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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. Outline. Introduction to Top Quarks Motivation for Single Top Search The Experimental Challenge Analysis Techniques

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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


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


The Tevatron Collider

  • Tevatron is worlds highest energy Collider (until 2008)

  • Proton Anti-proton Collisions at ECM=1.96 TeV


Top Production at the Tevatron

Once every 10,000,000,000 inelastic collision..


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 = 6.7±0.8 pb

mt=175GeV/c2

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


>10 orders of magnitude!

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’


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

Direct measurements

Ratio from Bs oscillations

Not precisely measured

Why measure Single Top Production ?

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).

Vtb

s-channel

t-channel


1.25 t (pb)

s (pb)

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

Z

c

t

W,H+

  • s-channel single top has the same final state

  • as WHlbb

  • => benchmark for WH!

Tait, Yuan PRD63, 014018(2001)

CMSSM Study:

Buchmuller, Cavanaugh, deRoeck, S.H., Isidori,

Paradisi, Ronga, Weber, G. Weiglein’07]

(WH ~ 1/10s-channe))


Experimental

Challenge


Top Pair Production with decay

Into Lepton + 4 Jets final state

are very striking signatures!

Jet3

Electron

Jet1

Single top Production with decay

Into Lepton + 2 Jets final state

Is less distinct!

Jet2

Jet4

Event Signatures

MET


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!


CDF II Detector

Central calorimeters

Central muon

Endplug

calorimeters

Drift chamber tracker

Silicon detector


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


Electron

Jet2

Jet1

Single Top Selection

Event Selection:

  • 1 Lepton, ET >20 GeV, |e()|< 2.0 (1.0)

  • Missing ET, (MET) > 25 GeV

  • 2 Jets, ET > 20 GeV, ||< 2.8

  • Veto Fake W, Z, Dileptons, Conversions, Cosmics

  • At least oneb-tagged jet, (displaced secondary vertex tag)

CDF W+2jet 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


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

  • 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

NN Output


Background Estimate

  • W+HF jets (Wbb/Wcc/Wc)

    • W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo

  • 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

Z/Dib

tt

non-W

Wbb

Mistags

  • W+HF jets (Wbb/Wcc/Wc)

  • W+jets normalization from data and

  • heavy flavor (HF) fraction from MC

Wcc

Wc

  • Mistags (W+2jets)

    • Falsely tagged light quark or gluon jets

    • Mistag probability parameterization obtained from inclusive jet data


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:

After b-tagging:

Signal Region

Signal Region


Note: Similar for W+charm background

Correct data for non W+jets events

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%)

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


Single top swamped by background and hidden behind background uncertainty.

 Makes counting experiment impossible!

Signal and Background Event Yield

CDF RunII Preliminary, L=1.51 fb-1Predicted Event Yield in W+2jets


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


Analysis Techniques

Likelihood Discriminant

Matrix Element Analysis

More Tevatron Results


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)


Wbb

ttbar

Kinematic Variables

HT =ET(lepton,MET,Jets)

Background

Signal

Background

Signal

tchan

schan

Wbb

ttbar

tchan

schan

Wbb

ttbar

tchan

schan


Analysis Techniques

Likelihood Discriminant

Matrix Element Discriminant

More Tevatron Results


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

  • Calculate an event-by-event probability (based on fully differential cross-section calculation) for signal and background hypothesis


c

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 Φ4

Input only lepton

and 2 jets 4-vectors!

Parton distribution function (CTEQ5)


Double Gaussian parameterization:

Eparton

Ejet

Transfer Functions

Full simulation vs parton energy:

Eparton

Ejet

Double Gaussian parameterization:

where:

 E = (Eparton–Ejet)


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

Background

Signal


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!


Cross-Checks


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


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


Monte Carlo Modeling Checks


Template Fit

to the data


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± = ±1σshift in bin k


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


Sources of Systematic Uncertainty

CDF RunII Preliminary, L=1.51fb-1


Results


Matrix Element Analysis

  • Matrix Element analysis observes excess over background expectation

  • Likelihood fit result for combined search:


ME Separate Search

  • Perform separate likelihood fit for

  • s-channel and t-channel signal

  • Both signal templates float independently

s-channel

s=1.1 pb

+1.0

−0.8

t-channel

t=1.9 pb

+1.0

−0.9


Likelihood Function Discriminant

  • Likelihood Function analysis also observes excess over background expectation

  • Observed deficit previously in 0.955 fb-1


Likelihood Function 2D Fit


Signal

Significance


3.1

Evidence

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


Hypothesis Testing

Less signal like

More signal like


Signal Features


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

Run: 211883, Event: 1911511

Jet1

Lepton

Jet2


Single Top Signal Features

Look for signal features

in high score output

EPD>0.95

EPD>0.90


QxEta Distributions in Signal Region

EPD>0.9

EPD>0.95

3)

4)


m(W,b) Distributions in Signal Region

EPD>0.9

EPD>0.95


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%


Using the Matrix Element cross

Section PDF we measure |Vtb|

Assume Standard Model V-A coupling

and |Vtb| >> |Vts|, |Vtd|

Direct |Vtb| Measurement

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


Single Top

Results from DØ


3.4

Evidence

D0 Results

Boosted Decision Tree

First direct limit on Vtb:

0.68 <|Vtb|< 1 @ 95%CL or

|Vtb| = 1.3± 0.2

PRL 98 18102 (2007)


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


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 < 17.5 pb at 95 % C.L.


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!


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) > 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)


LHC is the Future

Large Hadron

Collider


LHC is the Future

Additional single top process at the LHC! (negligible at the Tevatron)

Wt- production

  • LHC will be a top quark factory

    • σtt ~ 800 pb

    • 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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