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Top quark production and properties at D0

Top quark production and properties at D0. E. Shabalina for the D0 collaboration RAS conference November 26-30, 2007. Top quark. Decays as a free quark  = 5×10 -25 s <<  QCD -1 Passes spin information to its decay products Allows to test V-A structure of SM.

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Top quark production and properties at D0

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  1. Top quark production and properties at D0 E. Shabalina for the D0 collaboration RAS conference November 26-30, 2007

  2. Top quark • Decays as a free quark •  = 5×10-25 s <<QCD-1 • Passes spin information to its decay products • Allows to test V-A structure of SM • Special place in FNAL physics program • The only place where top quarks are produced • 12 years since the discovery • The heaviest fundamental particle • mt= 170.9±1.8 GeV (~1% precision) • Close to a gold atom • Mass close to scale of electroweak symmetry breaking • May shed light on EWSB mechanism

  3. Top quark properties W helicity Top Mass Production mechanism l Top Width Anomalous Couplings Production cross-section Top Spin W+ CP violation Top Charge Resonant production p  t b Production kinematics _ X b Top charge asymmetry _ _ p t q Rare/non SM Decays W _ q’ Branching Ratios |Vtb| • Produced mostly in tt pairs at the Tevatron • 85% qq, 15% gg •  = 6.8 ± 0.6 pb at NLO • Dataset: ~1 fb-1, ~10 times more than Run I • Precise measurements of top quark properties are possible • Most of the current knowledge comes from the studies of top quark pairs • Production of single top quarks: next talk by E. Boos

  4. Top decay channels Top pair final states are classified based on W decays • SM decay: • Dilepton (ee, μμ, eμ) • Both W’s decay leptonically • Lepton (e or μ) + jets • One W decays leptonically, one hadronically • All-hadronic • Both W’s decay hadronically • τhad +X E. Shabalina RAS conference

  5. B-jet identification • Top, Higgs signal contain b-jets • Most of backgrounds do not • B-hadronlifetine~ 1 ps • B-hadron travels Lxy~1 mm before decay • Combine properties of reconstructed secondary vertexes and displaced tracks in 7-variable network • Working point: efficiency ~54%, fake rate ~1% • tt event tagging probability ~70% E. Shabalina RAS conference

  6. Dilepton channel 57 candidates with nj>=2, 43.9 expected signal ,13.3 background use events with 1 jet in emu channel to increase acceptance ee,eµ,µµ:  = 6.8  (stat) (syst) ± 0.4(lum) pb +1.2 1.1 +0.9 0.8 For mtop = 175 GeV 22.5% E. Shabalina RAS conference

  7. Dilepton combination • Lepton+track channel • Looser selection – apply b-tagging • Orthogonal to dilepton • Use events with nj=1 and nj>=2 16 candidates with nj>=2 18 expected signal 3.2 background Combined (L=1 fb-1) @ mtop = 175 GeV dilepton:  = 6.2± 0.9 (stat) (syst) ± 0.4(lum) pb +0.8 0.7 19.5% CDF best (L=1 fb-1) in lepton+track (no dilepton veto): 19% E. Shabalina RAS conference

  8. Hadronic+lepton channel • Important channel to expand top program (searches for non-SM top decays, H+) • Cross section: use all available top events, assume SM branching • Cross section×Br for ttµ(e)+h (other top contributions are considered to be background) = 8.3± (stat) (syst) ± 0.5(lum) pb +2.0 1.8 +1.4 1.2 Br= 0.19± 0.08 (stat) ± 0.07(syst) ± 0.01(lum) pb SM: ×Br = 0.126 After tagging µ+ e+ Selected 29 18 Expected 5.6 4.7 Pretag CDF (350 pb-1) 5 events selected 2.7±0.4 bckg E. Shabalina RAS conference

  9. Cross section (L=0.9 fb-1, mtop = 175 GeV) Lepton+jets channel (topology) Uncertainty 18.5% Update is expected soon, will include events with nj=3 E. Shabalina RAS conference

  10. Top branching fractions =1 in SM • Assumed value of R changes the fraction of events with 0,1 and 2 tags • Perform simultaneous fit of R and ttbar cross section 76 double tags 179 single, 58 double tags 0 tags, >=4 jets =3 jets >=4 jets E. Shabalina RAS conference

  11. Using unitarity of CKM matrix 12% R=1: Results Simultaneous fit result in good agreement with SM: CDF best (L=1.1 fb-1): 13% E. Shabalina RAS conference

  12. Cross section summary The most precise single measurement in the world E. Shabalina (UIC) D0 collaboration meeting

  13. FC limit: B(t→H+b) < 0.35 @95% C.L. Cross sections ratio • Extract as much physics as possible from the existing measurements Ratio is sensitive to the non-W decays of top beyond SM tXb Simplified model: charged Higgs tH±b with a mass close to W and exclusive H±cs (leptophobic higgs in MHDM: hep-ph/9509203, hep-ph/9401311, radiative corrections in MSSM: hep/ph/9907422) E. Shabalina RAS conference

  14. tt=6.8 pb Z’ with mass 750 GeV Top resonances • No resonant production is predicted in SM • Some models predict ttbar bound states: topcolor assisted technicolor predicts leptophobic Z’ with strong coupling to 3rd generation • Narrow width: width is dominated by detector effects • Use lepton+jets events with >=1 b-tag Z’ Search for bumps in ttbar reconstructed mass spectrum No evidence for a narrow resonance Set upper limit on the X×B(Xtt) Excluded mass range: MZ’ < 680 GeV @ 95% CL Expected MZ’ < 740 GeV CDF MZ’ < 725 GeV E. Shabalina RAS conference

