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“Top Physics studies at LHC in SM and BSM with the ATLAS detector”

“Top Physics studies at LHC in SM and BSM with the ATLAS detector”

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“Top Physics studies at LHC in SM and BSM with the ATLAS detector”

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  1. “Top Physics studies at LHC in SM and BSM with the ATLAS detector” DISCRETE '08Symposium on Prospects in the Physics of Discrete Symmetries 11–16 December 2008, IFIC, Valencia, Spain SUSANA CABRERA (IFIC) On behalf of the ATLAS collaboration

  2. OUTLINE • ATLAS PROSPECTS ON (1) : • SM: • L=100 pb-1: σ(pptt). • L=1 fb-1: M TOP , σ(single top). • BSM: L=1 fb-1 • W polarization and Wtb anomalous couplings. • Rare Top quark decays FCNC • Resonant tt-bar production Z´tt • Brief current status from TEVATRON (2) and (3) • ATLAS Collaboration, Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics, CERN-OPEN-2008-020, Geneva, 2008, to appear. • (*) All the information presented in this talk is PRELIMINARY. • (**) All results presented here evaluated at √s=14 TeV • http://www-cdf.fnal.gov/physics/new/top/top.html (CDF Top quark Physics results) • http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_public.html (DØ Top quark Physics results)

  3. σtt (SINGLE LEPTON CHANNEL) L=100 pb-1 tt(LHC) = 883.90 pb(Phys. Rev. 2008, D78, 034003 ) • Triggers: e/μ isolated & high pT • Dominant backgrounds: W+jets & Single top • Excellent sample for: • Commissioning of b-tagging algorithms. • Determination of the light jet energy scale using the decay Wjj. ≥ 4 jets pT >20 GeV & ≥ 3 jets pT> 40 GeV PT(lep) > 20 GeV No b-tagging Missing ET > 20 GeV

  4. σtt (SINGLE LEPTON CHANNEL) TT-BAR RECONSTRUCTION Hadronic Top Candidate: j1,j2,j3 / ΣpTmax Hadronic W candidate: j1,j2 / ΣpTmax S/B~2.2 • |m(jj)-mW|<10 GeV N b-tag =1,2 S/B~10 (Nb-tag=1) S/B~21 (Nb-tag=2)

  5. σtt (SINGLE LEPTON CHANNEL) METHODS AND SYSTEMATICS Gauss. Chebychev • Counting method: • Very sensitive to the uncertainty in the total background rate, especially W+jets. • Likelihood fit method: mjjj of hadronic top candidate • Sensitive to the uncertainty in the tt-bar reconstruction efficiency

  6. σtt (DI-LEPTON CHANNEL) L=100 pb-1 : 3 METHODS. 2 isolated leptons, Etmiss > 30 GeV, 85 < M ll <96 GeV MAIN BACKGROUNDS: ttbar(semileptonic)(17%) Zee/μμ/ττ,(57%) W(e/μ+ν)+jets, WW,WZ,ZZ,single top. “CUT & COUNT” METHOD “INCLUSIVE TEMPLATE” METHOD: 2D binned likelihood: Etmiss vs N jets tt WW Z

  7. σtt (DI-LEPTON CHANNEL) L=100 pb-1 : SYSTEMATICS. “LIKELIHOOD METHOD”: Df(l,Etmiss) Df(l,MET) ‘signal’ Df(l,MET) ‘background’ Df(l,MET) ‘data’ • Overview of systematic uncertainties • Jet energy scale largest contributor(Template method has included systematic uncertainties into fit)

  8. σttATLAS PROSPECTS L=100 pb-1 VS CDF L=2.8 fb-1 CDF L=2.8 fb-1 Δσ/σ(stat)=4% Δσ(syst)/σ=6% Δσ(lumi)/σ=6%Δσ/σ=9%

  9. MTOP ATLAS L=1fb-1 χ2 minimization method (I) • General strategy: • Single lepton channel: 4 jets, p T >40 GeV, 2 b-jets • Select the hadronic W candidate within a mass window of ±3 σMjj (30 GeV) around the peak value of the dijet mass distribution in events with only two light jets. • For each preselected light jet pair in the event, perform a χ2 minimization: • For each event, the hadronic W boson candidate is taken as the pair of light jets with minimum χ2 . Only events with: |Mjj- MWPDG| ≤ 2 ΓMw PDG (4.2 GeV) are kept. • Advantage: Event per event in-situ rescaling of the light jet energy scale reduces miscalibrations in the default jet energy scale and JES systematics that affects MTOP Hadronic W reconstruction: χ2 minimization

