CDF Measurements of Top Quark Cross Section and Mass

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CDF Measurements of Top QuarkCross Section and
Mass

• Jason Nielsen
• (LBNL)
• for the CDF Collaboration

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Why Study Top Quark Physics?
Fits into third generation - CKM
Large mass interesting in itself

• Just lighter than tungsten nucleus

CDF/D0 2 fb-1goal
Mass near EWSB scale

• Central role in these theories
• Corrections to MW ? mt2, ln(MH)

Heavy enough to decay to exotics

• On-shell charged Higgs, SUSY

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Top Quark Production and Decay
Pair production
Central, spherical events Large transverse energy
Cross section increases 30 with Tevatron vs
increase to 1.96 TeV
(Single top production is factor of 2 smaller)
Top event signatures (from W bosons)
Expect 2 b jets from top pair production
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Tevatron Run 2 at Fermilab
Tevatron successes in early 2004

• Record luminosity 5.9E31
• 3pb-1 integrated in a single store
• First store w/ antiprotons from Recycler

New CDF results use 200pb-1 collected through
Oct. 2003 (cf. 110 pb-1 from Run 1)
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CDF II Detector at the Tevatron

• Continuing work to incorporate upgraded detectors
  in data analysis
• Accurate detector simulation vital to precision
  physics measurements

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Top Data Samples
Define samples counting leptons and
jets Establish component contributions to samples
Optimize event selections for top physics and new
physics
In both cases, the sample composition is important
Develop tools to increase purity
Cross section results validate top-enriched
samples

• can also point toward new physics

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Top Dilepton Cross Section
Tight e/m selection complemented by e/m tracks
selection
Lepton track sample has looser ID requirements
for second lepton
Sensitive also to t lepton final states
2 lepton 2 jets sample is small but very clean
for top signal
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Tagging Tools Vertexing and Soft Muons
B hadrons in top signal events
are long-lived and massive
may decay semileptonically
Identify low-pt muon from decay
Vertex of displaced tracks
55 0.5
Top Event Tag Efficiency False Tag Rate (QCD jets)
15 3.6
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Cross Section Results using Tagging
Counting experiments with vertex tag and soft
muon tag in 3,4-jet bins

• Estimate backgrounds in the lepton jets sample
  from first principles
• Using data as much as possible (non-W QCD, fake
  tags)
• Some MC calculations for diboson and W heavy
  flavor backgrounds

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Kinematic Fits to Lepton Jets Sample
Provides another way of estimating the background
contribution
Sample before tagging
Vertex-tagged sample
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CDF Top Cross Section Summary
108 pb-1
126 pb-1
108 pb-1
4.7 1.6(stat.) 1.8 (syst.)
195 pb-1
202 pb-1
7.6 3.4(stat.) 1.5 (syst.)
126 pb-1
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Comparison with Theory
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Search for Single Top Quark Production
Best hope for constraining Vtb at the Tevatron
1.980.24 pb
0.880.11 pb
Final state is W exactly 2 jets

• Tag one b jet

Search strategy employs

• Likelihood fit to HT (combined search)
• Likelihood fit to b Qh (t-channel only)

• Q of lepton, h of light quark jet

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Single Top Search Results
Sample composition
Cross section limits at 95 confidence level

• Combined search s lt 13.7 pb (a priori
  sensitivity 14.1 pb)
• T-channel search s lt 8.5 pb (a priori
  sensitivity 11.3 pb)

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Top Mass Measurement Challenges
Many combinations of leptons and jets which one
is correct?

• Choose assignment kinematically most consistent
  with top
• Use all combinations, but weight them

Link observables to parton-level energies

• Detector - Generation -Simulation
• Largest uncertainties come from this
• difficult relation

Final likelihood fit methods

• Derive mass templates from top MC and fit data to
  most likely template
• Add event kinematic information, possibly
  including matrix element

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Top Mass Results Dilepton Reconstruction
6 dilepton candidates in 125.8 pb-1
Kinematic fit is underconstrained
For each lepton-b pair assignment

• Calculate best raw mass, from
• most probable combination

Fit to signal and background templates
Likelihood fit result mtop 175 17(stat.)
8(syst.) GeV/c2
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Top Mass Results Vertex-Tagged Events
22 vertex-tagged events from lepton4 jet sample
Reconstruct mt via kinematic fit
Choose single combination with best ?2 from
over-constrained fit
Compare to templates from

• Top signal MC
• Backgrounds (fixed relative)

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Summary

• Quality top physics results arriving now
• Early CDF passes on Run 2 data complete
• No unexpected physics seen so far
• Uncertainties will decrease no surprises here
• Tevatron success - more data - precise results!
• The near future is rich with opportunity.
• Improvements in current measurements
• Plenty of new measurements and searches
• W helicity, top branching ratios, top resonances,
  all-hadronic channels