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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

Top physics at the Tevatron

FleraRizatdinova (Oklahoma State University)

on behalf of CDF and D0 Collaborations

XV Lomonosov Conference

Moscow, August 24, 2011

outline
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

top quark production and decay
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

top pair production cross section
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

top pair cross section summary
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

electroweak single top production
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

single top production cross section latest news
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

top quark mass measurements
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

top quark mass many analyses
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

top quark mass combination
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

forward backward asymmetry a fb
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

a fb cdf results
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

a fb d0 results
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

top quark width
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

ttbar spin correlations
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

ttbar spin correlations results
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

w helicity
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

w helicity combination
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

conclusions
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