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Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN

Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN on behalf of the ATLAS and CMS collaborations. LHC is a tt factory. Total production cross section. (90%). +. (10%). tt production cross section at LHC: ~833 pb. tt production cross-section at Tevatron:

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Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN

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  1. Moriond QCD Early physics with top quarks at LHC P.Ferrari CERN on behalf of the ATLAS and CMS collaborations Pamela Ferrari

  2. LHC is a tt factory Total production cross section (90%) + (10%) tt production cross section at LHC: ~833 pb tt production cross-section at Tevatron: 6.7 pb 2 tt events per second ! > 8 millions tt events expected per year Pamela Ferrari

  3. Top Physics day one In 2008 ECM = 14 TeV few fb-1already negligible statistical err 1) Top properties and basic SM physics at s = 14 TeV : • Estimate of σtop ~ 20% accuracy • Start to tune Monte Carlo • Measure top mass  feedback on detector performance 2) Understand/calibrate detector and trigger: tt  bl bjj • Light jet energy scale selecting a pure sample of W jj in tt events (< 1%) • b-tag efficiency (~ 5%) • Missing energy calibration 3) Prepare for new physics: • Resonances, MSSM higgses, SUSY, FCNC • Measure differential cross sections (ds/dpT,ds/dMtt) sensitive to new physics (provides also an accurate test of SM predictions) Pamela Ferrari

  4. « Data » a = 1 Best fit Light jet energy calibration Template histograms with different E scales a and relative E resolutions b: W  qq in ~106 PYTHIA tt events Simple tt  lnb jjb selection with MC@NLO tt events : 1(e/m) pT>20 GeV, ETmiss>20 GeV, = 4 jets pT>40 GeV (2 b-tagged), 150 GeV< mjjb< 200 GeV W purity ~83% Fit each template histogram to mjj in the « data », find best c2 a = 0.937 ± 0.004, b = 1.47 ± 0.05 • Statistics limited. Unknown syst limit< 0.5% from combinatorial backg. and templates shape JES as a function of energy ( n energy bins and nxn matrix template) Ej/Eq ±2% @ 1.3 fb-1 • b-jets • Light jets Pamela Ferrari Eq

  5. W CANDIDATE TOP CANDIDATE Calibrating b-tagging • Using semi-leptonic (and fully leptonic) tt events • Optimize the jet pairing efficiency via mass constraints in kinematic fits and likelihoods. • Only one jet is tagged as b-jet (on Whad side) CMS NOTE 2006-013 Isolate jet samples with a highly enriched b–jet content, on which the b–jet identification algorithms can be calibrated. Main systematics :ISR/FSR For 1 fb-1 (10fb-1) relative accuracy on the b–jet identification efficiency is ~ 6% (4%) in barrel region and about 10% (5%) in the endcaps. Pamela Ferrari

  6. No b-tag W =2 jets maximising pT W in jjj rest frame Hadronic top=3 jets maximising pT top |mjj-mW| < 20 GeV 100pb-1 100 pb-1 Siginficance (s) Combinatorial only with 100 pb-1 (few days) W+jets Luminosity (pb-1) Day one: can we see the top? We will have a non perfect detector: Let’s apply a simple selection • No b-tag • relaxing cut on 4th jet: pT>20 GeV: doubles signal significance! 4 jets pT> 40 GeV 600 pb-1 Isolated lepton pT> 20 GeV ETmiss > 20 GeV Pamela Ferrari

  7. Refining the selections: semi-leptonic case More refined selection studied with the aim of applying it to x-section, mass, polarization studies.. Example: CMS NOTE 2006/064 • 1 isolated lepton pT>20 GeV • ≥4 jets ET>30 GeV |h|<2.4 • 2 b-tagged jets • Coverging kin. fit to mW stat total w/o lumi total w lumi esel~ 6.3% S/B~26.7 @5 fb-1stt(m)=0.6% (stat)± 9.2% (syst)±5.0%(lumi) Exploiting new topological variables from D0? • Sphericity S and Aplanarity A • Centrality C • HT = • Df(lep,n) • KTmin=min D(h,f) between 2 jets 1fb-1 Not very useful to separate from W+jets after selection Pamela Ferrari

  8. Summary of cross-section • The cross-section has also been extracted from in the di-leptonic and fully hadronic channels here examples from: CMS NOTES 2006-064/ 2006-077 Pamela Ferrari

