Transcript Evidence for Single Top Quark Production at CDF Bernd Stelzer

Evidence for Single Top Quark
Production at CDF
Bernd Stelzer
University of California, Los Angeles
HEP Seminar, University of Pennsylvania
September, 18th 2007
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)
5. Measurement of |Vtb|
6. More Tevatron Results
7. Summary / Conclusions / Outlook
2
The Tevatron Collider
• Tevatron is worlds highest energy
Collider (until 2008)
• Proton Anti-proton Collisions at
ECM=1.96 TeV
3
Top Production at the Tevatron
Once every 10,000,000,000 inelastic collision..
4
Top Production at the Tevatron
• At the Tevatron, top quarks are primarily
produced in pairs via the strong interaction:
NLO = 6.7±0.8 pb
mt=175GeV/c2
• Single top quark production is also predicted
by the Standard Model through the
electroweak interaction: (st ~ ½ tt)
QuickTime™ and a
TIFF (LZW) decompres sor
are needed to see this picture.
Quick Time™ and a
TIFF (LZW) decompressor
are needed to see this picture.
s-channel
NLO = 0.88±0.07 pb
t-channel
NLO = 1.98±0.21 pb
Cross-sections at mt=175GeV/c2, B.W. Harris et al., Phys.Rev. D70 (2004) 114012, Z. Sullivan hep-ph/0408049
5
Top Quark in the Standard Model
• 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’
>10 orders of magnitude!
6
Why measure Single Top Production ?
• Source of single ~100% polarized top quarks:
Direct measurements
– Short lifetime, information passed to decay products
– Test V-A structure of W-t-b vertex
Ratio from Bs
oscillations
•Allows direct Measurement of CKMMatrix Element Vtb:
single top ~|Vtb|2
– indirect determinations
of Vtb enforce 3x3 unitarity
Not precisely measured
–
Ceccucci, Ligeti, Sakai PDG Review 2006
Precision EW rules out “simple”
fourth generation extensions,
but see
Vtb
s-channel
t-channel
J. Alwall et. al., “Is |Vtb|~1?”
Eur. Phys. J. C49 791-801 (2007).
7
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)
Z
c
- s-channel signature: Heavy W boson,
charged Higgs H+, Kaluza Klein excited WKK
t
• s-channel single top has the same final state
as WHlbb
=> benchmark for WH!
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
(WH ~ 1/10 s-channe))
1.25 t (pb)
W,H+
s (pb)
Tait, Yuan PRD63, 014018(2001)
CMSSM Study:
Buchmuller, Cavanaugh, deRoeck, S.H., Isidori,
Paradisi, Ronga, Weber, G. Weiglein’07]
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Experimental
Challenge
9
Event Signatures
Top Pair Production with decay
Into Lepton + 4 Jets final state
are very striking signatures!
Jet3
Jet1
Single top Production with decay
Into Lepton + 2 Jets final state
Is less distinct!
MET
Jet2
Jet4
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CDF II Detector (Cartoon)
■ Silicon tracking
detectors
 = 1.0
■ Central drift
chambers (COT)  = 2.0
■ Solenoid Coil
■ EM calorimeter

■ Hadronic
 = 2.8
calorimeter
■ Muon scintillator
counters
■ Muon drift
chambers
■ Steel shielding
  log(tan(  2))
Single top analysis
needs full detector!

