Top Quark Physics at the Tevatron

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Top Quark Physics at the Tevatron

• Kevin Lannon
• The Ohio State University
• For the CDF and DØ Collaborations

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Top Quark is Special

• Top is really massive
• Comparable to gold nucleus!
• In Standard Model Mass related to coupling to
  Higgs
• Special relationship between top and Higgs?
• Top quark decays very quickly (10-24 seconds)
• Decays before hadronization
• Momentum and spin transferred to decay product
• Tevatron is currently the only place in the world
  to study top

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Tevatron Performance

• Tevatron status covered next session
• Quick summary
• Performance is great
• Thanks to the accelerator division!
• Integrated luminosity critical for top physics
• Most analyses statistically limited

• Total for Run II (per experiment)
• Delivered 2.8 fb-1
• Recorded to tape 2.5 fb-1 (gt 20 x Run I!)
• Results for summer conferences will use Jan-Feb
  of this year

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CDF and DØ Detectors

• General purpose detectors capable of many
  different physics measurements
• Top physics uses almost all detector systems

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Top Quark Production at Tevatron

• QCD pair production
• ?NLO 6.7 pb (for mTop 175 GeV)
• First observed at Tevatron in 1995

85
15
s-channel
t-channel
???
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SM Top Quark Decays

• Lepton Jets Relatively clean with manageable
  backgrounds
• Dilepton Cleanest but lowest statistics
• All Hadronic Large branching fraction but
  formidable backgrounds

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Top Quark Physics is Rich
charge
, production mechanism
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Top Production Cross Section

• Important indirect test of many top quark
  properties
• NLO calculation 6.7 pb with 13 uncertainty
• Looking for excess of events with top-like
  signature

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Cross Section Summary

• Similar experimental and theoretical uncertainty
• Working on Tevatron combination

CDF Run II Preliminary
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Why Measure the Top Mass?

• Its the most striking feature of the top quark!
• Consistency of mass and cross section ? Standard
  Model Top?
• Related to the Higgs mass through radiative
  corrections to the W mass
• Provides indirect constraint on Higgs mass
• Consistency test when combined with limits from
  direct searches

?MW ? M2top
?MW ? ln MHiggs
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Measuring the Top Mass is Challenging
What a theorist sees
What an experimentalist sees

• Measure jets, not partons
• Calibrate the energy scale
• Associating jet with correct parton
• Extra jets add confusion

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Results Lepton Jets Channel

• Exceeding original Run II expectations because of
  in-situ jet energy scale calibration

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Tevatron Combination

• Many more measurements than can be discussed here
• Combine for better precision
• Best individual measurement 1.5
• Combination 1.1 uncertainty!

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Searches for Resonant Production

• Look for bump in invariant mass distribution ?
  New particle that decays to top pairs
• Uses many of the same techniques as top mass
  analyses
• No evidence for resonance
• Updates planned for 2 fb-1

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The Search for Single Top
t-channel
s-channel

• Standard Model
• Rate ? Vtb2
• Background for other searches (Higgs)
• Beyond the Standard Model
• Sensitive to a 4th generation
• Sensitive to new particles or interactions
• W, H, FCNC
• Main challenge Backgrounds
• Signal to background ratio 1/20 in best
  channels
• Expect single top signal smaller than background
  uncertainties

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The Search for Single Top
t-channel
s-channel

• Standard Model
• Rate ? Vtb2
• Shares many common issues with Higgs searches
• Beyond the Standard Model
• Sensitive to a 4th generation
• Sensitive to new particles or interactions
• W, H, FCNC
• Main challenge Backgrounds
• Signal to background ratio 1/20 in best
  channels
• Expected single top signal smaller than
  background uncertainties

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Multivariate Analysis Techniques
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Multivariate Analysis Techniques

• Both CDF and DØ used multiple multivariate
  techniques (Six separate analyses)
• Boosted decision trees (DØ), matrix elements
  (CDF, DØ), multivariate likelihood (CDF), neural
  network (CDF), Bayesian neural network (DØ)
• Can combine results for greater sensitivity

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Single Top Results
Matrix Element
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All Single Top Results
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More Top Properties

• See public webpages for CDF and DØ
• http//www-cdf.fnal.gov/physics/new/top/top.html
• http//www-d0.fnal.gov/Run2Physics/top/top_public_
  web_pages/top_public.html

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Summary

• Very exciting times in top physics at the
  Tevatron
• Top mass uncertainty 1.1!
• Aiming for 1 GeV uncertainty before end of Run II
• Already have evidence for single top production
  gt 3?!
• Pushing for 5? observation
• Cross section Uncertainty comparable to theory
• Just beginning to gain sensitivity to many other
  top quark properties
• Great place to search for new physics!
• Stayed tuned for new results this summer and
  beyond

Published so far. 6 more submitted. Other
analyses in progress.