RECENT CDF RESULTS ON THE TOP QURK Nikos Giokaris University of Athens On behalf of the CDF...

RECENT CDF RESULTS ON THE TOP QURK

Nikos GiokarisUniversity of Athens

On behalf of the CDF Collaboration

September 21, 2006

Crimea 2006, 9/21 N. Giokaris 2

OUTLINEI. TEVATRON PERFORMANCEII. CDF DETECTORIII. TOP QUARK

I. DiscoveryII. Production & DecayIII. Top quark Properties

I. Cross sectionII. Top massIII. W helicity in Top decaysIV. Search for Single TopV. Search for Resonant Production

IV. Conclusions


• Run II with

• Record peak Luminosity: 2.3x1032cm-2s-1

• Integrated delivered luminosity: 1.5 fb-1

• CDF recorded luminosity: 1.3 fb-1

TEVATRON PerformanceTeV96.1=s

Expected:

• 2fb-1 by 2006

• 4fb-1 by 2007

• 8fb-1 by 2009


CDF Detector


Top Quark History

• Searched for since the late ‘70s, after the discovery of the b quark

• Discovered by CDF and D0 in 1995 (Run I)– ∫Ldt~100pb-1

– Mass ~175 GeV – Decays to Wb, as Mtop>MW– ttbar Cross Section 7.6pb


Top-Antitop Production and Decay

Half-life of top: ~10-25s•Top decays before hadronizing!

ttqq ttgg

e-e(1/81)

mu-mu (1/81)

tau-tau (1/81)

e -mu (2/81)

e -tau(2/81)

mu-tau (2/81)

e+jets (12/81)

mu+jets(12/81)

tau+jets(12/81)

jets (36/81)

W bosons decay either hadronically or leptonically.W decays define channel:

Dilepton: 12% Lepton+jets: 44% All-hadronic: 44%

Strong Production (6.7pb @ NLO) dominates at Tevatron Energy 85%: 15%:

TeV96.1=s


Top Cross Section

Mtop sNLO(pb) ± ds

170 7.8±1175 6.7±0.8180 5.7±0.7

Theoretical predictions

TeV96.1=s

•Measured in all topologies.•Use complementary techniques:topological (counting) vs shape fit.

Deviation from SM expectations could indicate non-SM production mechanism or new physics in top sample.


How we measure Top

Top decays before hadronizing:Observed through W decay products leptons (Lepton +Jet and Dilepton channels)

Jets (problem with jet combinations)

Two methods to measure top:

1. Use b-tagging Good S/B Lose ~50% in efficiency

2. Use kinematics (NN, etc) Also good S/B


L+jets channel:

• Large BR ~44%

and

• Good S/B

bbvlqqbbWW tt

Lepton+Jets Cross Section

Results in 750 pb-1

Lepton + MET + ≥1b-tag

pb1.1±6.0±0.6=)notag(σpb0.1±6.0±2.8=)btag(σ

syststattt

syststattt


Dilepton Cross Section

pb5.0±0.1±5.1±3.8=σ )lumi()syst()stat()notag(tt

Result in 750 pb-1

Dilepton channel:

• Small BR ~5% for e and μ leptons

BUT

• Easy to identify e and μ

• Very good S/B

bbvvllbbWW tt -+


All Hadronic Cross Section

Result in 1 fb-1

pb5.0±1.5-0.2+.1±3.8=σ )lum()syst()stat()notag(tt

L+jets channel:

• Large BR ~44%

but

• Poor S/B

bbqqqqbbWW tt


Cross Section Summary

NOW REACHED 12% uncertainty @760 pb-1

Expected

10% uncertainty/experiment with 2fb-1

We are doing much better than predicted in TDR!!


Top Mass

)L.C%95(GeV175<M)L.C%68(GeV89=M

H

42+30H

• Fundamental parameter of SM.• Recent Tevatron combination: Mtop=171.4 ± 2.1 GeV• EWK fit gives:

• In Run II, expect δMW= ±25MeV and δMtop= ± 2 GeV35% constraint on the Higgs Mass.• Sensitive to new physics through radiative corrections


Top Mass MeasurementMass measurement is hard• Jet combinatorics• ISR/FSR jets• Jet Energy Scale (JES) uncertainty

Two methods•Template Methods•Matrix Element Method (ME)The idea in both methods is try to reconstruct the parton level quantities


Top Mass with Template Method1. Evaluate event-by-event best “reconstructed mass”, Mrec, by using observed kinematics

of ttbar event (e.g.: χ2 fitter)

2. Create “templates”, i.e. MC predictions for Mrec using different true masses of Mtop.

3. Measure top mass with likelihood fit of data Mrec to signal + background template.

Background


Top Mass in lepton+jets• JES uncertainties are the largest source of systematics:

±1σJES σMtop = ±3GeV

• Fit simultaneously for MWjj and Mbjj using 2D templates of true Mtop and σJES

• (940 pb-1) achieves world single best measurement with Matrix Element Analysis Technique (MEAT)

Mtop = 170.9 ± 2.2 (stat+JES) ± 1.4 (syst) GeV/c2=170.9 ± 2.6 GeV/c2


Top Mass with ME• Calculate event-by-event signal probability curve (rather than single Mrec) using

decay matrix element and transfer functions.• Calculate event-by-event background probability (no dependence on Mtop!).• Combine signal and background probability in one likelihood vs Mtop for entire

sample

ME method uses maximal information per event at a price of simplifiedassumptions.Final mass result and uncertainty is calibrated against simulatedevents.


