Tevatron Top Physics

Tevatron Top Physics

Meenakshi Narain

Brown University(for the D0 and CDF collaborations)

Fermilab User’s Meeting – June 4, 2008

6/4/2008 Meenakshi Narain - Fermilab User's Meeting 2

top at Fermilab• 13 years ago…

…we observed a few handfuls of top quark decays

• today…

…we are performing detailed studies of 1000s of top decays

4.2 fb-1

3.7 fb-1


outline• strong production

– cross section– branching fractions

• mass• couplings

• new physics?– FCNC decays– tt resonances– tb resonances– H+

– ….

• electroweak production– |Vtb|


W W

t

b

W W

H0

mtop2

log(mH)

why is the top quark important?• most massive elementary particle

– dominant contributor to radiative corrections

– how is its mass generated?• topcolor?

– does it couple to new physics?• massive G, heavy Z’, H+, …


top-antitop production • strong interaction top-antitop pairs

= 7.6±0.6 pb (Kidonakis & Vogt, arXiv:0805.3844)• mt = 171 GeV, NNLO(approx)+NNNLL, CTEQ6M

= 7.6 ±0.6 pb (Cacciari et al., arXiv:0804.2800) • mt = 171 GeV, NLO+NLL, CTEQ6.5

= 7.8 ±0.6 pb (Moch & Uwer, arXiv:0804.1476) • mt = 171 GeV, NNLO(approx.), CTEQ6.5


standard model top decay• tWb with B ≈ 100%

– Wqq with B ≈ 67 %– Wℓ with B ≈ 11%

e with B ≈ 17 %

• final state signatures for top-antitop pairs

• b-tagging

dileptons6%had+e/μ

4%

lepton+jets34%

had+jets10%

all jets46%

primaryvertex

secondary vertex


dileptons lepton+jets

why measure the ttbar cross section?• cross section analysis

– basic understanding of signal and background necessary for further study

– consistency between channels– decay branching fractions– are there non-standard decays?

B(tH+b) = 0 B(tH+b) = 0.1 B(tH+b) = 0.2 B(tH+b) = 0.3 B(tH+b) = 0.4 B(tH+b) = 0.5 B(tH+b) = 0.6 all jets


ttbar cross section in dilepton channel• characteristics

– small branching fraction– two neutrinos– kinematically underconstrained + signal:background = 3:1

• selection– two isolated leptons/lepton+track 2 jets

– missing pT

– Z rejection in ee/ channels

Dilepton(BR~6%)

e,

b-jet

e,

b-jet

miss pT

Dilepton(BR~6%)

e,

b-jet

e,

b-jet

e,

b-jet

e,

b-jet

miss pT


ttbar cross section in dilepton channel• CDF (2 fb-1)

– b-tag = 9.0±1.1(stat)±0.7(syst)±0.5(lum) pb

– no b-tag = 6.8±1.0(stat)±0.4(syst)±0.4(lum) pb

• D0 (1 fb-1)– dileptons

= 6.8+1.2-1.1(stat)+0.9

-0.8(syst)0.4(lum) pb

– lepton+track = 5.1+1.6

-1.4(stat)+0.9-0.8(syst)0.3(lum) pb

– combined = 6.2 0.9(stat) +0.8

-0.7(syst)0.4(lum) pb



ttbar cross section in channels• interesting because of

tH+b, H+• 3 types of hadronic decays

• require– 1 , 1 e/, ≥2 jets, missing pT

– 1 jet is b-tagged

• neural networks distinguish decays from background

1 trackno em cluster12%

1 trackem cluster38%

≥2 tracks15%

= 7.4+1.4–1.3(stat)+1.2

–1.1(syst)±0.4(lum) pb (2.2 fb-1)

B(ttl) = 0.190.08(stat)0.07(syst)0.01(lum) pb (1 fb-1)


ttbar cross section in all-jets channel• characteristics

+ large branching fraction+ no neutrinos+ complete reconstruction– signal:background 1:16

• selection– 6-8 well separated jets– no significant miss pT

– at least one b-tag– neural network

ET, event shape, angles

• backgrounds– pretag data x tagging

probability from 4-jet control region

jet

b-jetb-jet

jet

jet jet

All-hadronic(BR≈46%)

= 8.3±1.0(stat)+2.0-1.5(syst)±0.5(lum) pb

1 fb-1

CDF PRD 76 072009


ttbar cross section in l+jets channel• characteristics

– large branching fraction– one neutrino– kinematically overconstrained – signal:background = 1:2, 2:1(b-tag)

