Recent Measurements of the Top Quark from Fermilab

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Recent Measurements of the Top Quark from Fermilab

• Kevin Lannon
• The Ohio State University
• For the CDF and D0 Collaborations

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Note to Slide Readers

• This presentation makes heavy use of animations.
  Several slides to do make sense unless viewed in
  animated form. I recommend viewing this
  presentation as a slide show.

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The Top Quark and the Standard Model
Top quark needed to complete the period table
of the Standard Model

• Top quark discovery
• Late 1970s Existence suggested by discovery of
  b quark
• 1980s Existence required for consistency of
  Standard Model
• Eluded experimental observation for two decades
• 1995 Observed at Tevatron

• Properties of top quark that made discovery
  difficult also make study interesting!

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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 (Yukawa coupling)
• Top Yukawa coupling near unity (natural value?)
• Why are couplings for other quarks so small in
  comparison?
• Special relationship between top and Higgs?
• Top quark decays very quickly (10-24 seconds)
• Decays before hadronization
• No hadron spectroscopy
• Momentum and spin transferred to decay product

5 orders of magnitude between quark masses!
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The Tevatron Accelerator

• Highest energy accelerator in the world (Ecm
  1.96 TeV)
• World record for hadron collider luminosity
  (Linst 2.86E32 cm-2s-1)
• Only accelerator currently making top quarks

• Run I (1992-1995)
• Integrated 105 ? 4 pb-1 luminosity
• Discovery of the top quark
• Run II (2001-present)
• Integrated gt 2.5 fb-1 and counting!
• Precision study of top quarks

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Tevatron Performance
Integrated Luminosity
Peak Luminosity
Todays Presentation 1 fb-1
Analyzed by Summer

• Integrated luminosity at CDF and D0
• Total delivered 2.7 fb-1 to each experiment
• Total recorded 2.2 fb-1 ( 20? Run I!) at each
  experiment
• So far for top analyses, used up to 1 fb-1
• More analyses with 1.2-2.0 fb-1 in progress for
  summer
• Doubling time currently 1 year
• Future 4 fb-1 by end of 2007, 8 fb-1 by 2009

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CDF and D0 Detectors
D0
CDF

• 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
• First observed at Tevatron in 1995

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s-channel
t-channel

• EWK single-top production
• s-channel ?NLO 0.9 pb
• t-channel ?NLO 2.0 pb
• First evidence!

???

• Other?

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SM Top Quark Decays
BR(t?Wb) 100

• Particular analyses usually focus on one or two
  channels
• New physics can impact different channels in
  different ways
• Comparisons between channels important in
  searching for new physics

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Top Signatures
Electron or muon
Jet shower of particles
Neutrino Missing ET
Dilepton
Lepton Jets
All Hadronic
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Top Production Rates
Needle in haystack (approx.)

• Efficient Trigger
• 90 for high pT leptons
• Targeted event selection
• Distinctive final state
• Heavy top mass
• Advanced analysis techniques
• Artificial Neural Networks

• Like finding a needle in a haystack . . . .

One top pair each 1010 inelastic collisions at ?s
1.96 TeV
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Top Quark Physics is Rich
Parallel Sessions

• Systematically limited measurements
• Cross section (12 precision)
• Mass (1 precisions)
• Statistically limited measurements
• Most other measurements of top quark properties
• Top quark charge
• Top quark production mechanism
• Searches
• Single top production
• Resonant production
• Top to charged Higgs

J14, R14
J14, R14
C14, F1, X13
F1, J14, K14, R14, T14
K13, K14, J14
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Measuring the Top Cross Section

• Agreement between theory and experimental
  important test of top quark properties (spin,
  couplings, mass)
• Techniques form basis for top properties
  measurements
• Key separating top from backgrounds
• Two main techniques

Event Kinematics central, spherical events with
large transverse energy
HT ? scalar sum of lepton, jet, and missing ET
Presence of b-jets Detected through long
life-time of the B hadrons. Decays at displaced
vertex
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Recent Cross Section Results
Lepton Jets
Individual Measurements approaching same
precision as theoretical calculation
Session R14 (Monday)
Excess of events with ? 3 energetic jets ? 1
b-tag
Dilepton Channel
Excess of events with ? 4 energetic jets and
top-like kinematics (determined by a
multivariate discriminant technique

• Excess of events with
• Two high pT leptons
• Two energetic jets
• Missing ET

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Cross Section Summary

• Measurements in many different channels
• Experimental precision approaching theoretical
  uncertainty (12)
• Working on Tevatron combination

Several cross section talks in Session R14
(Monday)
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
• More precision ? Tighter constraint
• Tevatron Run II goal
• Uncertainty lt 3 GeV/c2 with 2 fb-1 data
• New Goal Uncertainty 1 GeV/c2 by end of Run II

?MW ? M2top
?MW ? ln MHiggs
Summer 2006
Updated Result in Next Talk
Already exceeded!
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Measuring the Top Mass is Challenging
What a theorist sees
What an experimentalist sees

• Measure jets, not partons
• Account for bias and resolution ? Jet Energy
  Scale
• Determine which jet should be assigned to which
  parton ? Combinatorics (up to 720 permutations
  for all hadronic decay!)
• Dont measure neutrino momentum ?Infer pT
  indirectly
• Extra jets from radiation confuse things

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Jet Energy Scale

• Determine parton energy from measurements in
  calorimeter
• Correct for
• Detector effects
• Fragmentation/Hadronization
• Underlying event
• Energy scale determined from data and MC
• Uncertainties in jet energy scale directly affect
  top mass uncertainties
• Leading uncertainty without special treatment!

