Evidence%20for%20Single%20Top%20Quark%20Production%20at%20CDF

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Evidence for Single Top Quark Production at CDF

• Bernd Stelzer
• University of California, Los Angeles

HEP Seminar, University of Pennsylvania September,
18th 2007
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Outline

• Introduction to Top Quarks
• Motivation for Single Top Search
• The Experimental Challenge
• Analysis Techniques
• Likelihood Function Discriminant (1.51fb-1)
• Matrix Element Analysis (1.51fb-1)
• Measurement of Vtb
• More Tevatron Results
• Summary / Conclusions / Outlook

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The Tevatron Collider

• Tevatron is worlds highest energy Collider (until
  2008)
• Proton Anti-proton Collisions at ECM1.96 TeV

4
Top Production at the Tevatron
Once every 10,000,000,000 inelastic collision..
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Top Production at the Tevatron

• At the Tevatron, top quarks are primarily
• produced in pairs via the strong interaction
• Single top quark production is also predicted
• by the Standard Model through the
• electroweak interaction (?st ½ ?tt)

Discovered 1995!
?NLO 6.70.8 pb mt175GeV/c2
s-channel ?NLO 0.880.07 pb
t-channel ?NLO 1.980.21 pb
Cross-sections at mt175GeV/c2, B.W. Harris et
al., Phys.Rev. D70 (2004) 114012, Z. Sullivan
hep-ph/0408049
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Top Quark in the Standard Model

• Top Quark is heaviest particle to date
• mt170.9 ? 1.8 GeV/c2 March 2007
• Close to the scale of electroweak symmetry
  breaking
• Special role in the Standard Model?
• Top Quark decays within 10-24s
• No time to hadronize
• We can study a bare quark

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Why measure Single Top Production ?

• Source of single 100 polarized top quarks
• Short lifetime, information passed to decay
  products
• Test V-A structure of W-t-b vertex
• Allows direct Measurement of CKM- Matrix
  Element Vtb
• ?single top Vtb2
• indirect determinations
• of Vtb enforce 3x3 unitarity

Ceccucci, Ligeti, Sakai PDG Review 2006
Precision EW rules out simple fourth generation
extensions, but see J. Alwall et. al., Is
Vtb1? Eur. Phys. J. C49 791-801 (2007).
Vtb
s-channel
t-channel
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Sensitivity to New Physics and Benchmark for WH

• Single top rate can be altered due to the
  presence of New Physics
• t-channel signature Flavor changing
  neutral currents (t-Z/?/g-c couplings)
• - s-channel signature Heavy W? boson,
  charged Higgs H, Kaluza Klein excited WKK

Z
c
t
W?,H

• s-channel single top has the same final state
• as WH?l?bb
• gt benchmark for WH!

Tait, Yuan PRD63, 014018(2001)
CMSSM Study Buchmuller, Cavanaugh, deRoeck,
S.H., Isidori, Paradisi, Ronga, Weber, G.
Weiglein07
(?WH 1/10 ?s-channe))
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Experimental Challenge
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Event Signatures
MET
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CDF II Detector (Cartoon)

• Silicon tracking
• detectors
• Central drift
• chambers (COT)
• Solenoid Coil
• EM calorimeter
• Hadronic
• calorimeter
• Muon scintillator
• counters
• Muon drift
• chambers
• Steel shielding

h 1.0
h 2.0
?
h 2.8
Single top analysis needs full detector!
Thanks to great work of detector experts and
shift crew!
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CDF II Detector
Central muon
Central calorimeters
Endplug calorimeters
Drift chamber tracker
Silicon detector
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Data Collected at CDF
This analysis uses 1.51 fb-1 (All detector
components ON)
Delivered 3.0 fb-1
Collected 2.7 fb-1
Tevatron people are doing a fantastic
job! 3fb-1 party coming up!
Design goal
CDF is getting faster, too! 6 weeks turnaround
time to calibrate, validate and process raw data
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Single Top Selection

• Event Selection
• 1 Lepton, ET gt20 GeV, ?e(?)lt 2.0 (1.0)
• Missing ET, (MET) gt 25 GeV
• 2 Jets, ET gt 20 GeV, ?lt 2.8
• Veto Fake W, Z, Dileptons, Conversions, Cosmics
• At least one b-tagged jet, (displaced
  secondary vertex tag)

