LEP, SLC and the Standard Model · 2003. 11. 17. · LEP, SLC and the Standard Model Final - and...

LEP, SLC and the

Standard ModelFinal - and not so final - electroweak results

e+

e−γ /Z

f−

f

e+

e−

γ

Z

f−

f

e+

e−

νeW

We+

e− γ WW

e+

e− Z WW

e+

e− Z(∗) Z(∗)

H

Dave CharltonThe University of Birmingham

SLAC Summer Institute Topical Conference, August 2002

Most of the averages and plots shown were prepared by the LEP electroweak

working group – particular thanks are due to them

High Energy e e

�

Colliderse+

e−γ

f−

f

e+

e− Z

f−

f

10

10 2

10 3

10 4

10 5

0 20 40 60 80 100 120 140 160 180 200 220

Centre-of-mass energy (GeV)

Cro

ss-s

ecti

on (

pb)

CESRDORIS

PEP

PETRATRISTAN

KEKBPEP-II

SLC

LEP I LEP II

Z

W+W-

e+e−→hadrons

SLC 1989-1998

�� ,� 20 pb

� �

� �

polarisation� 75%

LEP-1 1989-199588 � � � 94 GeV

� 160 pb

� � 4

LEP-2 1996-2000130 � � � 209 GeV

� 700 pb

� � 4

Millions of Z’s, tens of thousands of W’s

Dave Charlton LEP/SLD Electroweak (2) August 2002

The Z Lineshape

Ecm [GeV]

σ had

[nb]

σ from fitQED corrected

measurements, error barsincreased by factor 10

ALEPHDELPHIL3OPAL

σ0

ΓZ

MZ

10

20

30

40

86 88 90 92 94

� �� measured over�� 3 GeV

Initial-state radiation (ISR): bigeffect

�� 91.1875� 0.0021 GeV

� � 2.4952� 0.0023 GeV

��

� � � � � ��

gives access to � �� via

� ��

Using ! � �� " � ! ! �$# � we find

! �% &'( � � )% ) )'*


The Z Lineshape: High Statistics!

peak-2

9.9

10

10.1

10.2

89.44 89.46 89.48Ecm [GeV]

σ had

[nb]

ALEPH

DELPHI

L3

OPAL

1990-1992 data

1993-1995 datatypical syst. exp.luminosity error

theoretical errors:QEDluminosity

peak

30

30.2

30.4

30.6

30.8

91.2 91.25 91.3Ecm [GeV]

peak+2

14

14.25

14.5

14.75

92.95 92.975 93 93.025Ecm [GeV]


Z Decay Widths: Heavy Quarks

Decay Length Significance

DELPHI

bcuds

0 1 2 3 4 5 60

200

400

600

800

1000

1200

1400

1600

1800

M / GeV

SLD

bc

� � ��

Z lineshape � � e, � , � , � � �

Also measure Z � , Z �

Main b tags: � � , � , � � High performance multivariate b tags:

� from data – “double tag” method

0.16

0.17

0.18

0.19

0.214 0.216 0.218 0.22

R0b

R0 c

Preliminary

68% CL

95% CL

SM

By convention � � � � � ��

� � )% � � �( ( � )% ) ) ) ��Dave Charlton LEP/SLD Electroweak (5) August 2002

Asymmetries at the Z

e−

FB

µ−

� � ��

��

Asymmetry parameters �

� � � ��

� ��

� �

��

� �( � �� Various asymmetries:

� forward-backward

� ��

� tau polarisation � � , � � � � �

� left-right polarisation � � �

� forward-backward left-right

Superscript 0 denotes “Z-pole” quantities

� � measured for e, � , � , b and c


Left-Right Polarization Asymmetry

Strained Lattice Cathodefor 1993 SLD Run

SourceLaserWavelenghtOptimized


1996 Run


→

→

→

→

→

Z Count

Beam Polarization SLD 1992-1998 Data

Pol

ariz

atio

n of

Ele

ctro

n B

eam

(%

)

0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000 4000 5000x 10

2

0LRA

0.10 0.12 0.14 0.16

92

93

94-95

96

97-98

Average

0.004 0.044 ±0.100

0.0028 0.0071 ±0.1656

0.0011 0.0042 ±0.1512

0.0010 0.0057 ±0.1593

0.0010 0.0024 ±

±

±

±

±

±0.1491

0.0022±0.1514

/DOF=7.4/4 Prob.=11.4%2χ

Measured by SLD, polarized� �

beam

Counting experiment:

