±: massless b taken from b-pdf • gg tH±b: massive b from initial gluon splitting • Cross sections for both processes as the NLO gbtH±: cross section

  38. Search for 4 b-jets • Signal properties • Exponential decrease with mA • Quadratic increase with tanb in interesting region tanb > 20 • Final state: bbbbqq’ln • Isolated lepton to trigger on • Charged Higgs mass can be reconstructed • Only final state with muon investigated • Background simulation • ttbb • ttjj • (large mistag rates, large cross section) • b’s from gluon splitting passing theshold of ttbb generation)

  39. Significance and Reach • Kinematic fit in top system • Both W mass constraints • Both top mass constraints • Neutrino taken from fit • Event selection and efficiencies 4 4

  40. Significance and Reach • Significance as function of cut on signal-background • Due to low statistics interpolation of number of background events as function of number of signal events • Optimization performed at each mass point

  41. H±tb • Fast simulation • 4 b-jets analysis • No systematics (apart uncertainty on background cross sec) • Runninng mb • B-tagging e static L = 30 fb-1

  42. ttH The Yukawa coupling of top to Higgs is the largest.  It is a discovery mode of the Higgs boson for masses less than 130 GeV  Measuring the coupling of top to Higgs can test the presence of new physics in the Higgs sector  Very demanding selection in a high jet multiplicity final state 0.7 pb (NLO) mH=120GeV ttjj: 507 pb ttZ: 0.7 pb ttbb: 3.3 pb

  43. Higgs boson reconstruction  Reconstruct ttH(h)  WWbbbb  (l)(jj)bbbb  Isolated lepton selection using a likelihood method  Jet reconstruction: 6 jets at least, 4 of which b-tagged  Reconstruct missing ET from four-momentum conservation in the event (+W mass constraint in z)  Complete kinematic fit to associate the two bs to the Higgs (can improve the pairing efficiency to 36%, under investigation) results can be extrapolated to MSSM h gttH/gttH~16% for mH=120 GeV hep-ph/0003033

  44. Probabilistic approach • Preselection • General criteria: • ≥ 1 lepton (pT > 25 GeV and |h| < 2.5) • ≥ 2 jets (pT > 20 GeV and |h| < 2.5) • Only 1 b-tagged jet • ETmiss > 20 GeV • Events classified into different channels (qZ, qg or qg) • Specific criteria for each channel • After the preselection, probabilistic analysis:

  45. tqZ • Specific criteria: • ≥ 3 leptons • PTl2,l3 > 10 GeV and |h|<2.5 • 2 leptons same flavour and opposite charge • PTj1 > 30 GeV • 453.8 backgnd evts,e x BR = 0.23% L = 10 fb-1 Mjl+l- Mlnb

  46. tqg • Specific criteria: • 1 photon • PT > 75 GeV and |h|<2.5 • 20 GeV < mgj < 270 GeV • < 3 leptons • 290.7 backgnd evts,e x BR = 1,88% L = 10 fb-1 Mgj PTg

  47. tqg • Specific criteria: • Only one lepton • No g with PT > 5 GeV • Evis > 300 GeV • 3 jets (PT1 > 40 GeV, PT2,3 > 20 GeV and |h| < 2.5) • PTg > 75 GeV • 125 < mgq < 200 GeV • 8166.1 backgnd evts,e x BR = 0,39% L = 10 fb-1 Mlnb Mgq

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