Thanks to great work of
detector experts and shift crew!
11
CDF II Detector
Central calorimeters
Endplug
calorimeters
Central muon
Drift chamber tracker
Silicon detector
12
Data Collected at CDF
This analysis uses 1.51 fb-1
Delivered : 3.0 fb-1
Collected : 2.7 fb-1
(All detector components ON)
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:
CDF W+2jet Candidate Event:
• 1 Lepton, ET >20 GeV, |e()|< 2.0 (1.0) Close-up View of Layer 00 Silicon Detector
• Missing ET, (MET) > 25 GeV
• 2 Jets, ET > 20 GeV, ||< 2.8
• Veto Fake W, Z, Dileptons,
Conversions, Cosmics
Jet2
• At least one b-tagged jet,
(displaced secondary vertex tag)
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
Jet
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 Lxy~3mm
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
W+HF jets (Wbb/Wcc/Wc)
Top/EWK (WW/WZ/Z→ττ, ttbar)
•W+jets normalization from data and
heavy flavor (HF) fractions from
ALPGEN Monte Carlo
•MC normalized to theoretical cross-section
Non-W (QCD)
•Multijet events with
semileptonic b-decays or
mismeasured jets
non-W
•Fit low MET data and
extrapolate into signal region
tt
Wbb
W+HF
jets (Wbb/Wcc/Wc)
Mistags
Wcc
Wc
• W+jets normalization from data and
heavy flavor (HF) fraction from MC
Mistags (W+2jets)
• 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 W+jets contribution -> Likelihood Fit
Before b-tagging:
Signal Region
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
Note: Similar for W+charm background
data
N Wbb
NW bb MC
(
)  K HF  NWdata
 jets
NW  jets
Correct data for non W+jets events
data
NWdata
 jets  NCandidates  N nonW  N EWK

Heavy 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
KHF=1.4 ± 0.4

Large uncertainties from Monte
Carlo estimate and heavy flavor
calibration (36%)
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Signal and Background Event Yield
CDF RunII Preliminary, L=1.51 fb-1
Predicted Event Yield in W+2jets
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
Single top swamped by background and
hidden behind background uncertainty.
 Makes counting experiment impossible!
1078
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CDF Data
Monte Carlo
Signal/Background
Result
Cross Section
Analysis Event Selection
Analysis Flow Chart
Analysis
Technique
Apply MC
Corrections
Template
Fit to Data
Signal
Background
Discriminant
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Analysis Techniques
Likelihood Discriminant
Matrix Element Analysis
More Tevatron Results
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The Likelihood Function Analysis
LF(x ) 



Nsig
Nbkg
Unit Area
Bkgr
n v ar
i1
n v ar
i1
i
psig
(x i )
p (x i )  
i
sig
Wbb
ttbar
sig
N
pisig  sig i bkg
Ni  Ni
i, index input variable
n v ar
i1
Signal
i
pbkg
(x i )
tchan
schan
Discriminant
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)
22
Kinematic Variables
HT =ET(lepton,MET,Jets)
Background
Wbb
ttbar
Signal
Background
tchan
schan
Signal
tchan
schan
Wbb
ttbar
tchan
schan
Wbb
ttbar
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Analysis Techniques
Likelihood Discriminant
Matrix Element Discriminant
More Tevatron Results
24
Matrix Element Approach
• 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
P(x) 
d ( pi )


1

2
M d
Calculate an event-by-event probability (based on fully
differential cross-section calculation) for signal and
background
hypothesis

25
Matrix Element Method
Event probability for signal
and background hypothesis:
Leading Order
matrix element
(MadEvent)
P( pl , pj1, pj 2 ) 
Input only lepton
and 2 jets 4-vectors!
1

 d j1d j 2dpz
Integration over part
of the phase space Φ4
| M(pi ) |2
comb
W(Ejet,Epart) is the probability of
measuring a jet energy Ejet when
Epart was produced
f (q1) f (q2 )
4 W jet (E jet , E part )
| q1 || q2 |
Parton distribution
function (CTEQ5)
c
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Transfer Functions
Full simulation vs parton energy:
Double Gaussian
parameterization:
Double Gaussian parameterization:
1
(E  p1 ) 2
(E  p4 ) 2
W jet (E jet , E parton) 
[exp
 p3 exp
]
2
2
2 p2
2 p5
2 ( p1  p2 p5 )
where:
pi  ai  bi E parton
 E = (Eparton–Ejet)
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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 Psingletop
EPD 
b Psingletop  b PWbb  (1 b) PWcc  (1 b) PWcj
Background
;b = Neural Network b-tagger output
Signal
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Event Probabilty Discriminant
• S/B~1/17 over full range
• Likelihood fit will pin down
background in low score region
S/B~1/1
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
• 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
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:
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
All sources of systematic uncertainty included as nuisance parameters
Correlation between Shape/Normalization uncertainty considered (δi)
35
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
36
Sources of Systematic Uncertainty
CDF RunII Preliminary, L=1.51fb-1
Source
Rate Uncert.
Shape Uncert.
W + bottom
36%