� � � � � � �

� � � � ��

� � � �

� � � � mean polarization, three measure-ments

With � � � , convert to

�� % � � �� % � ��

Overall, SLD obtain

�� % � � �� % � � ��

Most precise measurement of ��


Leptonic Couplings

-0.041

-0.038

-0.035

-0.032

-0.503 -0.502 -0.501 -0.5

gAl

g Vl

68% CL

l+l−

e+e−

µ+µ−

τ+τ−

mt

mH

∆α

Combining leptonasymmetry results

� )% �� ) � � )% ) ) � �

Combine also with leptonicpartial widths

Test of NC leptonuniversality

Strong sensitivity to radiativecorrections

Preference for small H massin SM framework


Heavy Quark Asymmetries

0

200

400

600

800

1000

1200

1400

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1Q tag class 4

even

ts /

0.02

OPAL1994 databbcuds

b

b

uds c

�� and �� measured at LEP/SLD

Needs good flavour and charge tagging

For �� , powerful lifetime tags need

charge measure – multivariate techniquescalibrated from data

Lepton tags probe b charge via �

0.055

0.065

0.075

0.085

0.09 0.1 0.11

A0,bfb

A0,

cfb

Preliminary

68% CL

95% CL

mHmt

SM

For � �� , sensitivity to ��

mainly via � � � ��

SLD also measure � , � via polarized� �


Effective Weak Mixing Angle

10 2

10 3

0.23 0.232 0.234

Preliminary

sin2θlept

eff = (1 − gVl/gAl)/4

mH [

GeV

]

χ2/d.o.f.: 10.2 / 5

A0,l

fb 0.23099 ± 0.00053

Al(Pτ) 0.23159 ± 0.00041

Al(SLD) 0.23098 ± 0.00026

A0,b

fb 0.23217 ± 0.00031

A0,c

fb 0.23206 ± 0.00084

<Qfb> 0.2324 ± 0.0012

Average 0.23148 ± 0.00017

∆αhad= 0.02761 ± 0.00036∆α(5)

mZ= 91.1875 ± 0.0021 GeVmt= 174.3 ± 5.1 GeV

Asymmetries mostly measure��

A posteriori, see that two most

precise �� measurementsagree only at 2.9 � level

Old problem: discrepancy around 3 � for six

years, though errors improved by factor 1.5

In context of SM: �� prefers

� � 400 GeV — unlike mostother observables which prefer low

�


Fermion Pairs at LEP-2

10 2

10 3

10 4

10 5

80 100 120 140 160 180 200

120 140 160 180 200

√s / GeV

Cro

ss-s

ecti

on /

pb

OPALpreliminary

e+e-→hadrons

e+e-→hadrons; s′/s>0.7225

√s

´ (GeV)

Num

ber

of e

vent

s / 4

GeV

Data 207 GeV

hadrons(γ)

e+e−hadrons

W+W−

others

L3preliminary

0

100

200

300

400

50 75 100 125 150 175 200

��

e+

e−(γ/Z)*

f−

f

e+

e−

γ

Z

f−

f

radiative return

non-radiative


LEP-2 Fermion Pair PropertiesNon-radiative events � � � � � � � � �

� � � � � � �

Cro

ss s

ectio

n (p

b)

√s

´/s

> 0.85

e+e−→hadrons(γ)e+e−→µ+µ−(γ)e+e−→τ+τ−(γ)

LEPpreliminary

√s

(GeV)

σ mea

s/σ S

M

1

10

10 2

120 140 160 180 200 220

0.8

0.9

1

1.1

1.2

120 140 160 180 200 220

For

war

d-B

ackw

ard

Asy

mm

etry √s

´/s

> 0.85

e+e−→µ+µ−(γ)e+e−→τ+τ−(γ)

LEPpreliminary

√s

(GeV)

AF

B

mea

s -AF

B

SM

0

0.2

0.4

0.6

0.8

1

120 140 160 180 200 220

-0.2

0

0.2

120 140 160 180 200 220

SM continues to describe � � at LEP-2Dave Charlton LEP/SLD Electroweak (12) August 2002

W Pair Productione+

e−

νeW

We+

e− γ WW

e+

e− Z WW

√s (GeV)

σ WW

(pb

)

YFSWW and RacoonWW

LEP PRELIMINARY

13/07/2002

0

10

20

160 180 200

16

17

18

190 195 200 205

Around 12000 WW / experiment

46% WW � � � � �

� 4 jets

44% WW � � � � �

2 jets, charged lepton, missing p

10% WW � ��

��

2 leptons, missing p

Selection � � � typically 80-90%

� � � � � to� 1%

New O( � )-corrected predictionsshift � � � by � 2.5� 0.5%

� � � well-described byO( � ) calculations


Gauge Boson Self-Couplings

0

10

20

30

160 180 200

√s (GeV)

σ WW

(pb

)

YFSWW/RacoonWWno ZWW vertex (Gentle)only νe exchange (Gentle)

LEPPRELIMINARY

16/07/2002

Direct measurements possible at LEP-2

Several channels, WW is most powerful

e+

e− γ /Z WW?

