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Top Physics Results from the ATLAS E xperiment

First Results w ith 2011 Data!!. Top Physics Results from the ATLAS E xperiment. Marina Cobal University of Udine / INFN Udine On behalf of the ATLAS Collaboration. PLHC2011 Perugia 6-10 June 2011. LHC status. 2010 / 2011 @ 7 TeV L peak = 2.1x10 32 / 1.3x10 33 cm -2 s -1

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Top Physics Results from the ATLAS E xperiment

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  1. First Results with 2011 Data!! Top PhysicsResultsfrom the ATLAS Experiment Marina Cobal University of Udine / INFN Udine On behalf of the ATLAS Collaboration PLHC2011 Perugia 6-10 June 2011

  2. LHC status • 2010/2011 @ 7 TeV • Lpeak = 2.1x1032/1.3x1033 cm-2s-1 • ∫Ldt ~ 45 / ~736pb-1 • Pile-up: <nvtx> = 2 / 6<nvtx> • Lumi uncertainty: 3.4%/4.5% • Very high ATLAS data taking efficiency(93.7%) and fraction of good quality data from all sub-detectors 2010 ATLAS recorded 45 pb-1 2011 ATLAS recorded 736 pb-1

  3. Top quark pair production @ LHC - • stt(7 Tev LHC) = 165+11-16 pb • @ 172.5 GeV, Moch, Uwer, Langenfeld (Phys. Rev. D78 (2008) 034003, arXiv:0907.2527) = 25 x stt (Tevatron) • Few fb-1 expectedbyendof 2011 • 500k ttpairs(6 x Tevatron) - - - LHC Tevatron Dominant @ LHC: ≈80% M.Cacciari, S. Frixione private communication Nason, Dawson, Ellis, 1988

  4. Resultscovered • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults • Top quark production cross section(35 pb-1, 2010 data) • Single leptonpretag(ATLAS-CONF-2011-023) • Single leptonbtag(ATLAS-CONF-2011-035) • Dilepton (ATLAS-CONF-2011-034) • Combination (ATLAS-CONF-2011-040) • Allhadronicchannel(ATLAS-CONF-2011-066) • Top quark properties(35 pb-1, 2010 data) • Top quark mass (ATLAS-CONF-2011-033) • Whelicity (ATLAS-CONF-2011-037) • tt+ anomalousETmiss (ATLAS-CONF-2011-036) • Searchfor FCNC top quark processes(ATLAS-CONF-2011-061) • Mass from the ttcross section(ATLAS-CONF-2011-054) • Single top (ATLAS-CONF-2011-027) • Single top t-channel (ATLAS-CONF-2011-088) • Search for ttresonances (ATLAS-CONF-2011-087) Adam Roe, Thursday 4F - V. Dao - New R. Schwienhorst - D. Cinca, Friday SF

  5. - σttsingle lepton (no b-tagging) • Fit of projective likelihood based on uncorrelated variables • Three variables chosen: • Binned maximum likelihood to 4 channels (3-jets, ≥4-jets; e,µ) Leptoncharge m + jets Leptonh m + jets exp(-8*aplanarity) m + jets uncert. ~15% • Independent from b-tagging: • Avoids related systematics, but worse S/B • Cross checked by cut&count and 1 D-c2 likelihood - σtt = 171 ± 17(stat) +20-17(syst) ± 6(lumi) pb

  6. - σttsingle lepton (b-tagging) • Multivariate method → split in 6 channels (=3,=4,≥5 jets; e/µ) • Input variables: lepton η, exp(-8*aplanarity), exp(3*HT, 3p ), b-tag weight • Profile likelihood fit extracts 16 parameters including σtt • Main systematics: • W+jets HF content (7%) • Tagger calibration (7%) - uncert. ~13% σtt = 186 ± 10(stat) +21-20(syst) ± 6(lumi) pb

  7. Single leptonanalysissummary • Complementary and consistent results obtained from various analyses with and w/o b-tag info • Baseline and complementary cross sections are in agreement within the uncertainties • Consistent with theoretical prediction (165+11-16pb) Single leptonw/o btag Single lepton with btag

  8. - σttdilepton Can also simultaneously extract: • σttbar/σZ, lumi uncertainty cancels in ratio • σttbar, σWW, σZ→ττfrom fit ETmissvsNjets • σttbar and b-tagging efficiency • Cut-based method • require 2 OS high PT leptons, 20 GeV • at least two energetic jets, 20 GeV • Fake rate and Z+jets dominating background • ee/µµ: ETmiss > 40 GeV and |mll – mZ| > 10 GeV • eµ: HT > 130 GeV • MC-assisted data-driven estimation of remaining Z+jetsbackgnd • Uncertainty from ~100% to <50% • Fake lepton rate determined in a data driven way • matrix methos) • Main systematics • Jet energy scale (5%) • Parton shower model (5%) • Fakes (4%) Z mass low ETmiss