W + charm
36%
Mistags
15%
ttbar
21%
Non-W
40%

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%
Jet Energy Scale

QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
37
Results
38
Matrix Element Analysis
• Matrix Element analysis observes excess over background expectation
1.2
• Likelihood fit result for combined search:
 Single Top  3.01.1 pb
39
ME Separate Search
• Perform separate likelihood fit for
s-channel and t-channel signal
• Both signal templates float independently
s-channel
+1.0
s=1.1−0.8 pb
t-channel
+1.0
t=1.9 −0.9 pb
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Likelihood Function Discriminant
 Single Top  2.7
1.3
1.1
pb
• Likelihood Function analysis also observes excess over background expectation
• Observed deficit previously in 0.955 fb-1
41
Likelihood Function 2D Fit
 s  1.11.4
1.1 pb
 t  1.31.2
1.0 pb
42
Signal
Significance
43

Hypothesis Testing
L. Read, J. Phys. G 28, 2693 (2002)
T. Junk, Nucl. Instrum. Meth. A 434, 435 (1999)
•Calculate p-value: Faction of backgroundonly pseudo-experiments with a test statistic
value as signal like (or more) as the value
observed in data
•Define Likelihood ratio test statistic:
Q
More signal like
Less signal like
L(data | s  b)
L(data | b)
•Systematic uncertainties included in
pseudo-experiments
•Use median p-value as measure for the
expected sensitivity
Median p-value = 0.13% (3.0)
Observed p-value = 0.09% (3.1)
44
Hypothesis Testing
More signal like
Less signal like
Median p-value = 0.20% (2.9)
Observed p-value = 0.31% (2.7)
45
Signal Features
46

Single Top Candidate Event
Central Electron Candidate
Charge: -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=0
QxEta = 2.91 (t-channel signature)
EPD=0.95
u,d
d,u
Jet1
Run: 211883, Event: 1911511
Lepton

Jet2
47
Single Top Signal Features
Look for signal features
in high score output
EPD>0.95
EPD>0.90
48
QxEta Distributions in Signal Region
EPD>0.9
3)
EPD>0.95
4)
49
m(W,b) Distributions in Signal Region
EPD>0.9
EPD>0.95
50
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%
51
Direct |Vtb| Measurement
• Using the Matrix Element cross
Section PDF we measure |Vtb|
• Assume Standard Model V-A coupling
and |Vtb| >> |Vts|, |Vtd|
s-channel
t-channel
Flat prior 0 < |Vtb|2 < 1
|Vtb|= 1.02 ± 0.18 (experiment) ± 0.07 (theory)
|Vtb|>0.55 at 95% C.L.
Z. Sullivan, Phys.Rev. D70 (2004) 114012 52
Single Top
Results from DØ
53
D0 Results
Boosted Decision Tree
First direct limit on Vtb:
0.68 <|Vtb|< 1 @ 95%CL or
Expected p-value = 1.9% (2.1)
Observed p-value = 0.04% (3.4)
|Vtb| = 1.3 ± 0.2
PRL 98 18102 (2007)
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Summary of Results
Summary
Expected Observed
3.0
3.1
2.9
2.7
2.6
2.1
3.4
1.9
3.2
2.2
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
First Tevatron Run II result using 162 pb-1
single top < 17.5 pb at 95 % C.L.
2007: Evidence for single top quark production
using 1.5 fb-1 (expected and observed!)
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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
• Search for heavy W boson in W + 2, 3 jets
• Assume Standard Model coupling strengths
W
(Z. Sullivan, Phys. Rev. D 66, 075011, 2002)
• Perform fit to MWjj distribution
Previous Limits:
•CDF Run I: M(WR) > 566 GeV/c2 at 95% C.L.
•D0 Run II: M(WR) > 630 GeV/c2 at 95% C.L.
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)
58
LHC is the Future
Large Hadron
Collider
59
LHC is the Future
Additional single top process at the
LHC! (negligible at the Tevatron)
• LHC will be a top quark factory
σtt ~ 800 pb
Wt- production
σt-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)
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