Top Quark Measurements by the DØ Collaboration · 2008-01-15 · Michael Begel SMU Physics Seminar...

Top Quark Measurementsby the DØ Collaboration

Michael Begel

SMU Physics Seminar

March 5, 2007

University of Rochester

Outline

Why is the top quark interesting?

How do we produce and detect top quarks?

Top Quark Pair Production

Mass

Single Top Quark Production

Calorimeter

Shielding

Toroid

Muon Chambers

Muon Scintillators

η = 0 η = 1

η = 2

[m]

η = 3

–10 –5 0 5 10

–5

0

5

Michael Begel SMU Physics Seminar — March 5, 2007 2

Unique Physics Laboratory

The top quark exists at the interface betweenElectroweak and QCD physics. Its production andproperties are sensitive to the existence of newphysical phenomena and offerinsights on the Electroweak

symmetrybreakingmechanism.

QCD Electroweak

Higgsor

new physics

top


What is the Top Quark?Discovered in 1995 by CDF and DØ at Fermilab

mt ≈ 175 GeV vs mb ≈ 5 GeVTop-Higgs Yukawa coupling λt =

√2mt

v≈ 1

We still know very little about the top quark. Isit the particle required by the Standard Model?

If it is a normal quark, then it is still special:mt > mW so t → Wb

Γt ≈ 1.4 GeV

τt ≈ 4 × 10−25 sec

⇒ so top quarks decay before they interact

via the strong force(

1

ΛQCD≈ 3 × 10−24 sec

)

.

Top quarks are effectively bare quarks!


Top Quark PhysicsPrecision measurements of top quark properties are crucial tounderstanding its true nature

q

q

t

t

W +ℓ

ν

b

W −b

q

q

Top MassTop WidthTop SpinTop Charge

Rare/non-SM DecaysBranching Ratios|Vtb|

Production Cross SectionResonant ProductionProduction KinematicsTop Spin Polarization

W helicityAnomalousCouplingsCP violation

Higgsstrahlung

We are now transitioning between the discovery phase andmaking precision measurements of top quark properties


Classifying Top Events

tt Decay Modes

all hadronichigh BFhigh BG

lepton + jet(includes τ → e/µ)

high BFmanagable BG

dileptonlow BFlow BG

tW + ℓ+, q

ν, q

b

t → Wb is ≈ 100%

dilep

ton

τe/τµ

τe/τµ

lepton + jets

lept

on+

jets

ττ τ + jets

τ+

jets

all hadronic

e+µ+τ+ ud cs

W +

e−µ−τ−

ud

cs

W −

It is important to measure all decay channels asthey have different sensitivities to new physics

(eg, H± in τ )


Classifying Top EventsDileptonTwo high-pT leptonsMissing ET

Two high-pT jets

Lepton + JetsOne high-pT leptonMissing ET

Four high-pT jets

All HadronicSix high-pT jets

t

bW

ℓ

ν

t

b W

ℓ

ν

t

bWq

q

t

b W ℓ

ν

t

W bq

q

t

Wb q

q

Need to reconstruct electrons, muons, jets,b-jets, and missing transverse energy


DØ Detector

Calorimeter

Shielding

Toroid

Muon Chambers

Muon Scintillators

η = 0 η = 1

η = 2

[m]

η = 3

–10 –5 0 5 10

–5

0

5


Jet Energy Scale

Top quark measurements require a clean mappingbetween reconstructed objects and partons

Reconstruct jets using iterativecone algorithm with midpoints(Rcone = 0.5)