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In-Situ Jet Energy Scale Calibration

• W mass known very precisely from other
  measurements
• Use W mass reconstructed from jets to constrain
  jet energy scale

• Uncertainty decreases as data increases
• Key reason why were doing better than originally
  projected!

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Results Lepton Jets Channel
Worlds best
170.9 2.2 (statJES) 1.4 (syst) GeV/c2
Session T14 (Monday)

• Both use
• Matrix element technique
• In-situ JES calibration

170.5 2.4 (statJES) 1.2 (syst) GeV/c2
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Results All-Hadronic
Session T14 (Monday)
171.1 3.7 (statJES) 2.1 (syst) GeV/c2

• Combines matrix element and template techniques
• First incorporation of in-situ JES calibration in
  all-hadronic channel
• This measurement more precise than expected based
  on past performance!

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Tevatron Combination
Top mass measurements in Sessions F1 (Saturday),
J14, K14 (Sunday), and T14 (Monday)

• Many more measurements than can be discussed here
• Combine for better precision
• Best individual measurement 1.5
• Combination 1.1 uncertainty!
• See next talk for impact on indirect Higgs
  constraints

170.9 1.1 (stat) 1.5 (syst) GeV/c2
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Top Charge

• Are we observing Standard Model top?
• Standard Model top has charge 2/3
• Alternative hypothesis exotic quark with charge
  -4/3
• Difficult to measure (t?Wb or W-b)
• W charge measured through the lepton
  (straightforward)
• Bottom charge inferred from jet (difficult)
• Correctly pair the lepton and b jet (difficult)

Exclude top charge of -4/3 with 81 C.L.
Session K14 (Sunday)
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Top Production Mechanism
Session J14 (Sunday)
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• Does ratio of qq ?tt and gg ?tt match theoretical
  expectation?
• Depends on top mass, pdfs, etc.
• Could be modified by non-standard production
• Exploit correlation between low pT track
  multiplicity and number of gluons

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The Search for Single Top
t-channel
s-channel

• Standard Model
• Rate ? Vtb2
• Spin polarization probes V-A structure
• Background for other searches (Higgs)
• Beyond the Standard Model
• Sensitive to a 4th generation
• Flavor changing neutral currents
• Additional heavy charged bosons
• W or H
• New physics can affect s-channel and t-channel
  differently

Tait, Yuan PRD63, 014018(2001)
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Signal and Backgrounds
Backgrounds
Other EWK
Single-top Signature
tt
High pT e or ?
? MET
Multi-jet QCD
W Heavy Flavor
W Light Flavor (Mistags)
2 High ET jets, ? 1 b-tagged
Must use multivariate, kinematic techniques to
separate signal from background
Signal / Background 1/20 Signal size
background uncertainty
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Multivariate Analysis Techniques
Combine information from several variables into a
single, more powerful discriminant

• Six separate analyses
• Used many different multivariate analysis
  techniques Decision tree, matrix element,
  multivariate likelihood, neural network
• Only moderate correlations among discriminants ?
  Can combine results for greater sensitivity

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Single Top Results
Matrix Element
Neural Network
Expected Signal Significance 2.6?
Expected Signal Significance 2.5?
2.3?!
Session X13 (Tuesday)
Session F1 (Saturday)
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Single Top Results
Expected Signal Significance 1.8?
Expected Signal Significance 2.1?
2.9?!
3.4?!
Session X13 (Tuesday)
Session X13 (Tuesday)
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All Single Top Results
D0 Combination 3.5?
Session X13 (Tuesday)
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Limit on Vtb

• ?(single top) ? Vtb2
• First direct limit on Vtb
• No assumption about number of quark generations
• Assuming Standard Model production
• Pure V-A and CP conserving interaction
• Vtd2 Vts2 ltlt Vtb2 ? B(t ?Wb) 100
• Bayesian limits with flat prior between 0 and 1

Session X13 (Tuesday)
0.68 lt Vtb lt 1 at 95CL (f1L 1)
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Summary

• Many more top physics results available than
  could be covered here
• See public webpages for CDF and D0
• http//www-cdf.fnal.gov/physics/new/top/top.html
• http//www-d0.fnal.gov/Run2Physics/WWW/results/top
  .htm
• Very exciting times in top physics at the
  Tevatron
• Top mass uncertainty 1.1!
• First evidence for single top production gt 3?!
• Cross section Uncertainty on measurements
  approaching theoretical uncertainties
• Just beginning to gain sensitivity to many top
  quark properties
• Great place to search for new physics!
• Stayed tuned for new results this summer

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Backup Slides
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Weights in the Combination
CDF and D0 both crucial for best precision
Better than expected performance from
all-hadronic measurement ? In-situ JES calibration