Tevatron Top Physics

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Tevatron Top Physics

• Meenakshi Narain
• Brown University
• (for the D0 and CDF collaborations)

Fermilab Users Meeting June 4, 2008
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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
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outline

• strong production
• cross section
• branching fractions

• mass
• couplings

• new physics?
• FCNC decays
• tt resonances
• tb resonances
• H
• .

• electroweak production
• Vtb

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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,

? mtop2
? log(mH)
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top-antitop production

• strong interaction ? top-antitop pairs
• ? 7.60.6 pb (Kidonakis Vogt,
  arXiv0805.3844)
• mt 171 GeV, NNLO(approx)NNNLL, CTEQ6M
• ? 7.6 0.6 pb (Cacciari et al.,
  arXiv0804.2800)
• mt 171 GeV, NLONLL, CTEQ6.5
• ? 7.8 0.6 pb (Moch Uwer, arXiv0804.1476)
• mt 171 GeV, NNLO(approx.), CTEQ6.5

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standard model top decay

• t?Wb with B 100
• W?qq with B 67
• W?l? with B 11
• ??e??/??? with B 17
• final state signatures for top-antitop pairs

• b-tagging

primary vertex
secondary vertex
all jets 46
??hadjets 10
dileptons 6
leptonjets 34
??hade/µ 4
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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(t?Hb) 0
B(t?Hb) 0.1
B(t?Hb) 0.2
B(t?Hb) 0.3
B(t?Hb) 0.4
B(t?Hb) 0.5
B(t?Hb) 0.6
all jets
dileptons
leptonjets
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ttbar cross section in dilepton channel

• characteristics
• small branching fraction
• two neutrinos
• kinematically underconstrained
• signalbackground 31
• selection
• two isolated leptons/leptontrack
• ?2 jets
• missing pT
• Z rejection in ee/?? channels

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ttbar cross section in dilepton channel

• CDF (2 fb-1)
• b-tag
• ? 9.01.1(stat)0.7(syst)0.5(lum) pb
• no b-tag
• ? 6.81.0(stat)0.4(syst)0.4(lum) pb
• D0 (1 fb-1)
• dileptons
• ? 6.81.2-1.1(stat)0.9-0.8(syst)?0.4(lum) pb
• leptontrack
• ? 5.11.6-1.4(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

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ttbar cross section in ? channels

• interesting because of
• t?Hb, 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

? 7.41.41.3(stat)1.21.1(syst)0.4(lum
) pb (2.2 fb-1) ?B(tt?l?) 0.19?0.08(stat)?0.07(s
yst)?0.01(lum) pb (1 fb-1)
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ttbar cross section in all-jets channel

• characteristics
• large branching fraction
• no neutrinos
• complete reconstruction
• signalbackground 116
• 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

1 fb-1
CDF PRD 76 072009
? 8.31.0(stat)2.0-1.5(syst)0.5(lum) pb
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ttbar cross section in ljets channel

• characteristics
• large branching fraction
• one neutrino
• kinematically overconstrained
• signalbackground 12, 21(b-tag)
• selection
• one isolated lepton
• 4 jets
• missing pT

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ttbar cross section in ljets 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 ?6.6?0.8(s
tat)?0.4(syst)?0.4(lum) pb
CDF (0.8 fb-1) neural network ?6.0?0.6(stat)?0.9(
syst)?0.3(lum) pb
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ttbar cross section in ljets 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) ?8.10.5(stat)0.7(syst)
0.5(lum) pb
CDF (2 fb-1) ?8.20.5(stat)0.8(syst)0.
5(lum) pb ?8.71.1(stat)0.9-0.8(syst)0.6(lum)
pb ?7.82.4(stat)1.5(syst)0.5(lumi) pb
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implications of cross section

• compare different channels
• B(t?Wb)gt0.79 _at_ 95 CL
• B(t?Hb)lt0.35 _at_ 95 CL
• for mH mW and H?cs
• compare with theory
• combine likelihood and b-tag cross section
  measurements
• stt 7.62 0.85 pb
• for mtop 172.6 GeV
• mt 170 7 GeV

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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 ? 0.90.1 pb ? 2.00.3 pb
PRD70 (2004) 114012
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single top production

• D0 results (0.9 fb-1)
• CDF results (2.2 fb-1)

• 4.7 1.3 pb
• significance 2.3 s (expected)
• significance 3.6 s (observed)
• PRL 98, 181802 (2007) PRD (accepted)

• 2.2 0.7 pb
• significance 5.1 s (expected)
• significance 3.7 s (observed)

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single top production

• measure Vtb from total cross section
• D0 Vtbgt0.68 _at_ 95 CL CDF Vtbgt0.66 _at_ 95 CL
• disentangle s and t channels

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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

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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
173.75.4(stat)3.4(syst) GeV
171.22.7(stat)2.9(syst) GeV
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leptonjets

• 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 W?qq decay
• peak of joint likelihood top quark mass
• CDF 171.41.5(stat?jes)1.0(syst) GeV (1.9
  fb-1)
• D0 172.21.1(stat)1.6(syst?jes) GeV (2.1 fb-1)

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all jets (CDF)

• kinematic fitter
• leading 6 jets
• jj/jjj masses, jet pTs
• 2-dimensional template fit
• top/W masses with smallest ?2

mtop 177.0 3.7(stat?jes) 1.6(syst) GeV
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combination
for the first time ?m/mlt1 Run II
goal ?m ? 1 GeV
http//tevewwg.fnal.gov/top/ http//lepewwg.web.ce
rn.ch/LEPEWWG/plots/winter2008/
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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

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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)

68 CL 95 CL
Phys Rev Lett 100, 062004 (2008)
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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 gt 284 GeV
• no evidence
• limits on H ?tb,t?Hb

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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

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tt resonances

• D0 (2.1 fb-1)
• technicolor Z?tt
• Hill Parke, PRD 49 (1994) 4454)

• CDF (1.9 fb-1)
• massive gluon G?tt

MZgt760 GeV for ?Z/MZ 1.2
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FCNC decays of top quarks

• flavor changing neutral currents
• highly suppressed in sm
• select Z(?ee/??) 4 jets
• fit to

B(t?Zq) lt 3.7 _at_ 95 CL
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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
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thank you
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charged Higgs bosons

• MHgt mt ? H ? tb
• tb resonance

• MHlt mt ? t ? Hb
• H?cs

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top quark charge

• is it
• t?Wb (Qtop 2/3 e)
• t?W-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 lt 0.80  (90 CL)
  
• CDF result with 1.5/fb
• p-value for SM 0.31
• exotic model XM excluded with 87 CL

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FB charge asymmetry

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

CDF unfold reconstructed AFB to go to parton
level AFBpp 0.17 0.07(stat) 0.04(syst)
D0 AFB in parton rest frame AFB
0.120.08(stat)0.01(syst) consistent with sm
expectation
Phys. Rev. Lett. 100, 142002 (2008)
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t quark

• 4th generation heavy quark
• pair-produced via strong interaction
• more massive than top
• decays to Wb, Ws, Wd

Mt lt 284GeV at 95C.L.
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mass syst

• CDF dilepton ljets

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xsec syst
D0 ljets b-tag kinematic
likelihood