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D. PALLIN Blaise Pascal Univ./ LPC Clermont-FD Pragua 6/04/09

Top Quark Physics at LHC with ATLAS. D. PALLIN Blaise Pascal Univ./ LPC Clermont-FD Pragua 6/04/09. Top quark Discovery. 11/11/1974 J/ψ and charm quark c discovery at SLAC & BNL

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D. PALLIN Blaise Pascal Univ./ LPC Clermont-FD Pragua 6/04/09

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  1. Top Quark Physics at LHC with ATLAS D. PALLIN Blaise Pascal Univ./ LPC Clermont-FD Pragua 6/04/09 D Pallin Pragua 6/04/09

  2. Top quark Discovery • 11/11/1974 J/ψ and charmquark c discovery at SLAC & BNL • Two families of leptons and de quarks Leptons: e  Quarks: u c e d s • 1976-1977 third fermion family is discovered :   lepton & b quark • Is the b quark belonging to an isospin doublet? Quarks: u c ? d s b • 1978-1994 • measures suggest an iso-doublet • Direct Top searches are negatives • LEP electroweak fit MS M(Top) ~150-170 GeV • 1995 Top quark discovery at Tevatron D Pallin Pragua 6/04/09

  3. The Top quark in the MS • Complete the 3e quark family • SU2L weak isospin Partnair of bottom quark • Spin=1/2 • electricCharge=2/3 • Color Triplet • RQ: NO direct measurement of quantum numbers of theTOP quark, only indirect informations • The free parameters in the Top sector are • The Top Mass (free fondamental parameter of the MS) • CKM matrix elements unitarity => Vtb =0.9990 -0.9992 => t Wb Coupling fixed by the jauge structure • Width computable from the SM D Pallin Pragua 6/04/09

  4. Why the Top quark is so interesting ? • Large mass • The only fermion heavier than the W • Mt =MAU =35Mb • Top-Higgs Yukava coupling : t = 2 MT /v ~1 • Interact heavily with the higgs sector => Suggest that the Top quark play a specific role in the electro weak symetry breaking (EWSB). => All New Physics in connection with EWSB should couple preferentially to the Top quark : Top sector is an ideal laboratory to search for ‘New Physics’ • Short lifetime • The Top Quark decays before hadronisation • => We can study the properties of a « nude » quark (Top Mass) D Pallin Pragua 6/04/09

  5. Shopping list in the Top sector • Explore properties • Precise meass measurement => consistency test of the SM, and constraint for the Higgs boson • Search for new physics • Top is a BKG for New Physics searches, need to be understood ( X-sections) • In addition at LHC • Top is a Reference point => Re- establishment of the top • Tool for Detector commissionning D Pallin Pragua 6/04/09

  6. W helicity Top Mass l+ Top Width Anomalous Couplings Production cross-section Top Spin W+ CP violation Top Charge Resonance production n Y t b Production kinematics Top Spin Polarization _ t Rare/non SM Decays X Branching Ratios |Vtb| Top physics: Which measurements ? • Productions mechanisms • Production X-sections • Vtb • Spin correlations • Ttbar production by new resonances • Properties • Top mass • Charge • Decay properties • Electroweak (V-A) vertex: W helicity • Rare Top decays • Search for New physics using heavy flavour D Pallin Pragua 6/04/09

  7. Physique du Top: state of Art • Tevatron CDF & D0 the unique source of Top • Since 1995 quark Top properties studies • Run I • Run 2 • Measurement limited by statistics • Very good understanding of the detector • Transition towards précisionmeasurements • >4fb-1 ( up to 8fb-1) D Pallin Pragua 6/04/09

  8. Top and LHC : from rare to common • LHC ? • Top factory • Measurement limited by systematics very soon • New detector generation • Start-up Phase • Progressive ramping of the LHC (E, L) • Detector to be tuned and performances to be understood • But great potential for Top properties D Pallin Pragua 6/04/09

  9. The Large Hadron Collider • pp collision cm : 14 TeV (x7 Tevatron) • 25 ns bunch spacing • 1.1 1011 proton/bunch • Design luminosity 1034 cm-2s-1 100 fb-1 /year;  20 int./x-ing • Initial/low lumi L1033 cm-2 s-1 10 fb-1 /year ;  2 int./x-ing • 4 interaction regions ATLAS and CMS : pp, general purpose 27 km ring 1232 dipoles B=8.3 T D Pallin Pragua 6/04/09