  9. Top mass measurement in lepton + jets channel 1) minimization of c2 reconstruct mW hadronic & jet E rescaling (a1,a2) • keep W’s if |mW - 80.4 GeV| < 2GmW • chose b-jet that maximises top pT • W purity 56.5%, top purity 45%, e=1.1% 2) Kinematic fit: 10fb-1 MC@NLO Fullsim CMS NOTE 2006-066 mtop Kinematic fit to reconstruct entire tt final state: • c2 based on kinematic constraints (El,j & directions vary within resolution) c2 minimisation, event by event • Mtop fitted in slices ofc2 • Estrapolation from linear fit: mtop = mtop(2 = 0) Gaussian/Full Scan Ideogram estimator for mt: • Event by event likelihood method convoluting the event resolution function with expected theoretical template. mtop obtained from maximum likelihood method PYTHIA Fullsim Pamela Ferrari

  10. Top mass measurement (semileptonic channel) c) Selection of high pT top quarks pT(top) > 200 GeV/c: • t and t tend to be back-to-back  used as constraint to reduce bkg • 3 jets in 1 hemisphere tend to overlap: collect E in a cone around candidate top • less sensitive to jet calibration. Mass scale recalibration based on hadronic W, • independent systematic errors  gain in combination Comparing the 3 methods Pamela Ferrari

  11. PYTHIA 1fb-1 Di-lepton channel and Hadronic channels Dilepton channel:clean channel but need to reconstruct 2 n’s. Reconstruction via 0C fit assuming mW and 2 equal masses for top mt1=mt2 (6 eq. ,6 unknowns) • The different n solutions are weighted using the SM prediction for the n and n E spectra • The neutrino solution with the highest weight is chosen mtop S/B = 12 - • Hadronic channel:full kinematic reconstruction of both • sides but huge QCD multijet background: • 6-8 jets, ET>30 GeV • Centrality>0.68,aplanarity>0.024 • ETtot- ET of 2 leading j>148 GeV • 2 b-tagged jets • Best jet pairing obtained from likelihood based mainly on angular distrubution of jets. CMS NOTE 2006-077 Pamela Ferrari

  12. Resonances in Mtt pp  X  tt with tt decaying semi-leptonically Technicolor, Strong EW symmetry breaking models, Z’, SUSY: • usual semi-leptonic events preselection • use W and top mass constraint: • neutrino pZ from mW constraint, solution • giving best top mass is retained • |mjj-mW|  20 GeV • b-jet associated with hadronic top is the • one maximising pTtop • |mbjj-mT|  40 GeV 5fb-1 3xZ’ signal • Since pT of top from resonance decay is larger than in direct production Add lower cut on top pT 370,390,500 GeV/c for mZ’ =1,1.5,2 GeV/c2 to increase purity (s/B~0.06-0.08) Pamela Ferrari

  13. Flavour Changing Neutral Currents CMS NOTE 2006/093 u (c,t) No FCNC at tree level in SM: 5σ sensitivity u Z/γ Look for FCNC in top decays: SN-ATLAS-2007-059 tqg (+1l+1g+missing) tqg (+1l+missing) tqZ (2jets+3l+missing) t @ 10 fb-1 2 orders of magnitude better than Tevatron/LEP/HERA Pamela Ferrari

  14. Abt,lt=0.375 ±0.014(stat) (syst) Aqt,lt=0.346 ± 0.021(stat) (syst) +0.055 -0.096 +0.026 -0.055 Top spin correlations t and t are produced unpolarized, but spins are correlated anomalous coupling (technicolor), tH+b, spin 0/2 heavy resonance H/KK gravitons  tt, would move A away from SM expectation s(tLtL) + s(tRtR) - s(tLtR) - s(tRtL) A= s(tLtL) + s(tRtR) + s(tLtR) + s(tRtL) Fit to double differential distribution CMS NOTE 2006/111 • Semilep. (10 fb-1) • Fitting to distribution of • lepton angle vs b-quark angle in the tt rest frame • lepton angle vs lower energy quark angle from the W-decay in the tt rest frame Eur.Phys.J.C44S2 2005 13-33 Semilep. + dilep. (10 fb-1) Fitting to distribution of • angles bewteen top spin analyser in top rest frame versus angle of t spin • analyser in antitop rest frame • Syst. dominated by b-JES, top mass and FSR A=0.41 0.014(stat) 0.023(syst)