Top Mass in DileptonUnderconstrained system: two neutrinos but only one MET measurement.Remind that the major difficulty in reconstructing top is the parton-level cambinatoric problem

(1 fb-1) assumes highest two ET jets are the b-jets and integrate ME probability over 8 unknowns

Using 78 events (27.8 bkgr)

– Confirmed in b-tag dilepton sample(S:B~30:1) .– Consistent results in template measurements.

)tt(p),v(p),v(p T21

GeV9.3±9.3±5.164=M syststattop


Top Mass in All HadronicLow S/B (~1/8) and large combinatorial background (90 permutations for 6 jets)

Top Mass Measurement with Template method using NN Selection has measured @1fb-1:

Mt = 174.0 ± 2.2 (stat) ± 4.8 (syst) Gev/c2


Top Mass Summary

CDF Mtop@1fb-1:

Mtop= 170.9±1.4stat ±1.9syst


Top LifetimeTop in SM has very short lifetime (SM ct ~ 3x10-10 μm)

look for anomalous lifetime by fitting impactparameter of lepton in l+jets events

ct< 52.5μm(@95%C.L.)


W helicity in Top Decays

F0+F-+F+=1• Helicity states of the W:

• Because top is heavy:

• SM test: if V-A interaction F-˜ 0.3 F+ ˜ 0if V+A interaction F- ˜ 0 F+ ˜ 0.3

012.0±703.0=m+m+M2m

~F 2b

2top

2W

2top

0

Top in SM has V-A decay. Longitudinal Left-handed Right-handed

F0 F- F+

suppressed by factorsof order m2

b/m2t

Variables sensitive to W helicity are angular distributions of W products in W rest frame.

• cos(θ*) distribution• M2

l b

• Lepton pT spectrum


CDF has 3 measurements1. cos(θ*) with full tt reconstruction in l+jets

2. M2lb in dil and l+jets

3. cos(θ*) in l+jet samples using the mass χ2 fitter

W helicity measurements

.)L.C%95(@26.0<F06.0±19.0±85.0=F

+

syststat0

.L.C%[email protected]<F+

Assume V-A and measure F0/F- with other components fixed at SM value.Measure F+ and put limits on V+A/new physics.

318pb-1

)L.C%95(@11.0<F06.0±06.0±06.0=F

04.0±12.0±61.0=F

+

syststat+

syststat0

750pb-1

955pb-1


Single top

NLO σ

t-channel 1.98±0.25 pb

s-channel 0.88±0.11 pb

TeV96.1=s

• Single top is produced via weak interaction at a rate ~1/3 that of top. Allows direct measurement of Vtb.

B.W. Harris et al. Phys. Rev. D 66 054024 (2002) • Kinematically wedged between non-top and top signal, plus high backgrounds (S/B~1/20) requires very sophisticated analysis techniques.• Use l +MET+2jet (>=1 btag) events: same signature as )bb WH(Htt• s and t-channel searched jointly and separately (have different sensitivityto new physics).

s-channel production

(W*)

Vtb

t-channel production (Wg fusion)

Vtb

Vtb


Single Top LimitsCDF has 2 methods

1. Multivariate Likelihood Function2. Neural Network

Channel s+t t(pb) s(pb)SM σNLO 2.9±0.4 2.0±0.3 0.9±).1

Lhood σ95% 4.3(3.4) 2.9(2.6) 5.1(5.7)

NN σ95% 3.4(5.7) 3.1(4.2) 3.2(3.7)695 pb-1

95% observed (expected) exclusion limit getting close to SM expectations!

Statistical errors onlyBased on SM single-top cross section

Projections• 2.4 s excess with 1 fb-1 expected by end of ‘06• 3 s excess around 1.5 fb-1

(syst ignored)


Search for Resonant ProductionttXpp 0Look for bumps in the ttbar invariant mass spectrum

CDF looks for generic spin 1 resonance (X0)• Assume ΓX0 = 1.2%xMX0 (narrow resonance)• Test masses between 450 GeV and 900 GeV in 50 GeV increments.

Set 95% confidence level limit for σX0 ateach mass.

Exclude leptophobic Z’ with Mz’ < 725 GeV.


ConclusionsCDF II has used as much as x10 the RUN I statistics to perform several studies on the top quark

– ttbar production cross section has been measured to about 12% level

– Top mass precision is already known to ~1.5% level – expect to go down to ~1% by the end of RunII, thus further constraining the SM Higgs mass

– Single top should be detected soon– All the, up to now, measurements show that top

quark behaves as predicted by the Standard Model


Acknowledgments

R. ErbacherV. GiakoumopoulouT. MaruyamaM. Tecchio

RECENT CDF RESULTS ON THE TOP QURK Nikos Giokaris University of Athens On behalf of the CDF...

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