• selection– one isolated lepton – 4 jets – missing pT jet

e,

b-jet

b-jet

jet

Lepton+jets(BR~34%)

MET


ttbar cross section in l+jets channel• extract top fraction using event topology

– angles, momentum sums, and event shape variables– dominated by statistical uncertainties

D0 (0.9 fb-1) likelihood discriminant=6.60.8(stat)0.4(syst)0.4(lum) pb

CDF (0.8 fb-1)neural network=6.00.6(stat)0.9(syst)0.3(lum) pb


ttbar cross section in l+jets channel• count number of events with at least one b-tagged jet

– smaller statistical uncertainty– large systematic uncertainty from jet energy calibration and b-tagging

D0 (0.9 fb-1)

=8.1±0.5(stat)±0.7(syst)±0.5(lum) pb

CDF (2 fb-1)

=8.2±0.5(stat)±0.8(syst)±0.5(lum) pb=8.7±1.1(stat)+0.9

-0.8(syst)±0.6(lum) pb=7.8±2.4(stat)±1.5(syst)±0.5(lumi) pb



implications of cross section• compare different channels

– B(tWb)>0.79 @ 95% CL– B(tH+b)<0.35 @ 95% CL

for mH+ = mW and H+cs

• compare with theory– combine likelihood and b-tag

cross section measurements

– σtt = 7.62 ± 0.85 pb

for mtop = 172.6 GeV

– mt = 170 ± 7 GeV


single top production• electroweak production of top quarks

─ s channel (tb) ─ t channel (tqb)

– event selection• 1 isolated e/

• missing pT

• 2-4 jets, ≥1 b-tag

NLO = 0.9±0.1 pb = 2.0±0.3 pb PRD70 (2004) 114012

event yields (CDF 2.2 fb-1)

Njets 2 3

single top 103 32

backgrounds 1492 745

data 1535 712


single top production• D0 results (0.9 fb-1)

• CDF results (2.2 fb-1)

= 4.7 ± 1.3 pbsignificance: 2.3 σ (expected)significance: 3.6 σ (observed)PRL 98, 181802 (2007) PRD (accepted)

= 2.2 ± 0.7 pbsignificance: 5.1 σ (expected)significance: 3.7 σ (observed)


single top production• measure |Vtb| from total cross section

– D0: |Vtb|>0.68 @ 95% CL CDF: |Vtb|>0.66 @ 95% CL

• disentangle s and t channels


top mass measurement• template fits

– mass estimator (eg best mt from kinematic fitter)

– fit probability density functions from simulated tt events generated to data

• event-by-event likelihood – for each event determine

likelihood as a function of mt (eg by integrating over LO matrix element)

– extract mass from peak of joint likelihood

Event 3Event 2Event 1


dilepton channel • D0 (1 fb-1)

– compute weight curve as a function of top mass for each event

– template fit to mass distribution

• CDF (2 fb-1)– selection uses evolutionary

neural network that optimizes resolution

– compute event weight using LO matrix element

matrix weighting

173.7±5.4(stat)±3.4(syst) GeV 171.2±2.7(stat)±2.9(syst) GeV


lepton+jets• matrix element analysis

– integrate over LO matrix element to get likelihood for event as a function of top quark mass

– in situ jet energy calibration using Wqq decay – peak of joint likelihood = top quark mass

– CDF: 171.4±1.5(statjes)±1.0(syst) GeV (1.9 fb-1) – D0: 172.2±1.1(stat)±1.6(systjes) GeV (2.1 fb-1)


all jets (CDF)• kinematic fitter

– leading 6 jets

– jj/jjj masses, jet pTs

• 2-dimensional template fit– top/W masses with smallest 2

jet

b-jetb-jet

jet

jet jet

mtop = 177.0 ± 3.7(statjes) ± 1.6(syst) GeV


combination

http://tevewwg.fnal.gov/top/http://lepewwg.web.cern.ch/LEPEWWG/plots/winter2008/

for the first time m/m<1% Run II goal: m 1 GeV


top quark couplings• if top plays a special role in ewk symmetry breaking its

couplings to W bosons may differ from predictions

• sm predicts V-A coupling at Wtb helicity of W boson– F0 = 0.7, F = 0.3, F+ = 0.0