Calibrate jet energies toparticle level

Map jets to partons

Correct jet energies toparton level

EM

FH

CH

hadrons

γ

π

K

q g

q pp

Tim

e

part

onje

tpa

rtic

leje

tca

lorim

eter

jet


Jet Energy Scale


Response of calorimeter to jetsis a dominant systematicuncertainty

Jet energy scale derived fromsamples of

zero and minimum bias eventsphoton + jet eventsZ + jet eventsdijet events

in data and simulation

EM

FH

CH

hadrons

γ

π

K

q g

q pp

Tim

e

part

onje

tpa

rtic

leje

tca

lorim

eter

jet


Jet Energy Scale



Ecorr =Emeas − O

R × S

Response Correctionhadronic responseuninstrumented regions

Showering Correctionparticles in-&-out of cone

Offset Correctionmultiple interactionsunderlying event

EM

FH

CH

hadrons

γ

π

K

q g

q pp

Tim

e

part

onje

tpa

rtic

leje

tca

lorim

eter

jet


Jet Energy Scale



(GeV)uncorrT,jetE

10 210 (GeV)uncorr

T,jetE10 210

Fra

ctio

nal u

ncer

tain

ty

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

TotalResponseShowering

Offset

DØ Run II preliminary = 0.0

jetη = 0.5, coneR EM

FH

CH

hadrons

γ

π

K

q g

q pp

Tim

e

part

onje

tpa

rtic

leje

tca

lorim

eter

jet


b Jet Identificationb-jet identification is an importantdiscriminant between top signaland background

(GeV)Tjet p20 40 60

Effi

cien

cy

0.0

0.2

0.4

0.6

0.8 DØb-jet efficiency

c-jet efficiency

10)×mis-tagging rate (

jet

d0

L

secondary vertex

primary vertex

displacedtrack

Secondary Vertex Tagger

Neural Network Tagger

1.2 m


Fermilab Tevatron pp Collider

Highest energy in the world!

≈ 1 fb−1 incurrent analyses

DØ


Top Quark Pair Production


tt Cross SectionTop quarks are mainly produced in pairs via strong interactionsMeasurement of the tt production rateprovides precision tests of (N)NLO pQCDand is sensitive to the presence of newphenomena

resonant states

additional decay modes

anomalous couplings

compositeness

√s¬ (GeV)

σ (p

b)

σtot

σbb_

σWσZ

σtt_

pp_

pp

Tev

atro

n

LH

C

10 -1

10 0

10 1

10 2

10 3

10 4

10 5

10 6

10 7

10 8

10 9

10 10

10 11

10 12

103

104

10

orde

rsof

mag

nitu

de!

t

t t

t≈ 15%≈ 85%σNLL = 6.7+0.7

−0.9 pbat mt = 175 GeV


Methodology

Counting Experimentselect signal-like eventsmeasure number of background eventsbest when signal easily distinguishedfrom backgroundoptimize signal acceptance

Examples: eµ, ℓ+jet with b tagging

Shape-based Measurementsselect signal-like eventsidentify variables that distinguishsignal from backgroundoptimize separation between signaland background

Examples: ℓ+jet without b tagging, all hadronic

σ = 1L

Ncand−Nbkg

ǫ


tt in Lepton + Jets ChannelCounting experiment using b-tagged jets to separate signal frombackground

Jet multiplicity= 1 = 2 = 3 4≥

Num

ber

of ta

gged

eve

nts

0

10

20

30

40

50 DØ data

Signal

Background

-1 L=425 pb


Num

ber

of ta

gged

eve

nts

0

10

20

30

40

50


Num

ber

of ta

gged

eve

nts

0

50

100

150

200

250 DØ data

Signal

Background

-1 L=425 pb


Num

ber

of ta

gged

eve

nts

0

50

100

150

200

250

1 b tag ≥ 2 b tags

Top Mass (GeV)150 160 170 180 190 200

(pb

)t tσ

02468

10121416182022

-1DØ, L=425 pb

+Xt t→ppσ

Total uncertainty

Kidonakis et al.

Theoretical uncertainty

Top Mass (GeV)150 160 170 180 190 200

(pb

)t tσ

02468

10121416182022

σtt = 6.6 ± 0.9 (stat. + sys.) ± 0.4 (lum.) pb

Almost pure tt19 observed events0.6 ± 0.4 ± 0.1 BG events

t

bWq

q

t

b W ℓ

ν


tt in Lepton + Jets ChannelAvoid b ID to improve efficiency. Use kinematic distributions in alikelihood function to separate signal from background.