  10. MTOP ATLAS L=1fb-1 χ2 minimization method (II) Hadronic Top reconstruction: b-jet closest to the hadronic W boson • Combinatorial background rejection: • Increases the purity of MTOP for high luminosity. • C1: M(Whad,blep) > 200 GeV • C2: M(lepton,blep) < 160 GeV Fit to gaussian +3rd order polinomial Mtop=175.0±0.2 GeV σMtop=11.6±0.2 GeV Mtop=174.8±0.3 GeV σMtop=11.7±0.4 GeV MC tt-bar with MTOP=175 GeV/c2

  11. MTOP ATLAS L=1fb-1 Geometric method • General strategy: • Hadronic W boson reconstruction: • Choose the light jet pair with the smallest angular distance ΔR = sqrt( Δη2 + Δφ2) • Keep only events within a mass window of ±σMjj ( 20 GeV) the peak value of the Mjj distribution. Add C1 and C2 cuts to increase purity. • Hadronic Top quark reconstruction: b-jet closest to the hadronic W boson. • Rescaling: Mjjb –Mjj+Mjj peak to remove at 1st order the contribution of light jets to the top mass resolution. C1+C2+ rescaling C1+C2 Fit to gaussian + threshold function Mtop=174.6±0.5 GeV σMtop=11.1±0.5 GeV Mtop=175.4±0.4 GeV σMtop=10.6±0.4 GeV MC tt-bar with MTOP=175 GeV/c2

  12. CONCLUSIONS MTOPATLAS L=1fb-1 vs TEVATRON • The best estimator of MTOP is the invariant mass of the hadronic top candidate in the single lepton channel. • In-situ determination of the light jet energy scale using the sample of hadronic W candidate extracted from the tt-bar sample. • The systematic uncertainty will come from the b-jet energy scale, measured using data driven methods. • In high luminosity scenario: stringent selection requirements to reduce combinatorial background and thus systematics. • With L=1 fb-1 , if JES uncertainty is in the range 1-5% then a precision of 1-3.5 GeV should be achievable in MTOP . • RUN I & II TEVATRON (CDF+DØ) arXiv:0898.1089v1 MTOP = 172.4 ± 1.2 GeV/c2 ΔMTOP /MTOP ~ 0.7%

  13. SINGLE TOP PRODUCTION: FROM TEVATRON TO LHC t-channel • Tevatron claimed evidence of s+t channels: • CDF: L=2.2 fb -1  3.7 σ (4.9 σ expected) (arXiv:0809.2581v2) • Vtb = 0.88 +0.13-0.12 (stat+syst) ±0.07 (theo) • |Vtb |>0.66 @ 95% CL • DØ: L=0.9 fb-1 3.4σ(Phys.Rev.Lett.98:181802,2007.) • 5σobservation by Tevatron is around the corner: but measurements statistically limited. • LHC will have higher statistics. s-channel σTEVATRON σLHC Wt (1) Sullivan,Z.,PRD 70 (2004)114012, Campbell et al. PRD 70(2004)094012 (2) Campbell, Tramontano, Nucl. Phys. B726(2005) 109-130 (3) Moch and Uwer (Phys. Rev. 2008, D78, 034003 [arXiv : 0807.2794])

  14. SINGLE TOP SIGNATURES AT THE LHC In all channels : 1 high pT (~70 GeV) b-jet 1 W boson from the top quark decay t-ch (qtqbWqblν) σ.BR=26 pb (x2) 2 extra forward jets: b-jet and light jet (often very forward) Leptonic decay of W only : BR(W(τ)e/μ) = 25.4%  1 high-pT e or μ + missing ET s-ch (btbbWbblν) σ.BR=0.74 pb (x4) 1 second central b-jet Leptonic decay of W only : high-pT e or μ + missing ET Associated Wt production 1 second W boson - tW: 1 lepton: (tWbWWbqq´lν) : σ.BR=0.74pb (x4) 2 jets, 1 lepton, missing ET - tW: 2 leptons: (tWbWWblνl´ν´): σ.BR=1.1 pb (x2) 2 leptons, some missing ET, no extra jets not considered yet

  15. T-channel: MVA technique BDT BDT cut Leptonic Top mass M(lνb) • M.V.A techniques to reduce W+jets background at low pT and tt-bar background at high pT • Specific BDT against tt-bar with variables less sensitive to JES, for instance: • p T and cos(θ*) of leading jet • Centrality(j1,j2),HT(j1,j2,MET,l),MT(W) • ΔR(j1,j2),ΔR(j1,lep),ΔR(j1non-b,l)

  16. EVENT YIELD: Sequencial CUT versus MVA (1fb-1 ) Wt • t channel is the most promising one for single top observation at the LHC. • Statistical significance is good for t-channel and Wt • S/B is better than unity only for t-channel and BDT technique.

