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Top physics at the Tevatron

Top physics at the Tevatron. Flera Rizatdinova (Oklahoma State University) on behalf of CDF and D0 Collaborations XV Lomonosov Conference Moscow, August 24, 2011. Outline. Top quark: production and detection Top quark physics at the Tevatron Top quark pair production cross section

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Top physics at the Tevatron

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  1. Top physics at the Tevatron FleraRizatdinova (Oklahoma State University) on behalf of CDF and D0 Collaborations XV Lomonosov Conference Moscow, August 24, 2011

  2. Outline • Top quark: production and detection • Top quark physics at the Tevatron • Top quark pair production cross section • Single top quark production cross section • Top quark mass measurements • Other properties (AFB, top width, spin correlations, W helicity) • Conclusions Flera Rizatdinova XV Lomonosov Conference

  3. Top quark production and decay • Top quark pair production via strong interaction. For Mt=172.5 GeV, • Assuming unitarity of 3-generation CKM matrix B(tWb) ~ 100% • Distinguish top quark pair events by the consequent decays of the W bosons: • Both Wℓ :dilepton channels • One Wℓ and the other Wqq’ : lepton+jets channels • Both W qq’ : all hadronic channel Flera Rizatdinova XV Lomonosov Conference

  4. Top pair production cross section • Total number of ttbar events collected by end of Tevatron: O(104) • results are limited by systematic uncertainties • Production cross section measured in all channels, results consistent with SM l+>3jets, no b-tag l+3jets, one b-tag random forest (RF) of decision trees CDF, dileptons (5.1 fb −1) pretag: σ=7.4±0.6(stat)±0.6(syst)±0.5(lumi) pb b-tag: σ=7.3±0.7(stat)±0.5(syst)±0.4(lumi) pb D0, l+jets (5.3 fb −1) σ=7.78+0.77−0.64(stat+syst+lumi) pb  Flera Rizatdinova XV Lomonosov Conference

  5. Top pair cross section summary CDF Run II Experimental uncertainty ∆σ/σ~6 %; all results are consistent across channels, methods and experiments Dominant experimental uncertainties: JES, b-tagging Tevatron combination is coming soon Flera Rizatdinova XV Lomonosov Conference

  6. Electroweak single top production • Year 2009: observation of single top quark production at the Tevatron • Allows direct access to Vtb CKM matrix element (σ~|Vtb|2) • Combination based on 2.1-3.2 fb −1 of data ~33% s-channel σNLO=0.9 pb t-channel σNLO=2.0 pb (PRD 66, 054024 (2002) ~67% FleraRizatdinova XV Lomonosov Conference

  7. Single top production cross section: latest news • D0: measurement of model-independent t-channel single top production using 5.4 fb −1 of data (arXiv:1105.2788) σ(pptqb + X) = 2.90±0.59 (stat + syst) pb significance 5.5σ Flera Rizatdinova XV Lomonosov Conference

  8. Top quark mass measurements • We know the mass of top quark better than of any other quark, so why more precision? • top mass is a fundamental SM parameter • top mass measurement provides consistency test of SM, and may constraint BSM parameters / discriminate between SM/BSM • Since Mtop is large, quantum loops involving top quarks are important to include when calculating precision observables (e.g. sinθW2, Rb, MW, …) Flera Rizatdinova XV Lomonosov Conference

  9. Top quark mass: many analyses • matrix element, neutrino weighting, from tt cross section,.. CDF: l+jets with matrix element Probability of being signal or BG is calculated as a function of Mt + neural network to discriminate S from B + in situ JES calibration Mt = 173.0 ±1.2(stat+syst) GeV D0: dileptons with matrix element shown: normalized likelihood for data Mt = 174.0 ±1.8(stat) ±2.4(syst) GeV FleraRizatdinova XV Lomonosov Conference

