Evidence for Single Top Quark Production at CDF
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Transcript of Evidence for Single Top Quark Production at CDF
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
1. Introduction to Top Quarks
2. Motivation for Single Top Search
3. The Experimental Challenge
4. 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 ECM=1.96 TeV
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Top Production at the Tevatron
Once every 10,000,000,000 inelastic collision..Once every 10,000,000,000 inelastic collision..
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Top Production at the Tevatron
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•At the Tevatron, top quarks are primarily produced in pairs via the strong interaction:
•Single top quark production is also predictedby the Standard Model through theelectroweak interaction: (st ~ ½ tt)
NLO = 6.7±0.8 pb
mt=175GeV/c2
s-channelNLO = 0.88±0.07 pb
t-channelNLO = 1.98±0.21 pb
Discovered
1995!
Cross-sections at mt=175GeV/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
>10 orders of magnitude!
•Top Quark is heaviest particle to date–mt=170.9 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 ?
Vtb
Direct measurements
Ratio from Bs oscillations
Not precisely measured
s-channelt-channel
Ceccucci, Ligeti, Sakai PDG Review 2006
Precision EW rules out “simple”fourth generation extensions,but see
J. Alwall et. al., “Is |Vtb|~1?”Eur. Phys. J. C49 791-801 (2007).
• 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 ~|Vtb|2
– indirect determinations of Vtb enforce 3x3 unitarity
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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/γ/g-c couplings)
- s-channel signature: Heavy W boson, charged Higgs H+, Kaluza Klein excited WKK
Tait, Yuan PRD63, 014018(2001)
Z
ct
W,H+
s (pb)
1.2
5
t (p
b)
•s-channel single top has the same final state
as WHlbb=> benchmark for WH!
(WH ~ 1/10 s-channe))
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CMSSM Study:Buchmuller, Cavanaugh, deRoeck, S.H., Isidori, Paradisi, Ronga, Weber, G. Weiglein’07]
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ExperimentalChallenge
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Top Pair Production with decayInto Lepton + 4 Jets final stateare very striking signatures!
Jet4
Jet3
Event Signatures
Jet1
Jet2
Ele
ctron
MET
Single top Production with decayInto Lepton + 2 Jets final stateIs less distinct!
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€
η =−log(tan(θ 2))
ηη = 1.0= 1.0
ηη = 2.8= 2.8
ηη = 2.0= 2.0
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
Single top analysisneeds full detector!
Thanks to great work of detector experts and shift crew!
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CDF II Detector
Silicon detector
Central muonCentral calorimeters
Endplug calorimeters
Drift chamber tracker
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Data Collected at CDF
This analysis uses 1.51 fbThis analysis uses 1.51 fb-1-1 (All detector components ON)(All detector components ON)
CDF is getting faster, too!6 weeks turnaround time to calibrate, validate and process raw data
Tevatron people are doing a fantastic job!3fb-1 party coming up!
Design goal
Delivered : 3.0 fb-1
Collected : 2.7 fb-1
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Single Top Selection
Event Selection:•1 Lepton, ET >20 GeV, |ηe()|< 2.0 (1.0)
•Missing ET, (MET) > 25 GeV
•2 Jets, ET > 20 GeV, |η|< 2.8
•Veto Fake W, Z, Dileptons, Conversions, Cosmics
•At least one b-tagged jet, (displaced secondary vertex tag)
CDF W+2jet Candidate Event:CDF W+2jet Candidate Event:
Close-up View of Layer 00 Silicon DetectorClose-up View of Layer 00 Silicon Detector
Jet2
Jet1
Ele
ctron
12mm
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
Run: 205964, Event: 337705Electron ET= 39.6 GeV, Missing ET = 37.1 GeVJet 1: ET = 62.8 GeV, Lxy = 2.9mmJet 2: ET = 42.7 GeV, Lxy = 3.9mm
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B-quark Tagging and Jet Flavor Separation
•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
Neural Network Jet-Flavor Separator
NN Output
Charm tagging rate ~10%Mistag rate ~ 0.5%
• Exploit long lifetime of B hadrons (c ~450 m)+boost
• B hadrons travel LLxyxy~3mm ~3mm before decay with large track multiplicity
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Mistags (W+2jets)
• Falsely tagged light quark or gluon jets
• Mistag probability parameterization obtained from inclusive jet data
Background Estimate
W+HF jets (Wbb/Wcc/Wc)
•W+jets normalization from data and
heavy flavor (HF) fraction from MC
Top/EWK (WW/WZ/Z→ττ, 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
Wbb
WccWc
non-W
Z/D
ibMistag
s
tt
W+HF jets (Wbb/Wcc/Wc)
•W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo
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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 W+jets contribution -> Likelihood Fit
Signal Region
Before b-tagging: After b-tagging:
Signal Region
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W + Heavy Flavor Estimate
•Method inherited from CDF Run I (G. Unal et. al.)