  17. Top quark decay: W polarization measurements. • F0,F- and F+ := fractions of W-bosons produced from Top decays with the 3 helicity states: “longitudinal”, “right-handed” and “left-handed” (F0+F-+F+=1) [ Phys. Rev. D45 (1992) 124] • Correction to recover parton level after background substraction to account for distortions: particle radiation, quark fragmentation, final event reconstruction, detector acceptance and resolution. Fit 3rd pol. -0.9<cosΨ<0.9  derive event-by-event correcting weight

  18. W polarization: ATLAS prospects vs TEVATRON status. CDF Combination L=2 fb-1 (10% improvement): F0 = 0.66 ± 0.16  ΔF0/F0~24% F+ = -0.03 ± 0.07

  19. W polarization and anomalous Wtb couplings. • W polarisation sensitive to new anomalous couplings (VL,VR,gL,gR) • (taken real if CP is conserved) • In SM: VL ≡ V tb ≈ 0.999 and (V R,gL,gR ) vanish. • F0,F+,F- set bounds to (VL,VR,gL,gR). • More sensitive varibles: • ρR,L ≡ ΓR,L/Γ0 = F+,- / F0 • Simpler variables: • AFB , z=0; AFB = (3/4) ( F+ - F- ) • A± = ± 3 β [ F0 + (1+β) F± ] • β = 21/3 -1 • - z = ± ( 22/3 -1) Phys. Rev. D67 (2003) 014009; Eur. Phys. J. C50 (2007) 519 Z

  20. ATLAS PROSPECTS FOR ANOMALOUS Wtb COUPLINGS • L= 1 fb-1  Systematic uncertainties dominate all measurements • Constrains on the anomalous couplings using TopFit program • J.A. Aguilar-Saavedra et al. Eur.Phys. J. C50: 519-533,2007. • Expected precisión in anomalous couplings: • ΔVR /VR≈0.15, ΔgL /gL≈0.07, ΔgR /gR≈0.15

  21. TOP QUARK RARE DECAYS AND FCNC • SM: Top quark FCNC decays highly suppressed O(10-14) • BSM: SUSY models and Two Higgs doublet models: BR(tFCNC)~O(10-4)

  22. ATLAS prospects for Rare Top decays and FCNC. • Expected 95% CL limits: CDF Run 1: L=110 pb-1 BR(tqγ)<3.2%, BR(tqZ)<33% Phys. Rev. Lett. 80, 2525 - 2530 (1998) CDF Run 2: L=1.9 fb-1 BR(tqZ)<37% @ 95% C.L.

  23. Model independent search for “generic” narrow resonances decaying to tt-bar. Search for deviations in dσ/dMtt-bar using a counting method with sliding window: Window width is twice the detector resolution Standard reconstruction tt-bar efficiency decreases with M Z´ . At M Z´ > 1 TeV, boosted top quarks decaying hadronically produce collimated jets New reconstruction techniques for hadronic top “monojets” in development. Access to jet sub-structure with KT algorithm. Need a specific cell-level calibration. Adequate b-tagging performance for high pT jets. SEARCH FOR tt-bar RESONANCES in ATLAS Mtt @ 1 fb-1 SM+Z´ MZ´ =700 GeV/c2 σZ´ .Br=11pb ΓZ´ =16 GeV

  24. 5σ (stat+syst)sensitivity: Limit on σZ´ vs M Z´ σZ´.Br > 11 pb for MZ´ =700 GeV/c2 @ L= 1 fb-1 Main systematic on the discovery potential: an error in the reconstruction efficiency of 16.6% produces an effect of 8% in the discovery potential. Application to a specific model: DØ (CDF): In a top-color-assisted technicolor model, a leptophobic Z´ with M Z´ < 760 GeV is excluded @ 95% CL ATLAS SENSITIVITY TO RESONANT TOP PAIR PRODUCTION.

