Top Quark Results from D

1
Top Quark Results from DØ

• 40th Rencontres de Moriond
• Electroweak Interactions and Unified Theories

Herbert Greenlee Fermilab Mar. 8, 2005
2
Outline

• Overview.
• Top quark pair production cross section.
• Top quark mass.
• Single top production.
• Updated in 2005.

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The Top Quark

• Discovered at Fermilab in 1995.
• Feb. 24, 1995 papers submitted to PRL.
• Mar. 2, 1995 First public presentations at
  fnal.
• Heavier by far than any other fundamental
  fermion.
• mt Electroweak scale.
• Top quark research.
• Does the top quark exist with properties
  predicted by SM?
• Are top-like signals seen at the Tevatron due
  exclusively to SM top quark?
• Measure fundamental parameters
• mt, spin, couplings, etc.

4
Top Quark Pair Production Decay

• Top quarks are pair-produced by the strong
  interaction.
• At the Tevatron, top quark pairs are produced
  nearly at rest.

B(tWb) 100 (according to SM)

• Top Quark Pair Signatures
• Dilepton (ee, em, mm) 5
• Lepton (e,m) jets 30
• All jets 44
• ts 21
• All channels include 2 b quarks.
• Goal measure stt in as many channels
• as possible, with and without b-tagging.

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Run II Integrated Luminosity
D0 560 pb-1 on tape
230 pb-1
160 pb-1
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Pair Production Dilepton
150 pb-1
D0 Run II Preliminary

• Signature
• Two leptons (ee, em, mm).
• Missing ET
• Two jets.
• Major backgrounds.
• WW.
• Ztt.
• Z ee, mm (fake missing ET).
• Fake leptons (Wjets, multijets).

stt 14.35.1 (stat.)2.6 (syst.)0.9 (lum.) pb
-4.3
-1.9
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Pair Production Lepton jets

• Signature (preselection)
• One isolated lepton (e or m). pT gt 20 GeV/c,
  h lt 1.1 (e) or 2.0 (m).
• Missing ET. ET gt 17
  or 20 GeV.
• ³ 3 or ³ 4 jets. ET gt 15
  GeV, h lt 2.5.
• Major backgrounds.
• W jets.
• Jets misidentified as leptons.
• Final selection
• Event topology.
• Lifetime b-tag.
• Secondary vertex b-tag.
• Impact parameter b-tag.
• Cross Section Likelihood fit.

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Topological Lepton Jets Results
D0 Run II Preliminary

Combined stt 7.2 2.6 (stat.) 1.6 (syst.)
0.5 (lum.) pb
-2.4
-1.7
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b-Tagged Lepton Jets Results
Secondary vertex b-tag jet multiplicity
160 pb-1
Single tagged
Double tagged

SVT stt 8.2 1.3 (stat.) 1.9 (syst.) 0.5
(lum.) pb
-1.3
-1.6
IP stt 7.2 1.3 (stat.) 1.9 (syst.) 0.5
(lum.) pb
-1.2
-1.4
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Pair Production All Jets

• Signature (preselection)
• ³ 6 jets. ET gt 15 GeV, h lt 2.5.
• One sec vertex b-tag.
• Major backgrounds
• QCD multijets.
• Final selection
• Neural network event shape analysis based on 13
  variables.
• Final cut NN gt 0.75.

160 pb-1
220 events observed 186514 expected
background
stt 7.7 3.4 (stat.) 4.7 (syst.) 0.5 (lum.)
pb
-3.3
-3.8
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Pair Production Summary

• Top quark pair production has been observed in
  many channels.
• So far, all cross section measurements are
  consistent with each other and with SM top quark.

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Tevatron Average Top Quark MassSummer 2004
Update (Run I)

• Run I Tevatron average top quark mass.
• mt 178.04.3 GeV/c2.
• Updated April, 2004 to include improved D0
  leptonjets mass.

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Top Quark Mass Dilepton

• Mass estimator.
• Dilepton events can not be fully reconstructed.
  Usually, candidate events are consistent with a
  continuum of top quark masses.
• But different mass hypotheses are not equally
  likely.
• Use Dalitz-Goldstein-Kondo (DGK) likelihood to
  find the top quark mass that is most consistent
  with each event.
• Likelihood fit.
• Likehood fit with signal and background templates
  to extract top quark mass and error.
• Event sample 8 em, 5 ee, 0 mm.
• Expected background (1.0 em, 0.9 ee, 1.4 mm).

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DGK Candidate Likelihoods
em Candidates
Smearing
No Smearing
Likelihood Fit
D0 Run II Preliminary
ee Candidates
mt 155 14 7 GeV/c2
-13
Best mt extracted for each event
8 CL with respect to 175 GeV/c2
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Top Quark Mass Lepton Jets

• Template method (updated winter 2005, 230 pb-1).
• Event preselection W 4 jets.
• Final selection
• Topological analysis Low-mass-bias multivariate
  discriminator (four variables, not strongly
  energy-related).
• B-tagged analysis Require one b-tagged jet (SVT
  tag).
• Top quark mass is estimated by fully
  reconstructing candidate events to top-antitop
  hypothesis (kinematic fit).
• 12 possible permutations (6 with b-tag, 2 with
  double b-tag).
• Mass estimator taken from best fit.
• Top quark mass result based on likelihood fit of
  data to signal and background templates.