CDF W2jet Candidate Event Close-up View of
Layer 00 Silicon Detector
12mm
Run 205964, Event 337705 Electron ET 39.6
GeV, Missing ET 37.1 GeV Jet 1 ET 62.8 GeV,
Lxy 2.9mm Jet 2 ET 42.7 GeV, Lxy 3.9mm
Number of Events / 1.51 fb-1 Single Top Background S/B
W(l?) 2 jets 136 28300 1/210
W(l?) 2 jets b-tag 61 1042 1/17
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B-quark Tagging and Jet Flavor Separation

• Exploit long lifetime of B hadrons (c? 450
  ?m)boost
• B hadrons travel Lxy3mm before decay with large
  track multiplicity

Charm tagging rate 10 Mistag rate 0.5
Neural Network Jet-Flavor Separator

• Separate tagged b-jets from charm/light jets
  using a Neural Network trained with tracking
  information
• Lxy, vertex mass, track multiplicity, impact
  parameter, semilepton decay information, etc...
• Used in all single top analyses

NN Output
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Background Estimate

• WHF jets (Wbb/Wcc/Wc)
• Wjets normalization from data and heavy flavor
  (HF) fractions from ALPGEN Monte Carlo

• Top/EWK (WW/WZ/Z?tt, ttbar)
• MC normalized to theoretical cross-section

• Non-W (QCD)
• Multijet events with semileptonic b-decays or
  mismeasured jets
• Fit low MET data and extrapolate into signal
  region

Z/Dib
tt
Wbb
non-W
Mistags

• WHF jets (Wbb/Wcc/Wc)
• Wjets normalization from data and
• heavy flavor (HF) fraction from MC

Wcc
Wc

• Mistags (W2jets)
• Falsely tagged light quark or gluon jets
• Mistag probability parameterization obtained from
  inclusive jet data

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Non-W Estimate

• Build non-W model from anti-electron selection
• Require at least two non-kinematic lepton ID
  variables to fail
• EM Shower Profile ?2, shower maximum matching (dX
  and dZ), Ehad/Eem,
• Data is superposition of non-W and Wjets
  contribution -gt Likelihood Fit

Before b-tagging
After b-tagging
Signal Region
Signal Region
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W Heavy Flavor Estimate

• Method inherited from CDF Run I (G. Unal et. al.)
• Measure fraction of Wjets events with heavy
  flavor (b,c) in Monte Carlo
• Normalize fractions to Wjets events found in
  data

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Signal and Background Event Yield
CDF RunII Preliminary, L1.51 fb-1 Predicted
Event Yield in W2jets
s-channel 23.9 6.1
t-channel 37.0 5.4
Single top 60.9 11.5
tt 85.3 17.8
Diboson 40.7 4.0
Z jets 13.8 2.0
W bottom 319.6 112.3
W charm 324.2 115.8
W light 214.6 27.3
Non-W 44.5 17.8
Total background 1042.8 218.2
Total prediction 1103.7 230.9
Observed 1078 1078 1078
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Analysis Flow Chart
CDF Data
Analysis Technique
Analysis Event Selection
Apply MC Corrections
Monte Carlo Signal/Background
Result
Signal Background
Template Fit to Data
Cross Section
Discriminant
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Analysis Techniques
Likelihood Discriminant Matrix Element
Analysis More Tevatron Results
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The Likelihood Function Analysis
Bkgr
tchan schan
Signal
Wbb ttbar
Nsig
Unit Area
Nbkg
Discriminant
i, index input variable
Leading Jet ET (GeV)
Uses 7 (8) kinematic variables for t-channel
(s-channel) Likelihood Function e.g. M(Wb) or
kin. Solver ?2, HT, QxEta, NN flavor separator,
Madgraph Matrix Elements, M(jj)
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Kinematic Variables
HT ?ET(lepton,MET,Jets)
Background
Signal
Background
Signal
tchan schan
Wbb ttbar
tchan schan
Wbb ttbar
tchan schan
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Analysis Techniques
Likelihood Discriminant Matrix Element
Discriminant More Tevatron Results
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Matrix Element Approach

• No single golden kinematic variable!
• Attempt to include all available kinematic
  information by
• using Matrix Element approach
• Start from Fermis Golden rule
• Cross-sections Matrix Element2 ? Phase space
• Calculate an event-by-event probability (based on
  fully differential cross-section calculation) for
  signal and background hypothesis