  15. Conclusions • LHC startup will require a long period of development and understanding • LHC is a top factory, but before performing precision measurements, a huge effort is needed in order to • Understand the detectors and control systematics • Complete study using full simulations and NLO generators • Early top signal will help • We could get top signal with ~ 100 pb-1 • s(tt) to ~13% and Mtop to 1% with 1fb-1 • In addition our aim is, as soon as we get a large statistics (few fb-1), to be ready for early discovery of new physics! Pamela Ferrari

  16. BACK-UP TRANSPARENCIES Pamela Ferrari

  17. t t Calibrating the missing energy We can calibrate the missing Energy since in semileptonic tt events the momentum of the neutrino is constrained from kinematics : MW known amount of missing energy per event Calibration of missing energy vital for all (R-parity conserving)SUSY and most exotics! Example from SUSY analysis Miscalibrated detector or escaping ‘new’ particle Events Perfect detector Range: 50 < pT < 200 GeV Missing ET (GeV) Pamela Ferrari

  18. Top signal Jet 4 PT cut Fraction 40 GeV 42.6 %30 GeV 68.1 %20 GeV 87.7 % 42.6 % Effect of cut on PT of 4th jet Lowering PT requirement on 4th jet to 20 GeV increase in significance top signal by factor > 2 42.6 % Cut on 4th jet does not bring any advantage Actually … it cuts away important information: large fraction of jets associated to quarks from W decay Pamela Ferrari

  19. Gaussian Ideogram, full scan ideogram CMS NOTE 2006-066 • Build event by event from the covariance matrices of the kinematics of the 3 fitted jets, the relative compatibility of the reconstructed kinematics of the event with the hypothesis of the a heavy object of mass mt decays into 3 jets. This is usually called the ideogram of the event or resolution function of the event In the full scan the top mass is not fixed but varies 125<mt<225 GeV • To estimate the top mass, the ideogram is convoluted with the Theorectical expected probability density function and after combining all likelihoods from all events a maximum likelihood method is applied to get mt. Sources (5% On-Off) (1.5%) (2%) Pamela Ferrari

  20. Sensitive to mt m(3l) Mtop other 2 methods • Exclusive (J/ℓℓ) decays: detect clean high mass products, easy to reconstruct and little background 2) via cross-section: very much sensitive to the top mass: 5% on  gives mt~2 GeV/c2, error luminosity and the pdfs • different systematic contributions: top mass determined • via m(3l): • without using the b-tagging ! • almost ignoring jet reconstruction ! • large improvement in combination with direct reconstruction • Challenging because of the extremely low branching ratio: BR = 0.810-4 after selection+trigger efficiency CMS NOTE 2006-058 Pamela Ferrari

  21. Di-lepton event selection CMS NOTE 2006-077 • Cut based selection: • Single or di-lepton trigger • Two isolated oppositely charged leptons with ET>20 GeV,<2.5 • Missing ET>40 GeV • 2 jets with ET>20 GeV, <2.5 • 2 b-tagged jets Main background represented by Z+jets when no b-tagging is present ( efficiency 15% ) With b-tagging, efficiency about 5% with excellent background reduction S/B~5 (B mainly from leptonic  decays) Pamela Ferrari

  22. Systematic effects for top physics • Almost all SM measurements at LHC dominated by systematic errors. • Can be divided into instrumental and from theory/modeling • Dominant instrumental uncertainties for top physics: • Luminosity: • Reasonable goal is 3-5% •  measure number of interactions/bunch crossing (HF) and (pp) (TOTEM) • Reconstruction related: • Jet energy scale • need calibrated calorimetry (beam tests, MB, single particles, Z, W…) • need jet energy calibration to a few % (with Z()+jet) • need excellent energy flow (association tracker+calo+muon system) • b-tagging efficiency+fake rate • use tt for calibration: to 4-5% with 10fb-1 • Lepton identification and energy scale • use Z, other mesons. Less crucial than for the W mass measurement • Theory related systematics are as important as instrumental ones ! Pamela Ferrari

  23. Theoretical systematic uncertainties • ISR/FSR: vary QCD and Q2max from low to high radiation • light jets Fragmentation:vary only the fragmentation parameters within errors from LEP/SLD tunings. • “b jets” Fragmentation: error estimated changing the Peterson parameter (-0.006 ) within its theoretical uncertainty (0.0025) • Minimum bias and underlying event: extrapolate from low energy UA5/Tevatron and change main pT cut-off parameter • PDF parametrization:CTEQ6M, contrain using LHC data • Hard process scale:switch among reasonable definition of the Q2 scale Pamela Ferrari

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