Likelihood Discriminant0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

20

40

60

80

100

KS = 0.855DØ RunII Preliminary 900 pb -1 DATA 715

224tt

W+Jets 403

Multijet 88

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

20

40

60

80

100

Likelihood Discriminant0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

20

40

60

80

100

σtt = 6.3+0.9−0.8 (stat.) ± 0.7 (sys.) ± 0.4 (lum.) pb

Sphericity0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

10

20

30

40

50

60

70

80

KS = 0.999DØ RunII Preliminary 900 pb -1 DATA 715

224tt

W+Jets 403

Multijet 88

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

Sphericity0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

10

20

30

40

50

60

70

80

Centrality0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

10

20

30

40

50

60

70

80

90

KS = 0.598DØ RunII Preliminary 900 pb -1

DATA 715

224tt

W+Jets 403

Multijet 88

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

10

20

30

40

50

60

70

80

90

Centrality0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Eve

nts

0

10

20

30

40

50

60

70

80

90

L =Ptt(x1, x2, ...)

Ptt(x1, x2, ...) + PW (x1, x2, ...)

t

bWq

q

t

b W ℓ

ν


tt in Dilepton ChannelIncludes contributions from eµ (very pure), ee, and µµchannels. Also includes ℓ+track events where the track is alepton that failed to be included in the other dilepton analyses.

Jet multiplicity0 1 2

Num

ber

of E

vent

s

0

10

20

30

40

50

60

70

Jet multiplicity0 1 2

Num

ber

of E

vent

s

0

10

20

30

40

50

60

70

)-1

Data (370 pb

tt

Fake leptonsWW/WZ

*γZ/

DØ Preliminary

σtt = 8.6+1.9−1.7 (stat.) ± 1.1 (sys.) ± 0.6 (lum.) pb

eµ ℓ+track

t

bW

ℓ

ν

t

b W

ℓ

ν


tt in the All Hadronic ChannelUse b tagging to improve signal to background

signal requirestwo b-tagged jetsbackground derived fromuntagged data andnormalized to thekinematic peakreconstruct all jj and bjjcombinationssubtract background

Sphericity0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

com

bina

tions

/0.1

0

10

20

30

40

50 DØ Data CandidatesDØ Data Untagged Multi-Jet Events

tPythia t

)-1DØ Run II Preliminary (420 pbDØ Data CandidatesDØ Data Untagged Multi-Jet Events

tPythia t

DØ Data CandidatesDØ Data Untagged Multi-Jet Events

tPythia t

jj Mass (GeV)0 50 100 150 200 250 300

com

bina

tions

/10

GeV

-20

0

20

40

60

80

100

120

140DØ Data

tPythia t

)-1DØ Run II Preliminary (420 pb

bjj Mass (GeV)0 50 100 150 200 250 300 350 400 450

com

bina

tions

/15

GeV

-20

0

20

40

60

80

100

120

140

DØ Data

tPythia t

)-1DØ Run II Preliminary (420 pb

jj Mass (GeV)0 50 100 150 200 250 300

com

bina

tions

/5 G

eV

0

20

40

60

80

100

120

140

160

180

200DØ Data CandidatesDØ Data Untagged Multi-Jet Events


bjj Mass (GeV)0 50 100 150 200 250 300 350 400 450 500

com

bina

tions

/5 G

eV

0

20

40

60

80

100

120DØ Data CandidatesDØ Data Untagged Multi-Jet Events


σtt = 10.4 ± 4.2 (stat) ± 4.0 (sys) pb

t

W bq

q

t

Wb q

q


tt Production Cross SectionDØ is in the process of extendingthese measurements to the 1 fb−1

data set. A new combined crosssection will be available soon.

0 2.5 5 7.5 10 12.5 15 17.5

DØ Run II Preliminary

σ (pp → tt) [pb]

0 2.5 5 7.5 10 12.5 15 17.5

dilepton/l+jets combined

dilepton (topological)

ltrack/emu combined

l+jets (b-tagged) NEW

Kidonakis and Vogt, PRD 68, 114014 (2003)

Cacciari et al., JHEP 0404, 068 (2004)

230 pb–1

7.1+1.2

–1.2

+1.4

–1.1pb

370 pb–1

8.6+2.3

–2.0

+1.2

–1.0pb

370 pb–1

8.6+1.9

–1.7

+1.1

–1.1pb

420 pb–1

6.6+0.9

–0.9pb

mtop = 175 GeV

+0.4

–0.4

l+jets (µ-tagged) NEW420 pb

–1

7.3+2.0

–1.8pb+0.4

–0.4

Fall 2006

τ+jets (b-tagged) NEW350 pb

–1

5.1+4.4

–3.6pb+0.3

–0.3

l+jets (topological) NEW910 pb

–1

6.3+1.1

–1.1pb+0.4

–0.4

all-jets (b-tagged) NEW410 pb

–1

4.5+2.4

–2.2

+0.3

–0.3pb


Top Quark Mass


Top Quark Mass

Fundamental parameter of the Standard ModelImportant ingredient for Electroweakprecision analyses initially used toindirectly measure mt