(longitudinal, left-handed, right-handed)– different distributions of *

angle between lepton and top in W rest frame


top quark couplings• D0 (1 fb-1)

F0 = 0.42 0.17(stat) 0.10(sys)

F+= 0.12 0.09(stat) 0.05(sys)

• CDF (1.9 fb-1)

Phys Rev Lett 100, 062004 (2008)

68% CL95% CL

http://www-cdf.fnal.gov/physics/new/top/summaryplots/whel.eps


searches for non-standard physics• quarks with charge 4/3e • FB ttbar asymmetry • 4th generation t’ quarks • scalar top production • charged Higgs bosons• tb resonances • ttbar resonances• FCNC decays of top quarks

disfavored

consistent with sm

m > 284 GeVno evidence limits on H+ tb,tH+b


tb resonances• vector resonance: W’tb• left-handed couplings

– DØ include interference with SM W

• right-handed couplings – decay to Rl/qq, depending on m(R)

• lower limits on MW’: 731-825 GeV


tt resonances • D0 (2.1 fb-1)

– technicolor Z’ttHill & Parke, PRD 49 (1994) 4454)

• CDF (1.9 fb-1)– massive gluon Gtt

MZ’>760 GeV

for Z’/MZ’ =1.2%


FCNC decays of top quarks• flavor changing neutral currents

– highly suppressed in sm– select Z(ee/) + ≥4 jets

– fit to

B(tZq) < 3.7% @ 95% CL


conclusion• top physics has come a long way since 1995

– Tevatron is still the only place to do it– ttbar cross section measured to 11%– evidence for single top production

• 5 this summer?

– top quark mass measured to 0.8%• will reach uncertainties below 1 GeV

– top properties and possible non-standard physics studies in great detail

– in the last year: D0 7 papers and 13 theses– in the last year: CDF 8 papers and 16 theses

• it still looks like top

http://www-d0.fnal.gov/Run2Physics/top/http://www-cdf.fnal.gov/physics/new/top/top.html

thank you


charged Higgs bosons• MH+> mt H+ tb

• tb resonance

• MH+< mt t H+b

• H+cs


top quark charge• is it

– tW+b (Qtop = 2/3 e)– tW-b (Qtop = -4/3 e)

• Exotic model – doublet (–1/3e,–4/3e) ?– D. Chang et al., PRD59 (1999) 091503

• D0 PRL 98, 041801 (2007) – 4/3e excluded at 92% CL– fraction of exotic quark pairs

< 0.80 (90% CL)

• CDF result with 1.5/fb– p-value for SM: 0.31– exotic model XM excluded with

87% CL


FB charge asymmetry • Asymmetry:

– LO – no asymmetry– NLO – small (3-5%) asymmetry predicted in sm– new physics could lead to larger values (eg Z’)

Phys. Rev. Lett. 100, 142002 (2008)

D0 AFB in parton rest frame:AFB = 0.12±0.08(stat)±0.01(syst)consistent with sm expectation

CDF unfold reconstructed AFB to go to parton levelAFB

pp = 0.17± 0.07(stat) ± 0.04(syst)

http://www-cdf.fnal.gov/physics/new/top/2007/topProp/KA_Afb/Afb_plots_gif/DyQl_inclusive_mcatnlo_rewPythia.gif


t’ quark• 4th generation heavy quark

– pair-produced via strong interaction– more massive than top– decays to Wb, Ws, Wd

Mt’ < 284GeV at 95%C.L.

http://www-cdf.fnal.gov/physics/new/top/2008/tprop/Tprime2.3/public_files/bless-plots/HT-m280.gif


mass syst• CDF dilepton l+jets


xsec syst

source uncertainty

vertex 0.15 pb

e id 0.11 pb

μ id 0.08 pb

jet id 0.12 pb

non-W bkg 0.06 pb

jet response 0.30 pb

MC model 0.29 pb

b-tagging efficiency 0.48 pb

total 0.69 pb

source selection fit total

vertex 0.13 pb 0.13 pb

e id 0.10 pb 0.10 pb

μ id 0.06 pb 0.06 pb

jet id 0.10 pb 0.02 pb 0.12 pb

non-W bkg 0.10 pb 0.10 pb

jet response 0.35 pb 0.26 pb 0.11 pb

MC model 0.13 pb 0.09 pb 0.11 pb

template stats 0.15 pb 0.15 pb

total 0.36 pb

D0 l+jets b-tag kinematic likelihood

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Tevatron Top Physics