  10. The Large Hadron Collider • 1st beam in LHC September 10, 2008 • 1st collision at 450+450 GeV expected end september • Incident (dipole connection) on Sept 19. • No collisions in 2008 • Remember everyone that LHC machine represents a challenge • O(100 pb-1) expected in 2009-2010 @ 10 TeV D Pallin Pragua 6/04/09

  11. pair Production via Strong Interaction Top quark production in hadron colliders ‘Single Top’ Production via EW interaction t channel Wt channel s channel D Pallin Pragua 6/04/09

  12. LHC tt ~830 pb X100 Tevatron tt ~6,7 pb LHC Low L 1033 cm-2s-1 Tevatron 1032 Prod Rate X1000 X10 Top Production at LHC At low Luminosity (1033), 14 TeV ~ one top pair produced per second LHC is a Top factory But 108evts /s are produced D Pallin Pragua 6/04/09

  13. Pair production Single Top production t channel Wt channel s channel ~10% ~90% Tev 1.47 pb 0.15 pb 0.75 pb 6,7pb Opposite to Tevatron LHC 250 pb (4%) 60 pb (8%) 10 pb (8%) 833pb LHC, 14 TeV Production du Top au LHC/tevatron At 10 TeV, X-sections drop by a factor 2 100pb-1 => ~40000 Top pairs produced At 10 TeV with about 100 pb-1 the ATLAS top sample has the size of the complete Tevatron sample D Pallin Pragua 6/04/09

  14. t W+ l+ qq l+ qq b b b b b t W- l- l- qq qq b b b b b top decay and tt decay channels • MS: t  Wb domine tau+X 21% mu+jets 44% e+jets 15% e+e 15% e+mu mu+mu all hadronic 1% 30% e/ + jets 5% ee/e/ 3% 1% D Pallin Pragua 6/04/09

  15. hadronic Calorimeter EM Calorimeter Tracking Vertex detector Muons chambers ATLAS detector • Top quark detection and reconstruction Involves many detector properties : • Lepton reconstruction and Identification • Jet reconstruction and calibration • Missing transverse Energy evaluation • b-tagging (lower eff at beginning?) Complete detector capability at play D Pallin Pragua 6/04/09

  16. Which detector performance on day one ? D Pallin Pragua 6/04/09

  17. Some examples of studies • SM tests with Top • Establish Top signal ~10pb-1 • Top pair production X-section stat(5%)-syst(15-5%)-lumi(3%) ~100pb-1 • Top mass measurement (5%-2%) ~100pb-1, 1fb-1 • Top as a Tool light jet (2-1%) b tag eff 3% ~100pb-1, 1fb-1 • Single Top production t channel@5 ~1fb-1 • Top properties top charge 5 , W pol 5-10%, FCNC BR 10-3, ~ 1fb-1 • BSM • Search for New physics using Top  1fb-1 From the updated TDR (CSC BOOK) Expected Performance of the ATLAS Experiment : Detector, Trigger and Physics’ (arXiv:0901.0512 ; CERN-OPEN-2008-020) Studies @1033 14 TeV, 1fb-1 of data D Pallin Pragua 6/04/09

  18. tt→(Wb)(Wb) decay v l b-jet l b-jet jet jet • “full hadronic” • tt (jjb) (jjb) • large BR: 44% • Large multijet BKG • Lower Trigger efficiency • “Di-leptonic” l=e, μ • tt (lvb) (l’vb) • Low BR : 4% • Low BKG • Lepton trigger • “lepton+jets” l=e, μ • tt (lvb) (jjb) • BR : 30% • Reduced BKG (,W+jets, QCD, single Top, Z->ll) • Lepton trigger D Pallin Pragua 6/04/09 18

  19. W CANDIDATE TOP CANDIDATE Top pair x-sec measurement with 100pb-1 • Lepton+jet evts • lepton trigger pT lepton>20 Gev • 4 jets pT >20 Gev, 3 jets pT >40 Gev • ET miss>20 GeV • Top = 3 jets giving Highest Pt sum • No b tag • (W constraint Mw+- 10 GeV) for 1 jj comb. Default selection S/B~2.3, eff~24% With W constraint S/B~3.5 Top contribution visible even with 10 pb-1 D Pallin Pragua 6/04/09