•Measure fraction of W+jets events with heavy flavor (b,c) in Monte Carlo
•Normalize fractions to W+jets events found in data
Correct data for non W+jets events
€
NWbb
data = (NWbb
NW + jets
)MC ⋅K HF ⋅NW + jetsdata
€
NW + jetsdata = NCandidates
data − Nnon−W − NEWKHeavy flavor fractions
and b-tagging efficiencies from LO ALPGEN Monte
Carlo
Calibrate ALPGEN heavy flavor Fractions by comparing W + 1jet Data with ALPGEN jet Monte Carlo
Note: Similar for W+charm background
Large uncertainties from Monte Carlo estimate and heavy flavor calibration (36%)
KHF=1.4 ± 0.4
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Signal and Background Event Yield
CDF RunII Preliminary, L=1.51 fbCDF RunII Preliminary, L=1.51 fb-1
Predicted Event Yield in W+2jetsPredicted Event Yield in W+2jets
Single top swamped by background and hidden behind background uncertainty. Makes counting experiment impossible!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 prediction1103.
7±230.9
Observed 1078
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Analysis Flow Chart
Analy
sis
Event
Sele
ctio
nA
naly
sis
Event
Sele
ctio
n
CDF DataCDF Data
Monte CarloSignal/
Background
Monte CarloSignal/
Background
Apply MCCorrectionsApply MC
Corrections
Analysis TechniqueAnalysis
Technique
Result
Template Fit to Data
Template Fit to Data
DiscriminantDiscriminant
Signal
Background
Cross SectionCross Section
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Analysis Techniques
Likelihood Discriminant
Matrix Element AnalysisMatrix Element Analysis
More Tevatron ResultsMore Tevatron Results
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The Likelihood Function Analysis
€
pisig =
N isig
N isig + N i
bkg
Nsig
Nbkg
i, index input variable
€
LF(r x ) =
psigi (x i)i=1
nvar∏psig
i (x i)i=1
nvar∏ + pbkgi (x i)i=1
nvar∏Bkgr Signal
Unit
Are
a
tchanschan
Wbbttbar
Leading Jet ET (GeV)
Uses 7 (8) kinematic variables for t-channel (s-channel) Likelihood Functione.g. M(Wb) or kin. Solver 2, HT, QxEta, NN flavor separator, Madgraph Matrix Elements, M(jj)
Discriminant
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Kinematic Variables
Background Signal Background Signal
Wbbttbar
Wbbttbar
tchanschan
tchanschan
HT =ET(lepton,MET,Jets)
Wbbttbar
tchanschan
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Analysis Techniques
Likelihood DiscriminantLikelihood Discriminant
Matrix Element Discriminant
More Tevatron ResultsMore Tevatron Results
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Matrix Element Approach
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P(x) =dσ (pi
μ )
σ=
1
σM
2dΦ
•No single ‘golden’ kinematic variable!
•Attempt to include all available kinematic information by
using Matrix Element approach
•Start from Fermi’s Golden rule:
Cross-sections ~ |Matrix Element|2 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
€
P( plμ , p j1
μ , p j 2μ ) =
1
σdρ j1dρ j 2dpν
z | M(piμ ) |2
f (q1) f (q2)
| q1 || q2 |φ4 W jet (E jet , E part )
comb
∑∫Parton
distribution function (CTEQ5)
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 Φ4
Event probability for signal and background hypothesis:
Input only lepton and 2 jets 4-vectors!
c
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Transfer Functions
Eparton Ejet
Full simulation vs parton energy:
Eparton Ejet
Double Gaussian parameterization:
partoniii Ebap +=where:
E = (Eparton–Ejet)
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W jet (E jet , E parton ) =1
2π ( p1 + p2 p5)[exp
−(δE − p1)2
2p22
+ p3 exp−(δE − p4 )2
2p52
]
Double Gaussian parameterization:
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Event Probability Discriminant (EPD)
€
EPD =b ⋅Psin gletop
b ⋅Psin gletop + b ⋅PWbb + (1− b) ⋅PWcc + (1− b) ⋅PWcj
;b = Neural Network b-tagger output
•We compute probabilities for signal and background hypothesis per eventUse full kinematic correlation between signal and background events
•Define ratio of probabilities as event probability discriminant (EPD):
SignalBackground
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Event Probabilty Discriminant
S/B~1/1In most sensitive bin!