  25. CONCLUSIONS • Top Quark physics will play an important role in the early days of data taking of the ATLAS experiment at the LHC. • With only L=100 pb-1, σ(tt-bar) can be measured with an uncertainty (5-10)% dominated by systematics, apart from the luminosity uncertainty. • ATLAS will get this early data probably at √s=10 TeV, or perhaps even lower: prospects are being reevaluated. • Moch and Uwer (Phys. Rev. 2008, D78, 034003 [arXiv : 0807.2794]) • W+jets: roughly, 30% lower cross section. • t channel is the most promising one for single top observation at the LHC. Observation with 5σ may be possible for ~1fb-1. • With L=1 fb-1 , if JES uncertainty is in the range 1-5% then a precision of 1-3.5 GeV should be achievable in MTOP measurement. • The study of top properties may provide a hint of new physics.

  26. BACKUP

  27. PRESELECTION & SPECIFIC t-Ch,s-Ch & Wt SELECTION. • Single electron or muon (20 GeV threshold) trigger • 1 isolated electron or muon, pT>30 GeV, |η|<2.5 • No extra lepton with pT>10 GeV, |η|<2.5 • Missing ET > 20 GeV • 1 b-tagged jet, pT > 30 GeV, |η|<2.5 (b-tag eff ~60%, rej ~ 100) • At least 1 extra jet, pT > 30 GeV, |η|<5 • b-tagged jet cut: pT > 50 GeV • Forward light jet with |η|>2.5 • 2nd b-tagged jet with pT > 30 GeV • Veto on extra jet with pT > 15 GeV • Topologial cuts: • 0.5<ΔR(b1,b2)<4 • 80 < HT(jets) < 220 GeV • 60 <missing ET+lepton pT < 130 GeV • Harder b-tagged jet cut: p T > 50 GeV • Veto extra b-tagged jets with p T>35 GeV (looser b-tag) • 2jets(1b1j), 3jets(1b2j), 4jets(1b3j) • If at least 2 non-btagged jets : • j1,j2 highest pT 50 <Mj1j2< 125 GeV

  28. SUMMARY: Single top XS sensitivities in ATLAS For 1 fb-1 • Results • t-channel: observation with 5σ (ie discovery) may be possible for ~1fb-1 • Measurement of σ with Δσ/σ = 22% and |Vtb| with Δ|Vtb|/|Vtb|~12% • Wt channel: possible observation for ~10fb-1 • s-channel: require more stat. > 30fb-1 • Prospects: once ST signal is established • Study top properties (polarization) • New physics searches (non-SM cross section, modified kinematics): charged Higgs, W' …

  29. M.V.A: BDT (Wt) and LH(s-channel) 12 Boosted Decision Trees vs ttl+jets, ttdilepton, Wl+jets, st t-channel BDT for 2/3/4 jet multiplicity 5 Likelihood functions vs ttl+jets, ttll, ttl+τ/ττ vs Wl+jets, st t-channel Discriminating variables: Opening angles: ΔR(lepton, s-ch: b1,b2, Wt: b,j1,j2,j3) Invariant masses: Minv(2 jets), M(b,W lep), Wt: M(b,Whad), Global event shapes: sphericity, aplanarity,centrality,HT

  30. W polarization: TEVATRON methodology for ATLAS. • Binned likelihood fit: • Data to signal (F+,F0) and background templates. • Signal templates: theoretical principles corrections for detector efficiencies and resolutions. • Unbinned likelihood fit: • Data to parameterized signal and background functions. • Measured fractions corrected for acceptance effects to true fractions to be compared against the theory.

  31. MTOP ATLAS L=1fb-1 Geometric method(II) • Further combinatorial background rejection: • C3: |X1-μ1| < 1.5 σ1, • X1=E*W-E*b=(M2W-M2b)/M top • C4: |X2-μ2| < 2 σ2, • X2=2E*b=(M2top-M2W+M2b)/M top C1+C2+C3+C4 C1+C2 +C3+ C4 & Rescaling:-Mjj+Mjj peak Mtop=175.0±0.4 GeV σMtop=14.3±0.3 GeV Mtop=175.3±0.3 GeV σMtop=10.6±0.4 GeV

  32. MTOP ATLAS L=1fb-1 Geometric method(III) • Assuming no b-tagging: “commissioning analysis” • Hadronic W reconstruction through geometrical method • b-jet associated to the hadronic Top decay chosen among other jets in the event minimizing: • Sqrt( (X1-μ1)2+(X2-μ2)2) • Larger contribution of physics and combinatorial backgrounds C1+C2+C3+C4 C1+C2 +C3+ C4 & Rescaling:-Mjj+Mjj peak Mtop=175.0±0.4 GeV σMtop=11.7±0.5 GeV Mtop=175.2±0.5 GeV σMtop=12.4±0.8 GeV