Top Quark Physics

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Top Quark Physics

• Robin D. Erbacher
• University of California, Davis

Physics in Collision, Prague, Czech Republic --
Thursday July 7, 2005
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Top Quark Discovery 1995
The search for top lasted almost two decades.
Its unexpectedly heavy mass delayed discovery.
CDF D0 combined Mass (top) 174.3 ? 5.1
GeV/c2
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Why Is Top So Interesting?

• Well, top physics is different!
• Top quark lifetime is short decays before
  hadronizing
• No spectroscopy like other heavy flavor
• Top momentum and spin transferred
• to decay products
• Probes physics at higher scales
  than other known fermions
• Top (or heavy top) very hip in many
• EWSB models Higgs, Top Color,
• Little Higgs, SUSY mirror models

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Elucidating the Top Quark in Run 2

• Top pairs ?(tt) 7 pb
• Top production rate
• Mass of top
• W helicity in top events
• QCD tests
• New physics in X? tt
• Anomalous couplings,
• new particles
• Single top ?(tb) 3 pb
• Vtb
• QCD tests
• New physics?

Vtb
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of Physicists for Particle Discovery
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Physics of the Top Quark
Top physics is still one of the more sexy things
to study at the Tevatron
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Fermilab Tevatron Complex
Tevatron is pp collider with ?s 1.96 TeV

• Run 1 ended 1996
• Energy ?s 1.80 TeV
• Integrated 105 4 pb-1
• Peak Lum 2.4x1031 cm-2s-1
• Run 2 began 2001
• Record initial luminosity! 1.36x1032 cm-2 s-1 in
  May 05
• Record integrated L 22 pb-1 in
  one week

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Tevatron Performance
Delivered Luminosity achieved 1 fb-1
Luminosity collected up until Fall 04
shutdown. (Most of these results 230-350 pb-1)
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Tevatron Detectors

• New silicon and fiber tracker
• Solenoid (2 Tesla)
• Upgrade of muon system
• Upgrade of Trigger/DAQ

• New silicon and drift chamber
• Upgrade of calorimeter and muon system
• Upgrade of Trigger/DAQ

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Tevatron Experiments
19 countries, 83 institutions 664 physicists
13 countries, 58 institutions 798 physicists
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How is Top Produced?
Rarely!!
Standard Model Tevatron Pair Production Through
Strong Interaction
One top pair each 1010 inelastic collisions at ?s
1.96 TeV
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How Else is Top Produced?
Standard Model Tevatron Single Top Production
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How Does Top Decay?
Standard Model t?Wb 100

• Main usable top event topologies
• tt ? l?l?bb di-lepton 5 e?
• tt ? l?qqbb leptonjets 30 e?
• tt ? qqqqbb all hadronic 45

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Identifying Top Quarks
gt Signature-Based Analyses!
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Measuring Top Pair Production
Production Cross Section

• Why is measuring the rate of top production
  important?
• Higher cross section than predicted could be a
  sign of non-standard model production mechanisms
• Resonant state X? tt OR Anomalous couplings
  in QCD?
• It could also mean new physics in the top
  sample!

One of the first things to measure is the top
pair production rate.
Theory 6.7 pb at Mtop 175 GeV Cacciari, et al.
JHEP 404, 68 (2004)
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Finding Top Is Difficult!
In Run 1, we likely produced 500 top quark
pairs at each of CDF and D0. The problem was
finding them. CDF had only 34 ttbar pairs in
main mass samples!

• Separating Top from background
• Finding clean lepton samples
• Tagging b-jets
• Displaced vertices (silicon)
• Soft lepton tagging (SLT)
• Jet Probabilities
• Fitting to kinematical distributions using
  likelihood or neural network techniques

Challenges Acceptance for Top (improved in Run
2) Statistics Higher vs ? 30 more Separating
Top Events from Wjets and QCD Backgrounds
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b Vertex Tagging in Run 2
Both CDF D0 use Secondary Vertex b-tagging
algorithm to reduce the Wjet background for top
events
Both CDF D0 use Secondary Vertex b-tagging
algorithm to reduce the Wjet background for
top events

• b-quark lifetime c? 450?m
• ? B hadrons travel
• 3 mm before decay

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Run 1 Excess in the b-tagged Lepton Jets
Sample?

• Observed excess of
• b-tags in the 2 jet bin
• Too many SVX double tags (more than one b-tagged
  jet/event)
• Too many multiple tags (more than one b-tag/jet)
• A lot of speculation,
• but nothing solid.