  15. A = (12 ±8 (stat) ± 1 (syst))% Top charge asymmetry • No asymmetry in QCD LO, 510% at NLO, even larger at NNLO • Depends on the region of phase space, any extra jet production • Reconstruct ttbar pair using kinematic fitter • Extracted simultaneously with sample composition from the likelihood fit • Do not correct for acceptance and reconstruction effects Large positive asymmetry is predicted for Z’ production Leptophobic Z’ Not restricted to narrow Z’ Limits the fraction F of top pairs produced via Z’: F<0.44 (exp), F<0.81(obs) for Mz’ = 750 GeV E. Shabalina RAS conference

  16. W helicity • Helicity = the relative direction between the spin and the particle's motion • SM V-A vertex dictates the fractions • extract from cos(*) distribution • combine dilepton (two measurements per event and l+jets channels) • Model-independent measurement: • simultaneously fit f0 and f+ • measure angle between top quark and down-type fermion (lepton or d,s-quark) • allows to use hadronic W-boson decays dilepton l+jets Coming soon f+ = 0.017±0.048 (stat)±0.047 (syst) f+ < 0.14  @95% C.L. for f0=0.7 (fixed to SM value) E. Shabalina RAS conference

  17. Top quark mass measurement Require a clean mapping between reconstructed objects and partonsJet energy scale calibration is crucial Many techniques • Template • Matrix Element • Ideogram (hybrid of the above) Follow the same pattern: • Measure the observable sensitive to the top quark mass • Map the partons to reconstructed objects (combinatorics!) • Calibrate with pseudo-experiments • Extract the mass from the maximum likelihood W boson decay products allow to use the known W mass as an in-situ calibration tool E. Shabalina RAS conference

  18. Neutrino weighting Scan potential top quark masses and  rapidities Solve for the  4-vectors Assign weights based on comparison of calculated and reconstructed MET Form weight templates for each mass Fit signal and background templates to data Matrix weighting Scan potential top quark masses Solve for top quark momentum assume two leading jets are b-jets 4 solutions per ttbar Include detector resolution Calculate weight as a function of mass for each event Dilepton mass Underconstrained kinematics given two neutrinos 172.5±5.8(stat)±3.5(syst) GeV Dominant Systematics JES ±2.5 GeV b JES ±2.0 GeV Template statistics ±0.9 GeV 4% 175.2±6.1(stat)±3.4(syst) GeV E. Shabalina RAS conference CDF: 2.5% @1.8fb-1 ME

  19. Matrix Element method • Pioneered by D0 and provides the most accurate measurement of the top quark mass • Calculate per-event probability density for signal and background as a function of the top quark mass using 4-vectors of reconstructed objects • Multiply the event probabilities to extract the most likely mass • Maximizes statistical power by using all event information • Extremely CPU intensive (most recent result required >0.5 M grid-hours for integration) E. Shabalina RAS conference

  20. Lepton+jets mass (ME) • LO matrix element is used to calculate probability • Transfer functions: map measured quantities (x) to parton-level ones • The jet energy calibration (JES) is a free parameter in the fit, constrained in-situ by the mass of hadronically decaying W • Use b-tagging information to reduce combinatorics • Weight each jet-parton assignment with b-tagging event probability • 24 possible weighted assignments between jets and partons 0.9 fb-1 m=170.5± 2.4 (stat+JES) ±1.2 (syst) GeV 1.6% (CDF: 1.25% @1.7fb-1) E. Shabalina RAS conference

  21. Mass combination and summary Dilepton combined: 173.7±5.4 (stat)±3.4 (syst) GeV 3.7% 1.4% D0 combined: 172.1 ± 1.5 (stat) ± 1.9 (syst) GeV 1.3% CDF New Tevatron average top mass is planned for Moriond’08 Requires a lot of work on systematics together with CDF E. Shabalina RAS conference

  22. Conclusion • Top quark is the least knows quark and the most interesting for new physics • Top physics has entered the era of precision measurements, we (finally!) have plenty of top quarks • Many top properties measurements are just beginning to have sensitivity • There is still a lot to understand about top! E. Shabalina RAS conference

  23. Backup slides E. Shabalina RAS conference

  24. Use kinematic fitter to reconstruct events to ttbar hypothesis • Build discriminant to separate top from stop • regular kinematic variables • fit output Search for scalar top • Stop is predicted by SUSY • Consider mstop <= mtop • 1+ and 0masses - close to their experimental lower limits • mstop: 145-175 GeV • 1+: 105-135 GeV • Same final state as ttl+jets stop Limit is 7-12 times higher than MSSM E. Shabalina RAS conference

  25. Single top • Since evidence ME and BNN analyses were updated • All three show similar sensitivity • Results are combined (tb+tqb)=4.7 pb p-value 0.014% significance 3.6 E. Shabalina RAS conference

  26. Mass from cross section • What mass do we measure? Depends on the convention… • Pole mass? • MS? • Pmas(6,1) in Pythia? – probably the closest given the analysis techniques • Cross sections are less dependent on the details of signal simulation • Extract for dilepton and l+jets channels independently • Depends on theory prediction: • Cacciari et al • Kidonakis, Vogt m=166.9± (stat+sys) (theory) GeV +5.9 5.2 +3.7 3.8 (lepton+jets, Kidonakis and Vogt) E. Shabalina RAS conference

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