Use precision measurementsof mW and mt to constrainthe Higgs mass

W W

t

bδmW ∝ m2

t

W W

H

δmW ∝ ln mH

80.3

80.4

80.5

150 175 200

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV

]68% CL

∆α

LEP1 and SLD

LEP2 and Tevatron (prel.)

0

1

2

3

4

5

6

10030 300

mH [GeV]

∆χ2

Excluded Preliminary

All DataTheory uncertainty

mLimit = 153 GeV

mH = 85+39−28 GeV


Matrix ElementCalculate the probability that an event is either signal or backgroundas a function of the top mass

Ptt(x; mt, JES) = 1σ(mt)

∫

dq1dq2f(q)f(q) dσ(y; mt) T (x, y, JES)

Transfer Functionprobability to

measure x whenparton-level y was

produced

Differential Cross Sectionbased on LO Matrix Element

(qq → tt only)

Normalizationacceptance &

efficiency

Initial StateJES is a free parameter in the fit,constrained in situ by the mass ofhadronically decaying W boson

measurements takenfrom jets and leptons

PN tag

tt(x; mt, JES) =

∑

j W jtt

Pjtt(x; mt, JES)

Weight each jet–parton assignment with b-tagging event probabilities24 possible weighted assignments between jets and partons in a ℓ+jets event


Matrix Element: ℓ+jetsThe total event probability is

PN tagevent (x; mt, JES) = ft PN tag

tt(x; mt, JES) + (1 − ft)Pbkg(x, JES)

Minimize L(x; mt, JES) =∏

n

∏

i Pn tagevent(xi; mt, JES, fn tag

t )

mt = 170.3+4.1−4.5 (stat. + JES) +1.2

−1.8 (sys.) GeV

jet energy scale0.9 1 1.1

(G

eV

)to

pm

160

170

180

190 DØ Run II, 370 pb -1

für Grit, Lukas und Julia

(GeV)topm140 160 180

max

)/L

top

L(m

0

0.5

1

DØ Run II, 370 pb -1

jet energy scale0.9 1 1.1 1.2

max

L(JE

S)/

L

0

0.5

1

DØ Run II, 370 pb -1

Dominant Systematics

Relative b/light JES +0.63−1.43 GeV

b fragmentation ±0.56 GeVMC Calibration ±0.48 GeVSignal Modeling ±0.46 GeV

JES=1.027+0.035−0.040


Matrix Weighting: ℓℓ+jetsUnderconstrained kinematics given two neutrinos

Scan potential top masses

Solve for top momentum

assume two leading jets correspond to the b jets⇒ 4 solutions per tt

include detector resolution effects

Calculate weight as a function of mt for each

event using Dalitz-Goldstein-Kondo method

100 200

w = f(x)f(x) × P(E⋆ℓ |mt)P(E⋆

ℓ |mt)

PDF probability that observedlepton energy comes fromtop quark with mass mt

t

bW

ℓ

ν

t

b W

ℓ

ν


Matrix Weighting: eµ+jetsChoose value of mt with maximum likelihoodForm binned likelihood with signal and

background templates

Mean 176.5

RMS 35.22

m_peak GeV100 120 140 160 180 200 220 240 260 280 300

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Mean 176.5

RMS 35.22

backgroundττZ Preliminary∅DMean 177.5

RMS 41.95

m_peak GeV100 120 140 160 180 200 220 240 260 280 3000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Mean 177.5

RMS 41.95

WZ background Preliminary∅DMean 178.2

RMS 40.76

m_peak GeV100 120 140 160 180 200 220 240 260 280 3000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Mean 178.2

RMS 40.76

WW background Preliminary∅D

m_top150 160 170 180 190 200

-log(

likel

ihoo

d)