  20. Top pair x-sec measurement with 100pb-1 • Likelihood fit • gaussian+chebychev bkg • extract X-sec by scaling with efficiency • Counting Method • sensitive to BKG normalisation, #jets, JES, less to shape Muons 508 Top evts 100pb-1 D Pallin Pragua 6/04/09

  21. Top mass measurement with 1 fb-1 • ttb+jjbselection • avoid contribution from BKG, rely on well measured objects • Select events containing • at least 1 lepton pT>20 (25) GeV (trigger) • at least 4 jets pT>40 GeV to keep only well measured jets • Missing Et >20 GeV (for the escaping ) • All particles emitted in ||>2.5 to keep only well measured & Identified particles • Select sub-samples with • 0, 1 or 2 identified b-jets among all selected jets • eff(b) =60% ; light jet rejection factor ~ 130 D Pallin Pragua 6/04/09

  22. ttb+jjbselection • Physical BKG • Main background: W+n jets • Others • QCD bb • Z+jets • WZ • tt jets, tt+X, Single Top • partially counted as signal when only tt jjb is considered 1 fb-1 Eff= 14% (5%) Purity=75% (91%) D Pallin Pragua 6/04/09

  23. Top Mass measurement strategy • Top mass estimator built from the invariant mass of the hadronic top decay products • Mjjb provides the most natural way to measure the Top mass • Close to the pole mass O(QCD) ~ 100MeV (fragmentation effects ) • needs to reconstruct the Top decay chain • Reconstruction of the hadronic top t Wb  jjb • For early data: Use simple method and do not rely on MC • Find the jet pair originating from the W jj and the b jet forming the top • Wrong association => combinatorial BKG (reduced if jet b-tag used) • The invariant mass peak should be a gaussian distribution centered on the Top mass • The precision on the mass depend mainly on the accuracy to determine the Jet energy scale for light jets (JES) and b jets (JESb) D Pallin Pragua 6/04/09

  24. HadronicTop reconstruction2 b-jet case • Find first the W jets • Closest jets • Min(Mjj-MWpeak) 50% of the events have more than 2 light jets D Pallin Pragua 6/04/09

  25. HadronicTop reconstruction2 b-jet case • Find first the W jets • Closest jets • Min(Mjj-MWpeak) • Then the b jet in tWb • Closest b jet from W D Pallin Pragua 6/04/09

  26. HadronicTop reconstruction2 b-jet case • Comb bKG is made of • Wrong association chosen • One of the jet has not been selected => the right combination cant be selected (main contribution to comb BKG) (Wrong W mainly) => Purification cuts to remove the comb bkg D Pallin Pragua 6/04/09

  27. HadronicTop reconstruction2 b-jet case standard Purification cuts high Purification cuts Standard Purification cuts (eff=75%, 85% of bkg rejection) Mtop= 174.6 ±0.5 GeV =14.1±0.5 GeV High Purification cuts (eff=65%, 95% of bkg rejection) Mtop= 175.0 ±0.4 GeV =14.3±0.3 GeV D Pallin Pragua 6/04/09

  28. HadronicTop reconstruction2 b-jet case • Mtop Mjjb-Mjj+80.2 is a better estimator of the top mass • Uncetainties on the light jet energies (W jets) affect both Mjj &Mjjb • Uncertainties cancel at first order on the mass difference Mjjb-Mjj • Since MW is well known, measure only the mass difference • Impact from the light JES uncertainties is lower on the Top mass determination • The resolution on the Top mass measurement improves Mtop= 175.4 ±0.4 GeV =10.6±0.4 GeV Mtop= 175.3 ±0.3 GeV =10.6±0.2 GeV D Pallin Pragua 6/04/09

  29. Uncertainties on Top mass measurement 2 b-jet case 1fb-1 • Statistical uncertainty • Estimated for 1fb-1 using a bootstrap resampling technique • (Mtop)stat < 0.4 GeV • Systematics uncertainties • Dominant uncertainty after a few fb-1 of data • Main contribution to syst are JES & JESb • (Mtop)syst ~ 1 (3.5) GeV if JES accuracy is 1 (5)% 1fb-1 D Pallin Pragua 6/04/09