  9. - σttdilepton Stransverse mass: generalization of mTconcept with two neutrinos • Cross-section extraction • Profile likelihood method to combine channels • Events in SR are compatible with top quarks σtt = 173 ± 22(stat) +18-16 (syst) +8-7(lumi) pb - 105 events selected, 101 ± 9 expected Data well modeled by MC+DD methods HT: sum of lepton and jet momenta Distribution after requiring 1 b-tag uncert. ~17% • Additionally requiring • b-tag improves S/B - σtt = 171 ± 22(stat) +21-16 (syst) +7-6(lumi) pb

  10. - sttcombination • Combined dilepton and single lepton channels • Single lepton with b-tag, dilepton w/o b-tag • Statistical uncertainty ~4% • Systematic uncertainty ~8% • Luminosity uncertainty ~3% stat stat+sys - σtt = 180 ± 9(stat) ± 15(syst) ± 6 (lumi) pb δσ = 10%

  11. - stt summary • stt in full hadronicchannel • Challengingmeasurement: - Measured < 261 pb @ 95% CL Expected < 314 pb @ 95% CL

  12. Top quark mass • Single lepton selection • Main aim: reduce JES systematic • Template fit to R32 = mjjb(t) / mjj(W) • Systematics: (b-)JES , ISR/FSR • Cross-checked by • kinematic fit templates • 2D templates with Jet Scaling Factor µ+jets mtop = 169.3 ± 4.0(stat) ± 4.9(syst) GeV µ+jets µ+jets

  13. - mtop from stt • Use most accurate measurement • Multivariate analysis with b-tag • Exploit dependence of stt from mtop • Assume mtopMC = mtoppole • mtop from combineduncorrelatedth. and exp. likelihood: maxdeterminesthe extractedmtoppole • fitperformed for 3 theoreticalcalculations • 13% uncertaintyon stt~ 5 GeVuncertainty on mtop • Default analysis → NNLO Langenfeld • (Phys. Rev. D80 (2009) 054009) - - mtop = 166.4+7.8-7.3GeV

  14. New Result!! 156 pb-1 Single top: t-channel • Single lepton NN analysis (check with cut&count) • Background: QCD multijet, W+jets • 22 variables • mostimportant: Mlnb by M(jet1, jet2) σt = 76+41-21pb Statistical uncertainty (17%) B-tagging (+12-9 %), JES((+23-2 %) SignalModelling (+36-20 %) • Observed (Expected) Significance: 6.2 (5.7) s

  15. Single top: Wt-channel • Wt-channel: cut-based analysis • single lepton+dilepton • SM predicts sWt = 15 pb • Combine dileptonand l+jetschannel • Include systematic uncertainty correlations • Dominant: JES, b-tag modeling, ISR/FSR, MC stat • Expected (observed) limit on σWtcorrespondsto6.3 (10.5) * σWtSM σWt < 158 pb at 95%

  16. Whelicity • Wtbstructureprobed → set limits on new physics • V-A couplingpredicts: • F0 ≈ 0.7 (long. polarization) • FR≈ 0 (RH polarization) • FL≈ 0.3 (LH polarization) • Use e+jets and µ+jets channels • Can extract directly from cosθ* or unfold to parton level and calculate asymmetry b

  17. Whelicity • V-A couplingpredicts: • F0≈0.7 (long. Polarization) • FR≈ 0 (RH polarization) • FL≈ 0.3 (LH polarization) • Wtbstructureprobed→ set limitson new physics • Use e+jets and µ+jets channels • Can extract directly from cosθ* or unfold to parton level and calculate asymmetry FL F0 FR b

  18. tt + anomalous ETmiss • Search for anomalous ETmissin ttbar (lepton+jets) events • benchmark: TT pair, TtA0 • A0 dark matter candidate • Enhanced cross-section due to spin states • Signal region: • ETmiss>80 GeV, mT>120 GeV; dilepton veto: pT > 15 GeV, tracks, loose electrons Alwall, Feng, Kumar et al.(2010) Berger, Cao (2009) Exclude MT< 275 (300) GeV For MA0< 50 (10) GeV Observed: 17 evts Expected 17.2 ± 2.6 Observed 17