  10. Top quark mass combination FERMILAB-TM-2504-E • Using 5 published Run I results, 5 published Run II results, and 2 preliminary Run II results Mt = 173.2±0.9 GeV (5.2% precision) main contributions Flera Rizatdinova XV Lomonosov Conference

  11. Forward-backward asymmetry AFB • Leading order QCD: top production is symmetric • NLO QCD: small asymmetry (AFB~6%) • New physics (Z’, W’, axigluons, technicolor,…) may give rise to much larger asymmetries Flera Rizatdinova XV Lomonosov Conference

  12. AFB: CDF results Dileptons, 5.1 fb-1 AFB= 0.42 ± 0.15(stat) ± 0.05(syst) l+jets, 5.3 fb-1 (arXiv:1101.0034) lab. frame:  AFB= 0.150±0.055 (stat+sys) ttbar frame:  AFB= 0.158±0.075 (stat+sys) NLO QCD expectation: 0.058±0.009 l+jets, Mtt dependence: AFB(Mtt>450GeV)= 0.475±0.114 (stat+sys) MCFM prediction: 0.088±0.013 Flera Rizatdinova XV Lomonosov Conference

  13. AFB: D0 results l+jets, 5.4 fb-1 (arXiv:1107.4995): see asymmetry! (No Mtt dependence observed) reconstructed ∆y reconstructed charge-signed lepton rapidity Flera Rizatdinova XV Lomonosov Conference

  14. Top quark width • SM predicts Гt=1.3 GeV • CDF (4.3fb−1): direct measurement • Γt<7.6 GeV (95% C.L.) • 0.3<Γt<4.4 GeV (68% C.L.) • D0 (2.3 fb−1): extract Γt from partial decay width Γ(tWb) measured from t-channel single top, and Br(tWb) measured from ttbar • Γt=1.99+0.69−0.55GeV use reconstructed Mt shape main systematics: JES Flera Rizatdinova XV Lomonosov Conference

  15. ttbar spin correlations • Top decays before hadronization spin information preserved in decay products • SM predicts k = 0.78 • Parameter k is related to observables where Flera Rizatdinova XV Lomonosov Conference

  16. ttbar spin correlations: results • Measured ttbar spin correlations consistent with the SM predictions D0 method 1 (lepton angles) C=0.10±0.45 CDF measured together with W helicity in l+jets channel, L = 5.3 fb-1 C=0.72 ±0.64(stat) ±0.26(syst) D0 method 2 (matrix element) C=0.57±0.31 Flera Rizatdinova XV Lomonosov Conference

  17. W helicity • In SM, top decays to Wb via V—A interaction  have certain predictions for W helicity: • ƒ0=0.698 (fraction of longitudinal W’s) • ƒ−=0.301 (fraction of left-handed W’s) • ƒ+=4.1×10−4 (fraction of right-handed W’s) • Observation of deviation from these numbers  indication of BSM Flera Rizatdinova XV Lomonosov Conference

  18. W helicity combination • Contributions: • CDF l+jets, 2.7 fb−1: PRL 105, 042002 (2010) • CDF dileptons, 5.1 fb−1: preliminary • D0 l+jets/dileptons combo, 5.4fb−1: PRD 83, 032009 (2011) • Results (assuming ƒ++ƒ0+ƒ−=1): • Consistent with the SM ƒ0 = 0.732±0.063(stat) ±0.052(syst) ƒ+= −0.039±0.034(stat) ±0.030(syst) Flera Rizatdinova XV Lomonosov Conference

  19. Conclusions • Tevatron has a broad top quark physics program • ttbar production cross section is measured in all final states with precision close to the theoretical calculation • an important measurement which can’t be repeated at the LHC – different energy! • Top quark mass measured with 0.5% accuracy • Many studies of top quark properties have been done • Indications from both CDF and D0 that AFB asymmetry is higher than the SM predicts • Still a lot of data to be included in the measurements! Flera Rizatdinova XV Lomonosov Conference

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