68

69

70

71

72

73

74

Data -ln(L) Preliminary∅D

WW Z→ττ fake e

100 120 140 160 180 200 220 240 260 280 3000

1

2

3

4

5

6

7

8

9

Peak Values vs. Template Preliminary∅D

mt = 177.7 ± 8.8 (stat.) +3.7−4.5 (sys.) GeV

100 200

L = 0.8 fb−1


Neutrino Weighting: eµ+jetsUnderconstrained kinematics given two neutrinosScan potential top masses and neutrino rapiditiesSolve for neutrino 4-vectorsSum over weights for neutrino solutions and detector resolution

Fit signal and background templates to data

ω =1

Niter

Niter∑

i=1

exp

−(

6Ecalcx,i − 6Eobs

x

)2

2σ26Ex

exp

−(

6Ecalcy,i − 6Eobs

y

)2

2σ26Ey

L = 0.8 fb−1

Top Mass [GeV]

100 150 200 250 300

Sum

of W

eigh

ts

0

0.005

0.01

0.015

0.02

0.025 mt = 175 GeV

Test Top Mass [GeV]

150 160 170 180 190 200

-ln(L

ikel

ihoo

d)

225

226

227

228

229

230

231 7.9 GeV±= 171.6 topm

mt = 171.6 ± 7.9 (stat.) +5.1−4.0 (sys.) GeV

t

bW

ℓ

ν

t

b W

ℓ

ν


Top Quark Mass Summary

140 160 180 200


Top Quark Mass [GeV]

140 160 180 200

World average

ll, l+track (neutrino weighting, topo) NEW

ll (matrix weighting, b-tagged, topo) NEW

l+jets (template, b-tagged)

l+jets (template, topological)

l+jets (ideogram, b-tagged, topo) NEW370 pb

–1

173.7 +4.4–4.4

+2.1–2.0

GeV

230 pb–1

169.9 +5.8–5.8

+7.8–7.1

GeV

230 pb–1

170.6 +4.2–4.2

+6.0–6.0

GeV

370 pb–1

176.2 +9.2–9.2

+3.9–3.9

GeV

370 pb–1

179.5 +7.4–7.4

+5.6–5.6

GeV

171.4 +1.2–1.2

+1.8–1.8

GeV

l+jets (matrix element, topological)

l+jets (matrix element, b-tagged) BEST370 pb

–1

370 pb–1

170.3 +4.1–4.5

+1.2–1.8

GeV

169.2+5.0–7.4

+1.5–1.4

GeV

Fall 2006

ll combination (matrix and neutrino) NEW370 pb

–1

178.1 +6.7–6.7

+4.8–4.8

GeV

eµ (neutrino weighting, topological) NEW

eµ (matrix weighting, topological) NEW835 pb

–1

177.7 +8.8–8.8

+3.7–4.5

GeV

835 pb–1

171.6 +7.9–7.9

+5.1–4.0

GeV

Best Independent Measurementsof the Mass of the Top Quark (*=Preliminary)

CDF-I dilepton 167.4 ± 11.4

DØ-I dilepton 168.4 ± 12.8

CDF-II dilepton* 164.5 ± 5.5

DØ-II dilepton* 178.1 ± 8.3

CDF-I lepton+jets 176.1 ± 7.3

DØ-I lepton+jets 180.1 ± 5.3

CDF-II lepton+jets* 170.9 ± 2.5

DØ-II lepton+jets* 170.3 ± 4.5

CDF-I alljets 186.0 ± 11.5

χ2/ dof = 10.6/10

Tevatron Run-I/II* 171.4 ± 2.1

150 170 190

Top Quark Mass [GeV]

CDF-II alljets* 174.0 ± 5.2

CDF-II b decay length* 183.9 ± 15.8



b

t

q

t

t

W

s channel (tb)σNLO = 0.88 ± 0.11 pb

t channel (tqb)σNLO = 1.98 ± 0.25 pb

W t productionσNLO = 0.093 ± 0.024 pb

Why search for single top quarks?