  30. Top mass measurement with 1 fb-1 • The Top mass is measurable with an accuracy of ~1 GeV with 1fb-1 of data • Mainly driven by the reached precision on JES • For light jets => JES from Mw • For b jets => JES(b)/JES(light) MC modeling at start Z+bjet, (di-jets b / di-jets light), … when enough stat • At LHC start • analyses will try to rely as low as possible on MC since not tuned • Selections should be simple, non biased D Pallin Pragua 6/04/09

  31. Top as a tool • Light jet JES • B jet JES • B tag eff • Trigger eff D Pallin Pragua 6/04/09

  32. Reconstruction and Calibration Scheme Reconstruction Jet reconstruction and calibration Detector effects : detector response should reflect the real deposited energy Jet algorithm effects: Energy deposited in calorimeter cells are grouped in clusters to form a jet Most of the energy of the jet belong to the originating parton, but some extra energy comes from other particles So in principle Eparton = JES Ejet with JES # 1 and JES= f(Ejet,jet,…) Calorimeter Cells clustering Clusters EM scale Global Approach (Default Scheme) Jet Reco Alg Uncalibrated Jets Jet Energy Calib to Particle Level Calibrated Jets Particle level In situ calibration Physics Jets Parton level D Pallin Pragua 6/04/09

  33. j1 n W1 W2 l(e,m) j2 t1 t2 b1 b2 MW MTop 4000 jets for 750pb-1 purity 80% JES in-situ determination JES determination is the key point of the top mass measurement AIM : Rescale jet to parton energyJES = 1/ (Ejet / Eparton) • In-situ calibration : JES from Wjj in ttbar events • Select an almost pure sample of wjj candidates in ttb+jjbevents • calibration dedicated for TOP? D Pallin Pragua 6/04/09

  34. (1-cosjj)/(1-cospp) cosjj JES from Wjj in ttbar events method • The W mass is a precise reference MW30 MeV • The W mass depends on jet energies and opening angle between J1 & j2 • Angle (J1,J2) well measured (at the % level ) D Pallin Pragua 6/04/09 34

  35. MW MW MW Ejet (GeV) JES from Wjj in ttbar events method • The W mass depends mainly on JES • JES is a simple rescaling to the W PDG mass ! =>Global JES factor • BUT in general • JES (E=40 GeV) # JES (E=100 GeV) • EJ1 # EJ2 • => Simple rescaling to the PDG W mass not sufficient • => MW Spectra in jet energy, eta,.. windows • to allow the separation of both jet contribution to the JES: JES(J1) et JES(J2) • =>JES in function of Jet Energy,, .. D Pallin Pragua 6/04/09 35

  36. JES from Wjj in ttbar events 1fb-1 10fb-1 + JES THEO ATLAS + JES found jet JES uncertainty of 1% is achievable with 1 fb-1 D Pallin Pragua 6/04/09 36

  37. TTbar resonances • With increasing ttbar mass : • SM ‘BKG’ decrease • Comb BKG contribution decrease • Recons eff drops • Top decay particles mixed D Pallin Pragua 6/04/09 37

  38. Rare Top decays • FCNC D Pallin Pragua 6/04/09 38

  39. Rare Top decays • FCNC D Pallin Pragua 6/04/09 39

  40. conclusion • But before any measurement • Detector understanding • detector performances measurements • Trigger • Calibrations • alignement • B tagging • Background studies • MC tuning on data => Top events serve as a tool for these studies D Pallin Pragua 6/04/09

  41. BACKUP D Pallin Pragua 6/04/09

  42. JES from Wjj in ttbar events MW MW MW Ejet (GeV) • ‘Itérative’ Method • From MW spectra in Energy slices • extraction of MW peak • R(E) applied to each jet • Recompute MW=> new Mw spectra •  3 itérations D Pallin Pragua 6/04/09

  43. JES from Wjj in ttbar events • JES in function of eta jets Expected = squares Fitted = circles 10 fb-1 JES uncertainty of 1% is achievable with 1 fb-1 D Pallin Pragua 6/04/09 43

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