•S/B~1/17 over full range•Likelihood fit will pin down background in low score
region
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Cross-Checks
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Cross-Checks in Data Control Samples
•Validate method in various data control samples
•W+2 jets data (veto b-jets, selection orthogonal to the candidate sample)
•Similar kinematics, with very little contribution from top (<0.5%)
px py pz E
Second Leading Jet
Leading Leading Jet
Lepton (Electron/Muon)
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Cross-Checks in Data Control Samples
CDF Run II Preliminary
•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
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Monte Carlo Modeling Checks
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Template Fitto 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 (δi)
βj = σj/σSM parameter
single top (j=1)
W+bottom (j=2)
W+charm (j=3)
Mistags (j=4)
ttbar (j=5)
k = Bin index
i = Systematic effect
δi = Strength of effect
εji± = ±1σ norm. shifts
κjik± = ±1σ shift in bin k
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Rate vs Shape Systematic Uncertainty
DiscriminantDiscriminant
•Rate systematics give fit templates freedom to move vertically only
•Shape systematics allow templates to ‘slide horizontally’ (bin by bin)
Shape systematics
Rate and
Systematic uncertainties can affect rate and template shape
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Sources of Systematic Uncertainty
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%
CDF RunII Preliminary, L=1.51fb-1
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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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:
€
Single Top = 3.0−1.1+1.2 pb
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ME Separate Search
s-channels=1.1 pb+1.0
−0.8
t-channelt=1.9 pb+1.0
−0.9
•Perform separate likelihood fit fors-channel and t-channel signal•Both signal templates float independently
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Likelihood Function Discriminant
• Likelihood Function analysis also observes excess over background expectation• Observed deficit previously in 0.955 fb-1
€
Single Top = 2.7−1.1+1.3 pb
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Likelihood Function 2D Fit
1.41.1
1.21.0
1.1 pb
1.3 pb
s
t
+−
+−
=
=
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Signal Significance
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Hypothesis Testing
•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
Median p-value = 0.13% (3.0)
€
Q =L(data | s + b)
L(data | b)
More signal like Less signal like
Observed p-value = 0.09% (3.1)
L. Read, J. Phys. G 28, 2693 (2002)T. Junk, Nucl. Instrum. Meth. A 434, 435 (1999)
3.1 Evidence
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Hypothesis Testing
Median p-value = 0.20% (2.9)
More signal like Less signal like
Observed p-value = 0.31% (2.7)
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Signal Features
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Central Electron CandidateCharge: -1, Eta=-0.72 MET=41.85, MetPhi=-0.83 Jet1: Et=46.7 Eta=-0.61 b-tag=1 Jet2: Et=16.6 Eta=-2.91 b-tag=0QxEta = 2.91 (t-channel signature)EPD=0.95
Single Top Candidate Event
Jet1
Jet2
Lepton
Run: 211883, Event: 1911511
€
u,d
€
d,u
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Single Top Signal Features
EPD>0.90
EPD>0.95
Look for signal featuresin high score output
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QxEta Distributions in Signal Region
EPD>0.9EPD>0.9
3) 4)
EPD>0.95EPD>0.95
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m(W,b) Distributions in Signal Region
EPD>0.9EPD>0.9 EPD>0.95EPD>0.95
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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| >> |Vts|, |Vtd|
|Vtb|= 1.02 ± 0.18 (experiment) ± 0.07 (theory)
t-channel
Z. Sullivan, Phys.Rev. D70 (2004) 114012
Flat prior 0 < |Vtb|2
< 1
|Vtb|>0.55 at 95% C.L.
s-channel
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Single TopResults from DØ
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D0 Results
First direct limit on Vtb:0.68 <|Vtb|< 1 @ 95%CL or
|Vtb| = 1.3 ± 0.2
First direct limit on Vtb:0.68 <|Vtb|< 1 @ 95%CL or
|Vtb| = 1.3 ± 0.2
Boosted Decision Tree
PRL 98 18102 (2007)
Expected p-value = 1.9% (2.1)Observed p-value = 0.04% (3.4)
3.4 Evidence
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Summary of Results
Expected
3.0
2.9
2.6
2.1
1.9
2.2
Observed
3.1
2.7
3.4
3.2
2.7
Summary
•CDF analyses more sensitive•D0 observes upward fluctuationIn 900 pb-1 of data
Combined:2.3
/3.6
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CDF Single Top History
First Tevatron Run II result using 162 pb-1
single top < 17.5 pb at 95 % C.L.
2004: Simple analysis while refining Monte Carlo samples and analysis tools
2 Years
2006: Established sophisticated analysesCheck robustness in data control samples
2007: Evidence for single top quark production using 1.5 fb-1 (expected and observed!)
•Development of powerfulanalysis techniques (Matrix Element, NN, Likelihood Discriminant)•NN Jet-Flavor Separatorto purify sample•Refined background estimates and modeling•Increase acceptance (forward electrons)•10x more data
Phys. Rev. D71 012005
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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 >3 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
Limit at 95% C.L. M(W´) > 760 GeV/c2 for M(W´) > M(νR) M(W´) > 790 GeV/c2 for M(W´) < M(νR)
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(WR) > 566 GeV/c2 at 95% C.L.•D0 Run II: M(WR) > 630 GeV/c2 at 95% C.L.
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LHC is the Future
Large Hadron Collider
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LHC is the Future
•LHC will be a top quark factoryσtt ~ 800 pbσt-channel ~ 243 pb (153 pb for top and 90 pb for antitop
production)σs-channel ~ 11 pb (6.6 pb for top and 4.8 pb for antitop production)σWt ~ 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 (W+3jets signature)
Wt- production
Additional single top process at the LHC! (negligible at the Tevatron)
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