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Lepton Jets Mass (Template Method)
Topological Event Selection
B-tagged Event Selection
mt 170.6 4.2 6.0 GeV/c2
mt 169.9 5.8 7.8 GeV/c2
-7.1
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Lepton Jets Mass (Errors)
Source Topological (GeV/c2) b-tagged (GeV/c2)
Statistical 5.8 4.2
Jet Energy Scale 6.8 -6.5 4.7 -5.3
Jet Resolution 0.9 0.9
Gluon Radiation 2.6 2.4
Signal Model 2.3 2.3
Background Model 0.7 0.8
b-tagging 0.7
Calibration (fitting bias) 0.5 0.5
Trigger 0.5 0.5
MC Statistics 0.5 0.5
Total Systematic 7.8 -7.1 6.0

• Lepton jets top mass is currently
  systematic-limited.
• Largest systematic is Jet Energy Scale.

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Jet Energy Scale

• Calorimeter jets are corrected to have the same
  energy as hard scatter particle jets for the same
  jet algorithm (e.g. cone algorithm). Correction
  is the same for all D0 analyses.
• Offset correction (O). Remove energy not
  associated with hard scatter (spectators,
  multiple interactions, calorimeter noise).
  Luminosity dependent.
• Showering correction (S). Particles inside jet
  cone can deposit energy outside jet cone (or vice
  versa) due to showering in calorimeter or
  magnetic bend.
• Response correction (R).
• EM scale calibrated using resonances (Z, J/y,
  etc.).
• EM scale transferred to hadron calorimeter using
  ET balance in g jet events. Missing ET
  Projection (MPF) method.
• Largest correction and largest contributor to JES
  systematic error.
• Parton level correction. Correct particle jet
  energy to parton energy. Pure MC based.
• Not considered part of JES systematic error per
  se (included in modeling error).

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Jet Energy Scale Error

• For Run I, jet energy scale systematic error was
  2.5 0.5 GeV.
• For Run II top quark mass results (so far), jet
  energy scale error is (conservatively) 5 for
  jets with ETgt30 GeV (newer version of JES has
  significantly reduced uncertainty).

Run I 2.5 0.5 GeV
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Top Quark Mass Summary

• D0 Run II results are consistent with Run I world
  average.

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Single Top Quark Search
s-channel s 0.9 pb
t-channel s 2.0 pb

• Single top quarks produced by weak interaction
  are a direct probe of top quark weak couplings.
• Measure Vtb without assuming three-generation
  unitarity.
• Cross section is close to top quark pair
  production cross section (2.9 pb vs. 6.7 pb), but
  background is much larger because there are fewer
  jets.

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Single Top

• Event selection
• One isolated lepton (e or m). pT gt 15 GeV/c,
  h lt 1.1 (e) or 2.0 (m).
• Missing ET. ET gt 15
  GeV.
• Leading jet. ET gt 25
  GeV, h lt 2.5.
• 1-3 additional jets. ET gt 15
  GeV, h lt 3.4.
• 1 or 2 SVT b-tags.
• Major backgrounds.
• Wjets.
• Top quark pairs.
• Jets misidentified as leptons.
• Multivariate analysis using 11 variables
• Energy-related (pT, HT, etc.).
• Reconstructed top quark mass.
• Angular/spin-related.

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Single Top Event Shape Analysis

• Multivariate analyses (all using same variables)
• Cut-based selection.
• Neural network (best so far, results are reported
  here).

s-channel
t-channel
Signal vs. Wjets
Signal vs. tt
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Single Top - Results

• Previous Limits (95 CL)
• s-channel
• s lt 17 pb (D0 Run I)
• s lt 18 pb (CDF Run I)
• s lt 13.6 pb (CDF Run II)
• t-channel
• s lt 22 pb (D0 Run I)
• s lt 13 pb (CDF Run I)
• s lt 10.1 pb (CDF Run II)
• st combined
• s lt 14 pb (CDF Run I)
• s lt 17.8 pb (CDF Run II)

Limit from 2D binned likelihood (NN vs. NN)
s-channel s lt 6.4 pb (95 CL) (expected limit
4.5 pb)
t-channel s lt 5.0 pb (95 CL) (expected limit
5.8 pb)
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Summary and Conclusions

• So far, all top quark results are consistent with
  Standard Model predictions.
• Statistics are still limited.
• New results for top quark mass and single top
  production.
• The Tevatron is running very well. D0 currently
  has 560 pb-1 of data. Ultimately expect 4-8
  fb-1.
• Many opportunities for new top results with
  greatly increased statistics.
• Single top production should be seen in Run II.

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References

• Pair Production Cross Section.
• Dilepton http//www-d0.fnal.gov/Run2Physics/WWW/
  results/prelim/TOP/T03/T03.pdf
• Ljets (b-tag) http//www-d0.fnal.gov/Run2Physics
  /WWW/results/prelim/TOP/T04/T04.pdf
• All jets http//www-d0.fnal.gov/Run2Physics/WWW/
  results/prelim/TOP/T02/T02.pdf
• Top Quark Mass.
• Tevatron Average http//arxiv.org/pdf/hep-ex/040
  4010
• Dilepton http//www-d0.fnal.gov/Run2Physics/WWW/
  results/prelim/TOP/T11/T11.pdf
• Ljets http//www-d0.fnal.gov/Run2Physics/WWW/re
  sults/prelim/TOP/T12/T12.pdf
• Single Top.
• Cross Section http//www-d0.fnal.gov/Run2Physics/
  WWW/results/prelim/TOP/T13/T13.pdf