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Matrix Element Method
Event probability for signal and background
hypothesis
Leading Order matrix element (MadEvent)
W(Ejet,Epart) is the probability of measuring a
jet energy Ejet when Epart was produced
Integration over part of the phase space F4
Parton distribution function (CTEQ5)
Input only lepton and 2 jets 4-vectors!
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Transfer Functions
Full simulation vs parton energy
Eparton
Ejet
Double Gaussian parameterization
where
? E (EpartonEjet)
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Event Probability Discriminant (EPD)

• We compute probabilities for signal and
  background hypothesis per event
• ?Use full kinematic correlation between signal
  and background events
• Define ratio of probabilities as event
  probability discriminant (EPD)

b Neural Network b-tagger output
Signal
Background
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Event Probabilty Discriminant

• S/B1/17 over full range
• Likelihood fit will pin down
• background in low score region

S/B1/1 In most sensitive bin!
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Cross-Checks
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Cross-Checks in Data Control Samples

• Validate method in various data control samples
• W2 jets data (veto b-jets, selection orthogonal
  to the candidate sample)
• Similar kinematics, with very little contribution
  from top (lt0.5)

px
py
pz
E
Lepton (Electron/Muon)
Leading Leading Jet
Second Leading Jet
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Cross-Checks in Data Control Samples

• b-tagged dilepton 2 jets sample
• Purity 99 ttbar
• Discard lepton with lower pT

• b-tagged lepton 4 jets sample
• Purity 85 ttbar
• Discard 2jets with lowest pT

CDF Run II Preliminary
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Monte Carlo Modeling Checks
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Template Fit to the data
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Binned Likelihood Fit

• Binned Likelihood Function
• Expected mean in bin k
• All sources of systematic uncertainty included as
  nuisance parameters
• Correlation between Shape/Normalization
  uncertainty considered (di)

ßj sj/sSM parameter single top (j1) Wbottom
(j2) Wcharm (j3) Mistags (j4) ttbar (j5) k
Bin index i Systematic effect di Strength of
effect eji 1s norm. shifts ?jik 1s
shift in bin k
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Rate vs Shape Systematic Uncertainty
Systematic uncertainties can affect rate and
template shape

• Rate systematics give fit templates freedom to
  move vertically only
• Shape systematics allow templates to slide
  horizontally (bin by bin)

Rate and
Shape systematics
Discriminant
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Sources of Systematic Uncertainty
CDF RunII Preliminary, L1.51fb-1
Source Rate Uncert. Shape Uncert.
W bottom 36 ?
W charm 36
Mistags 15 ?
ttbar 21
Non-W 40 ?
Jet Energy Scale 1..13 ?
Initial State Radiation 3.2 ?
Final State Radiation 5.3 ?
Parton Dist. Function 1.4 ?
Monte Carlo Modeling 1.6 ?
Efficiencies/b-tag SF 5
Luminosity 6
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Results
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Matrix Element Analysis

• Matrix Element analysis observes excess over
  background expectation
• Likelihood fit result for combined search

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ME Separate Search

• Perform separate likelihood fit for
• s-channel and t-channel signal
• Both signal templates float independently

s-channel ?s1.1 pb
1.0
-0.8
t-channel ?t1.9 pb
1.0
-0.9
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Likelihood Function Discriminant

• Likelihood Function analysis also observes excess
  over background expectation
• Observed deficit previously in 0.955 fb-1

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Likelihood Function 2D Fit
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Signal Significance
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Hypothesis Testing
L. Read, J. Phys. G 28, 2693 (2002) T. Junk,
Nucl. Instrum. Meth. A 434, 435 (1999)

• Calculate p-value Faction of background-only
  pseudo-experiments with a test statistic value as
  signal like (or more) as the value observed in
  data
• Define Likelihood ratio test statistic
• Systematic uncertainties included in
  pseudo-experiments
• Use median p-value as measure for the expected
  sensitivity