Access t − b − W coupling

Sensitive to new physics

Source of polarized top quarks

Need to extract a verysmall signal out ofvery large background


Event Signatures

q′ q light quark(t channel only)

W

b high pT b quark

W

t

ℓ

ν

high pT e or µ

6ET

gb

b

b quark

Pt, [GeV/c]0 20 40 60 80 100 120 140 160 180 200

102

leptonb from tother b

Eta-5 -4 -3 -2 -1 0 1 2 3 4 5

0

100

200

300

400

500

600

700 leptonb from tother blight q

s channel

t channels- and t-channels are similar

lepton + 6ET + jets

t channel b jet tends to be forward

Modeled with CompHEP (matched to NLO)


Background Modeling

Based on data as much as possible . . .W + jets production

distributions from matched ALPGEN

normalization from pre-tagged sampleheavy-flavor fraction from data

top pair production

contribution from the ℓℓ+jets and ℓ + jets channels

estimated from matched ALPGEN

normalized to NNLO pQCD

multi-jet events

jet misidentified as lepton

semi-leptonic decay of HF jets (bb)estimated from data

b

b

e or µ

6ET

e or µ

6ET

b

b

q

q


Measurement Strategy

Event Selection

select W -like events

maximize signal acceptance

optimize sensitivity

Separate Signal from Background

find discriminating variables

multi-variate techniques

Determine Cross Section

likelihood

pseudo-experiments


Event Yields

Selection optimized to maximizeacceptance: tb = (3.2 ± 0.4)%,tqb = (2.1 ± 0.3)%

Use multi-variate techniques toseparate signal from background

Dominant Systematic Uncertainties

tt cross section: 18%W +jets & multi-jet: 18 − 28%

jet energy scale: 1 − 20%

b ID: 2 − 16%


Multi-variate Analyses

Use multi-variate analysis techniquesto separate signal from background

HT > 212

Mt < 352 pt < 31.6Mt < 352

Ia Ia Ia

F P

DecisionTrees

NeuralNetworks

MatrixElement

tq Discriminant0 0.2 0.4 0.6 0.80

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

tqb

WbbBNN_output

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

20

25CC_EqTwoTag_EqTwoJet

BNN_output0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

20

25Datas+tWbbWccWlpttbar+ljttbar+llFake lepton

D0 Run II Preliminary 910 pb -1


Measuring the Cross Section

d = S + B = σAL + B = σa +∑Nbkg

i=1 bi

d = predicted number of data eventsS = predicted number of signal eventsB = predicted number of background eventsσ = cross sectionA = signal acceptanceL = integrated luminositya = effective luminositybi = number of events in each background component

P(D|d) ≡ P(D|σ, a, b) =∏Nbins

i=1 P(Di|di)

D = observed number of data eventsb = vector of background components

Posterior Probability Density (σ|D) ∝∫

a

∫

bP(D|σ, a, b)Prior(a, b)Prior(σ)dadb

Nbkg = 6 (ttℓℓ, ttℓj, W bb, W cc, W jj, multi-jet), Nbins = 12 chann. ×100 bins

Shape and normalization systematic uncertainties treated as nuisance parameters

Correlations between uncertainties properly treated

Signal cross section prior is non-negative and flat


Measuring the Cross Section

-5 0 5 10 15

DØ Run II 0.9 fb -1

σ(pp_ → tb+tqb) [pb]

-5 0 5 10 15

Decision trees

Matrix elements

Bayesian NNs

Z. Sullivan PRD 70, 114012 (2004), mt = 175 GeV

4.9 +1.4-1.4

pb

4.6 +1.8-1.5

pb

5.0 +1.9-1.9

pb

tb+tqb Cross Section [pb]

0

0.1

0.2

0.3σ measured

= 4.9 pb

σ expected

= 2.7 pb

0 1 2 3 4 5 6 7 8 9 10

[pb]σ0 2 4 6 8 10 12 140

0.020.040.060.08

0.10.12

0.140.160.180.2

0.22tbtqb_LeptonsCombined_EqTwoJet,EqThreeJet,EqFourJet_TagsCombined

Bayes Ratio> 10

Xsec. Sig: 2.6

tbtqb_LeptonsCombined_EqTwoJet,EqThreeJet,EqFourJet_TagsCombined

Cross Section [pb]0 2 4 6 8 10

]-1

post

Pro

b. D

ensi

ty [p

b

0

0.05

0.1

0.15

0.2

0.25

w/ 2+3 Jets and >=1 TagµPosterior Density: e+

-1.5 +1.8 = 4.6s+tσ

= 0.35s+tσs+tσδ

w/ 2+3 Jets and >=1 TagµPosterior Density: e+

Boosted Decision Tree Bayesian Neural Network Matrix Eleme nt

mea

sure

men

tsar

e≈

50%

corr

elat

ed;

com

bina

tion

inpr

ogre

ss


Have we seen it?Use ensembles with no signal todetermine signficance of eachmeasurement