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Silicon b-tagged Cross Section
Results from D0 LeptonJets Channel.
Backgrounds estimated from data and MC. Top is
excess above these for 3 jets and 4 jets.
two b-tags
Result for 230 pb-1
Sample Events tt Fraction ?(tt )
1 b tag 141 89 8.6 ? 1.6 ? 0.6 pb
? 2 b tags 23 73 8.6 ? 1.6 ? 0.6 pb
Estimate b-tagging, c-tagging, and mistag
efficiencies
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Silicon b-tagged Cross Section
Results from CDF LeptonJets Channel.
Backgrounds estimated from data and MC. Top is
excess above these for 3 jets.
Results for 318 pb-1
two b-tags
Sample Events tt Fraction ?(tt )
? 1 b tag 138 81 7.9 ? 0.9 ? 0.9 pb
? 2 b tags 33 90 8.7 ? 1.7 ? 1.5 pb
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CDF Double-Tagged Event
b-jet taggers provide clean samples of single and
double b-tagged events, useful for single top,
top properties, and searches such as for WH.
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Kinematics to Find Top CDF
?(tt)6.0 0.8 1.0 pb
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Kinematics to Find Top Dzero
Result with 230 pb-1 6.7 1.4 1.4 pb
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Run 2 Top Cross Section Results
CDF Run II Preliminary
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Verge of Discovery Single Top
Not yet observed but we are getting close!
Tevatron ?NLO 0.88?0.11 pb
Tevatron ?NLO 1.98?0.25 pb
Predicted total cross section 3 pb (compare
ttbar production 6.7 pb)
Best Run 1 Search Limit ? lt 14 pb (CDF)
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Whats Interesting about Single Top?

• Predicted by theory via the Wtb interaction
• Allows direct access to Vtb CKM matrix element
• top polarization, V-A in EWK top interaction
• Probe b-quark PDF (t-channel)
• Look for physics beyond SM
• Different sensitivity for s t-channels
• 4th generation
• anomalous Wtb couplings
• FCNC (t?Z/? c)
• new charged gauge boson
• (W top flavor)

Tait, Yuan PRD63, 014018 (2001)
?t-channel (pb)
?s-channel (pb)
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Best Single Top Sensitivity
Neural Networks Separate Single top from
Backgrounds
Dzero has current best limits with 230 pb-1
95 C.L. ?s lt 6.4 pb ?t lt 5.0
pb (Expected cross sections ?s 1 pb ?t
2 pb)
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Standard Model Higgs?
LEP Direct Search Limit Mass (Higgs) gt 114 GeV
top and W masses constrain the Standard Model
Higgs
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History Top Mass Publications
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Top Mass Challenges

• Background contamination
• Events with real W, Z bosons Wjets, Zjets,
  WW, WZ, ZZ
• Events with misidentified isolated leptons
• Imperfect measurements
• Large energy resolution for Jets
• Small Statistics Depends on b-tags
• Different ways to assign jets to partons
• 2 jets (dilepton)-gt 2 combinations
• 4 jets (leptonjets)-gt 24 combinations
• 6 jets (all jets)-gt 720 combinations
• Jet energy scale of jets is known to a few
  percent

bias
statistical uncertainty
systematic uncertainty
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Top Mass Methodology
Matrix Element Analyses
Template Analyses

• Choose a variable strongly correlated with the
  mass of the top quark (Reconstructed Mtop)
• Create templates using events simulated with
  different Mtop values ( background)
• Perform maximum likelihood fit to extract
  measured mass

• Build a probability (MEPDFresponse functions)
  integrating over unmeasured quantities (signal
  and background)
• Evaluate the probability of each event as a
  function of the top mass
• Calibrate (or not) using the simulation

D0 and CDF Use Both Methods for Most Sensitive
Results
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Run 1 Top Mass Measurements
Run I World average
Top Mass Measurement is challenging !
Mtop 178.0 4.3 GeV/c2
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Most Precise Run 1 MeasurementDzero -- Matrix
Element Method

• Method similar to Dalitz, et. al., dilepton Mt
  measurement by DØ - PRD 60 52001 (1999).
• Each event has its own probability density as a
  function mass
• The probability depends on all measured
  kinematical quantities

D0 Run 1 Measurement Mt 180.1?3.6?3.9
GeV Nature Vol 429, Page 640
Compare D0 Run 1 Mt174.0 ?5.6 ? 5.5 GeV
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Current Worlds Best Top Mass CDF Run 2--
Template Method

• Key Ingredients
• Statistics Large sample of single and double
  tagged events (improved b-tagging)
• Systematics Improved jet energy scale
  Simultaneous fit to top mass and jet energy scale
  using W?jj decays

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Top Mass in Dilepton Events

• Pros
• Fewer combinations
• Cons
• Unconstrained kinematics 2 neutrinos in final
  state
• Small branching fraction (5)

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Top Mass Summary
Many Results on Top Mass Different Techniques
Channels
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New Tevatron Mass Combination
Preliminary Use Most Precise Results from Each
Experiment.
Just released! hep/ex 0507006
D0- Run 1 dilepton/LJ CDF- Run 2
dilepton/LJ
Mtop 174.3 3.4 GeV
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Future for Top Mass
With improvements on the dominant systematic,
the jet energy scale, ?Mtop scales with
statistics!
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Top Decay Properties
We said t?Wb, but really 100?