Less signal like
More signal like
Median p-value 0.13 (3.0?)
Observed p-value 0.09 (3.1?)
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Hypothesis Testing
Less signal like
More signal like
Median p-value 0.20 (2.9?)
Observed p-value 0.31 (2.7?)
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Signal Features
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Single Top Candidate Event
Central Electron Candidate Charge -1, Eta-0.72
MET41.85, MetPhi-0.83 Jet1 Et46.7
Eta-0.61 b-tag1 Jet2 Et16.6 Eta-2.91
b-tag0 QxEta 2.91 (t-channel
signature) EPD0.95
Run 211883, Event 1911511
Jet1
Lepton
Jet2
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Single Top Signal Features
Look for signal features in high score output
EPDgt0.95
EPDgt0.90
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QxEta Distributions in Signal Region
EPDgt0.9
EPDgt0.95
3)
4)
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m(W,b) Distributions in Signal Region
EPDgt0.9
EPDgt0.95
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Unconstrained Likelihood Fit
Remove all background normalization constraints
and perform a five parameter likelihood fit (all
template shapes float freely) ? Best fit for
signal almost unchanged. ? Uncertainty increased
by about 20
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Direct Vtb Measurement

• Using the Matrix Element cross
• Section PDF we measure Vtb
• Assume Standard Model V-A coupling
• and Vtb gtgt Vts, Vtd

s-channel
t-channel
Flat prior 0 lt Vtb2 lt 1
Vtb 1.02 0.18 (experiment) 0.07 (theory)
Vtbgt0.55 at 95 C.L.
Z. Sullivan, Phys.Rev. D70 (2004) 114012
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Single Top Results from DØ
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D0 Results
Boosted Decision Tree
First direct limit on Vtb 0.68 ltVtblt 1 _at_ 95CL
or Vtb 1.3 0.2
Expected p-value 1.9 (2.1?)
Observed p-value 0.04 (3.4?)
PRL 98 18102 (2007)
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Summary of Results
Summary
Expected 3.0? 2.9? 2.6? 2.1? 1.9? 2.2?
Observed 3.1? 2.7? 3.4? 3.2? 2.7?
Combined 2.3? / 3.6?

• CDF analyses more sensitive
• D0 observes upward fluctuation
• In 900 pb-1 of data

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CDF Single Top History
2006 Established sophisticated analyses Check
robustness in data control samples
2004 Simple analysis while refining Monte Carlo
samples and analysis tools
Phys. Rev. D71 012005
2 Years

• Development of powerful
• analysis techniques
• (Matrix Element, NN,
• Likelihood Discriminant)
• NN Jet-Flavor Separator
• to purify sample
• Refined background
• estimates and modeling
• Increase acceptance
• (forward electrons)
• 10x more data

2007 Evidence for single top quark production
using 1.5 fb-1 (expected and observed!)
First Tevatron Run II result using 162
pb-1 ?single top lt 17.5 pb at 95 C.L.
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Conclusion

• Evidence for electroweak single top quark
  production at the Tevatron established by CDF and
  D0 experiment!
• First direct measures of CKM matrix element Vtb
• Advanced analysis tools essential to establish
  small signals buried underneath large backgrounds
• Entering the era of single top physics. 4-5 sigma
  observation possible with gt3 fb-1 of data -
  Perhaps CDF is lucky this time..
• Separate s-channel from t-channel, measure more
  top properties, e.g. top polarization etc..
• Exciting times! The race for first observation is
  on..
• Important milestone along the way to the Higgs!

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Search for Heavy W? Boson
W?

• Search for heavy W? boson in W 2, 3 jets
• Assume Standard Model coupling strengths
• (Z. Sullivan, Phys. Rev. D 66, 075011, 2002)
• Perform fit to MWjj distribution

• Previous Limits
• CDF Run I M(W?R) gt 566 GeV/c2 at 95 C.L.
• D0 Run II M(W?R) gt 630 GeV/c2 at 95 C.L.

Limit at 95 C.L. M(W) gt 760 GeV/c2 for M(W) gt
M(?R) M(W) gt 790 GeV/c2 for M(W) lt M(?R)
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LHC is the Future
Large Hadron Collider
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LHC is the Future
Additional single top process at the LHC!
(negligible at the Tevatron)
Wt- production

• LHC will be a top quark factory
• stt 800 pb
• st-channel 243 pb (153 pb for top and 90 pb for
  antitop production)
• ss-channel 11 pb (6.6 pb for top and 4.8 pb for
  antitop production)
• sWt 50-60 pb (negligible at the Tevatron)
• First precision t-channel measurement (10)
  expected after
• 1st year of running (10 fb-1/year)
• s-channel measurement harder because of small
  cross section
• and large backgrounds (sounds familiar!)
• The associated Wt production is tough because of
  large
• top-pair background (W3jets signature)

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Backup Slides

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