  19. FCNC in decay • Look for tZqvertex (q =u,c): leptonicW/Zdecays to suppress QCD background • Cut & Count: • 3 leptons (> 25 GeV, > 20 GeV, > 15 GeV), twosameflavor and OS • 2 jets ( >30 GeV, >20 GeV), ETmiss>20 GeV Expected BR(tqZ) < 17% Observed BR(tqZ) < 12‐3+4% @ 95% CL

  20. FCNC in production • look for gqtvertex • anomalous single top production • Single leptonEvents • exactly1 b‐taggedjet • exactly1 lepton (e/μ) • Neural Network with 13 input variables+ binnedlikelihood ratio • No excessobserved: limiton σqg*BR(tWb) • Systematics: ISR, JES, HF content in W+jets Observedσqgt*BR(tWb) < 17.3 pb Expected σqgt*Br(tWb) < 17.4‐5.4+8.2 pb

  21. - New Result!! 200 pb-1 Search for high mtt • In manynew physicsmodelstop plays a special role • Narrowresonance: leptophobictopcolorZ' as benchmark model • Wide resonance: a Kaluza-Klein gluon gKK, which appears in Randall-Sundrummodels • mttfrom 3 or 4 jets, e/m, and n • DRminvariant (4 highestpTjetsconsidered, jet removediftoo far from otherobjects and hastoo high mass) • Normalizationuncertainties • W(35%), diboson (5%), QCD in e (30%) and m (50%) channels • Shapeuncertainties • b-tag (11%), JES (9%), ISR/FSR(7%) -

  22. - Search for high mtt For a narrowZ’, observed95% C.L. limitsrange from ≈38 pbto 3.2 pbfor massesfrom MZ’=500 GeVto 1300 GeV. In Randall-Sundrummodels, KK gluonswithmassesbelow 650 GeVexcludedat95% C.L.

  23. Conclusions • Top physicsin ATLAS just started. With only 35 pb-1 of data: • Cross sectionmeasurementalreadycompetitive ~10% uncertainty • Large number of measurementsalreadyperformed • New resultspresented on single top and Mttbar with 2011 data • Statisticlimitedanalysiswillsoonbecomeverycompetitive Already ≈0.7 fb-1 recorded . 1 fb-1 coming soon! • Focus to reduce impact of systematics • Improve even further detector understanding • Use advanced analysis techniques • 2011 year of precision → more top measurements and maybe discoveries! • Cross sections in various channels, differential cross sections and top mass • Charge asymmetry, spin correlation, top charge • t’t’ , same sign top, W’ search in single top

  24. Single top: t-channel To be updated • Single lepton cut based (check with likelihood) • 1 lepton, 1 b-jet, 1 light jet, ET mi ss>25 GeV • Background: QCD multijet, W+jets • Final cut M t o p ∈ [130, 210] GeV, h | (u-jet)| > 2.5 • Likelihoodratio using: cosDf(lep,Etmiss), DR(bjet,lep), h(untagged jet), reconstructedMtop σt = 53+27-24(stat) +38-27(syst) = 53 +46-36pb • Significance: 1.6

  25. Search for high Mttbar To be updated • Search for massive neutralbosonsinto a ttbarpair • leptophobictopcolorZ' as benchmark model (hepph: 9911288) • Search for anomaloushigh mass tt+ X production → • Quantum Black Holesas benchmark model (JHEP 0805(2008)003) • M reconstructionusing3 differentmethods. • DRminvariant (4 highestpTjetsconsidered, jet removediftoo far from otherobjects and hastoo high mass). M build from lepton, nand ¾ remainingjets • Mainsystematics: JES, JER, btagging • Bayesianapproachusingbinnedlikelihood to the Mttbar • No evidence for resonancefound Exclude MQBH< 2.35 TeV@ 95% CL Observed 95% CL limitfrom 55(2.2)pb at M=500(2000) GeV

  26. The Top quark: from theory… • Weakisospin partner of b-quark • Spin ½ fermion, charge 2/3 • Produced in pairs via qqannihilationor gg fusion • Lifetime~ 5 . 10-25 s • Decays (BR ~ 100%) in Wb Most precise predictions of production cross sectionby approximateNNLO calculations, precisionof 6% to 9% : For Mtop= 172.5 GeV/c2, tt~xxx

  27. …to experiment • Mass known with a precision < 1% • Top properties (charge, width, …) deeplyinvestigated • Single top identified • Top samplesnowland for searches for new physics Discoveryatthe Tevatron 1995 (67 pb-1) 2011 ( > 5 fb-1 analyzed), 10 fb-1by the end of RunII)

  28. Top decay

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