• Indirect measurement using the CKM matrix
• Elements Vub and Vcb measured to be very
  small from decay of B mesons
• Unitarity and only three generations
• implies Vtb is 0.998 _at_ 90 CL

• With top quark samples we can
• measure it directly as R
• The relative rates of ttbar events
• with 0/1/2 b-tags is very sensitive to R

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Measuring BR(t?Wb)/BR(t?Wq)

• Compare the expected top with the observed top
  in the 0/1/2 tag subsets and extract R by
  maximizing the likelihood.

CDF Result
Set F-C lower limit R gt0.61 at 95 CL Vtb
gt 0.79 at 95 CL (assuming unitarity)
Mild excess in double b-tags sample drives the R
value above 1
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R Consistent with Standard Model
So. This means that the top decays to a b quark
most of the time, as expected.
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Measurement of BR(t?Hb)

• Assume each top quark has 5 possible decay modes
• t ? Wb
• t ? Hb ? tbb ? Wbbb
• t ? Hb ? ??b
• t ? Hb ? csb
• t ? Hb ? Wh0b ? Wbbb
• Data use four CDF samples
• dilepton
• leptonjets (1 tag)
• leptonjets (2 or more tags)
• lepton?h

(CPsuperH, full SUSY EW/QCD corrections)
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Limits MH v. tan?, Min Stop Scenario
BRs predicted by MSSM in Minimal Stop Mixing
scenario
Typical search for h0 at LEP(hep-ph/9912223).
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What Can We Take from This?
There is no evidence within reach for top
decaying to charged Higgs. So. Assume that top
decays to Wb.

• But, is the nature of the tWb vertex as expected?

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W Helicity from t?Wb Decays

• Examines the nature of the tWb vertex,probing
  the structure of weak interactions at energy
  scales near EWSB
• Stringent test of SM and V-A interaction Expect
  F0 0.7 in the Standard Model,
• F- 0.3 and F 0.0

V-A Suppressed
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Dzero W Helicity Using cos?
cos? angle between charged lepton and top
direction in W rest frame
Events with b-tags in 230 pb-1
Both leptonjets events, and dilepton samples, in
three analyses using topological and b-tagged
analyses.
Combined b-tag Topological In dilepton and
leptonjets Result F 0.04 0.11 0.06 Or
0 lt F lt 0.25 _at_ 95 CL
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What About Production?

• We know that, within errors
• The top decays mostly to Wb
• The nature of the tWb vertex is whats expected.
• Measured Production Cross Sections Consistent
  with Standard Model, within errors

• Are some of the top-like events from a heavy top?

• Are some small number of top pairs coming from a
  resonance?

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Search for High ET Top-Like Events
HT distribution for W4p, ttbar, and t' where
M(t')225 GeV We can set limits on new physics
processes in top sample
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HT Plot with t' Signal, M(t') 225 GeV
Plot for fit result with t' signal included at
95 CL limit ?(ttbar) ?6.12 pb in this fit

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Result Limits (pb) Versus M(t')
Constraints vary with assumed top mass, but not
by much.
Expected 1? sensitivity
Mtop ?constraint
170 7.8 ? 1.0 pb
175 6.7 ? 0.9 pb
180 5.75 ? 0.7 pb
Taken from hep-ph/0303085
Limits on ??BR(t'?Wq)2
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Projected Limits Higher Luminosity
Large improvement in systematic errors
expected (Jet energy scale dominates!) Should do
better!

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Run 1 Searches for ttbar Resonances
Coming in Summer 05 Run 2 CDF Matrix Element
Analysis X?ttbar
Search for narrow model-independent X?tt
resonances in ljets
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Top Physics at the LHC
Top Factory Copious amounts of top will be
produced at the LHC at CERN (Geneva), with
center-of-mass energy of 14 TeV. Good
Precision top physics, such as top mass,
couplings, associated Higgs
production, kinematic distributions,
anomalous production, rare decays Bad --
Searches for exotic signatures and new physics.
Top will be a major background!
Understanding top quark production and properties
will be crucial for success at LHC. What we
learn at the Tevatron, both physics and analysis
tools, extends directly to LHC!
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Top Physics at the ILC
The Linear Collider will be important for
precision top physics, necessary for full
exploration of the EWK scale, and of electroweak
symmetry breaking.
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Summary

• We may soon discover whether the top quark plays
  a special role in the Standard Model. The top
  sector will be a hunting ground for hints of new
  physics at the Tevatron, and will distinguish new
  physics in the LHC era. Many top properties
  remain to be explored
• Top spin, top charge, top lifetime
• Spin Correlations in top events, Top
  Forward-Backward Asymmetry
• Consistency of top quark kinematics with the SM
• Rare top decays (t?Zc) and FCNCs
• Top production (q-qbar v. glue-glue)

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Conclusions
As we increase our datasets at the Tevatron in
Run 2, Dzero CDF will have much to say about
the possibility of new physics in our top quark
samples. Understanding top at the Tevatron will
also allow us to immediately distinguish new
physics at the LHC. With a few years of running,
the LHC will improve our understanding of the
top, and its role in and beyond the Standard
Model, and the ILC will follow with precision
measurements in the top sector.
Stay tuned top quark publications are
streaming out again