-5 0 5 10 15

DØ Run II 0.9 fb -1

σ(pp_ → tb+tqb) [pb]

-5 0 5 10 15

Decision trees

Matrix elements

Bayesian NNs

Z. Sullivan PRD 70, 114012 (2004), mt = 175 GeV

4.9 +1.4-1.4

pb

4.6 +1.8-1.5

pb

5.0 +1.9-1.9

pb

Boosted Decision Tree Bayesian Neural Network Matrix Eleme nt

Observed tbtqb cross section [pb]0 1 2 3 4 5 6 7 8 9

1

10

210

310

410

tbtqbEntries 68150Mean 0.525RMS 0.7963

-channelµe+

-1DØ Run II Preliminary 910, pb

24 entries aboveobserved cross section

p-value: 3.5e-04

sigma: 3.4

Full systematics

Cross Section [pb]0 1 2 3 4 5 6 7

Entri

es/0

.2 p

b

1

10

210

310

410


pb-1.5+1.8 = 4.6s+tσ

p-Value = 0.0022

σSig = 2.9

Observed

Decision Tree 0.035%

Neural Network 0.885%

Matrix Element 0.21%

Decision tree has 3.4σ excess(11% compatable with SM)


CKM Matrix Element Vtb

d′

s′

b′

=

Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

d

s

b

t

b

W

Vtb

Weak interaction eigenstates and mass eigenstates are not thesame; there is mixing between quarks described by the CKMmatrix

In the SM, top must decay to a W and d, s, or b quark

V 2td + V 2

ts + V 2tb = 1

constraints on Vtd and Vts yield Vtb = 0.999100+0.000034−0.000004

If there is new physics then,

V 2td + V 2

ts + V 2tb < 1

no constraint on Vtb


Measuring |Vtb|Use measurement of single top cross section to makefirst direct measurement of |Vtb|

ΓµWtb

= − g√

2Vtb

(

γµ[

fL1 PL + fR

1 PR

]

− iσµν

MW(pt − pb)ν

[

fL2 PL + fR

2 PR

])

2| Vtb |0 0.5 1 1.5 2 2.5 3 3.5 4

Pos

terio

r D

ensi

ty

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1DØ Run II Preliminary, 910 pb

Measurement:

-0.5 +0.6 1.7

s+t-channels

Additional theoretical uncertaintiestb tqb

Top Mass 13% 8.5%

Scale 5.4% 4.0%

PDF 4.3% 10%

αs 1.4% 0.01%

VtbfL1 = 1.3 ± 0.2

0.68 < |Vtb| < 1

at 95% C.L.

(assuming fL1 = 1)

AssumeSM top quark decay: V 2

td + V 2ts << V 2

tb

Pure V − A: fR1 = 0

CP conservation: fL2 = fR

2 = 0

No need to assume three quark familiesor CKM unitarity


SummaryDØ is pursuing a rich program of top quark studyTevatron is the only game in town. . .for now!

Studies of tt production

Precision measurements of top quark properties

Observation of single top quark production andfirst direct measurement of |Vtb|Searches for new physics(Higgs, Z′, W ′, etc)

The LHC will be a top factoryand discovery machine —the lessons learned andtechniques developed at theTevatron will enhanceour discovery opportunities 160 165 170 175 180 185

mt [GeV]

80.20

80.30

80.40

80.50

80.60

80.70

MW

[GeV

]

SM

MSSM

MH = 114 GeV

MH = 400 GeV

light SUSY

heavy SUSY

SMMSSM

both models

Heinemeyer, Hollik, Stockinger, Weber, Weiglein ’06

experimental errors 68% CL:

LEP2/Tevatron (today)

Tevatron/LHC

ILC/GigaZ


Top Quark Measurements by the DØ Collaboration · 2008-01-15 · Michael Begel SMU Physics Seminar...

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Transcript of Top Quark Measurements by the DØ Collaboration · 2008-01-15 · Michael Begel SMU Physics Seminar...