Sensitivity via � � � , production polarangle, W polarisation

O( � ) corrections � current precision – now included in analyses

Vertex coupling parameters: (preliminary)

� � � 0.943 � 0.055 SM: 1

� � � � 0.020 � 0.024 0

�� 0.998 �� 1

SM gauge structure of boson self-couplings demonstrated


W Mass from LEP-2

W mass (GeV/c 2 ) ev

ents

/ 1

GeV

/c 2

0

20

40

60

80

100

120

140

50 55 60 65 70 75 80 85 90 95 100

L3

minv [GeV]

Num

ber o

f Eve

nts

/ 1 G

eV

Data qqeνM.C. reweighted

M.C. background (a)

preliminary

MW = 79.99 ± 0.22 GeV

0

20

40

60

50 60 70 80 90

0

10

20

30

40

50

60

70

80

50 55 60 65 70 75 80 85 90

MW (GeV/c2)

Eve

nts

per

1 G

eV/c

2

ALEPHµνqq selection

√s = 188.6 GeV

Data

MC (mW = 80.27 GeV/c2)

Non-WW background

OPAL √s=189 GeV

0

10

20

30

40

50

60

70

70 80 90mrec/GeV

Eve

nts

WW→qqτν

Primarily from WW � � � � �

and � � � �Reconstruct (jet, ) angles and

energies

Kin. fit improves resolution:

� (E,p)�� ( � ,0)

� � � � �

Fit to extract � (+ � � )

Statistical power:

� � � � � � � � �

Systematic errors significant


Final-State Interactions

γ,Z

W–

W+

πo πo

K+ K+

π– π–

ColourReconnection

Bose-Einstein

WW � � � � � may suffer from “final-stateinteractions”: if the two hadronic W decays don’t

develop independently

Non-perturbative hadronisation process: needsmodels

Models of two types: “colour reconnection” (CR)and Bose-Einstein correlations (BE)

Ongoing work to compare models with data:

� BE between W decay hadrons small, so give low � shift

� CR studies focus mainly on particle flow - but several models givequite different effects


Particle Flow & Colour Reconnection

10-1

1

0 1 2 3 4rescaled angle (φresc)

1/N

evt d

n/dφ

189-207 GeV (preliminary)W1 W2

L3 Data- WW → qqqq

A C B D

A

BC D

jet 2jet 1

jet 3jet 4

� ��

� ��

� ��

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1particle flowparticle flowparticle flowparticle flowparticle flow

φr

R

ALEPH data (189-207 GeV)

KoralW (JETSET)

KoralW (SKI 100%)

ALEPHpreliminary

0

2

4

6

0.8 1 1.2 1.4

OPAL

L3

DELPHI

ALEPH

LEP 0.969±0.015

r at 189 GeV

SK

-I 100%

No C

R

� � � �

� � � ��

�

� � � ��

, � � ��

LEP combination now available, full LEP-2 data

Hints of CR effects, data-driven error on

� ( � � � � channel) from CR� 90 MeV


W Mass Results

80.3 80.4 80.5 80.6 80.7MW[GeV]

ALEPH 80.458±0.055

DELPHI 80.404±0.074

L3 80.376±0.077

OPAL 80.490±0.065

LEP preliminary 80.447±0.042

CDF 80.433±0.079

D0 80.483±0.084

Tevatron Run-1 80.454±0.059

World average 80.450±0.034

In LEP � average, weightof � � � � channel just 9%

Also find

��

� � ��

� � � �� GeV

2

2.1

2.2

2.3

80.2 80.3 80.4 80.5

MW [GeV]

Γ W

[GeV

]

Preliminary

68% CL

SMmHmt

∆α

Good agreement LEP-Tevatron,comparable precision per experiment


Global Electroweak TestsUse precise LEP/SLD and Tevatron EW data to probe SM

(other inputs from NuTeV and atomic parity violation)

SM predictions from ZFITTER and TOPAZ0 electroweak libraries

Parameters:

� measured precisely by LEP-1 Z data

� � � �� measured precisely by LEP-1 Z data

� � � � �� calculated from low-energy measurements

� , � may be either predicted, or put in as measured

� may be predicted in this framework


Predicting and

�

80.2

80.3

80.4

80.5

80.6

130 150 170 190 210

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

Preliminary

68% CL

∆α

LEP1, SLD Data

LEP2, pp− Data

LEP-1/SLD Z data, � � � � �� ,NuTeV and APV results used to

predict � , �Compare with direct

measurements (Tev/LEP-2),and with SM relation between

� , � , �

Electroweak fit correctlypredicts the masses of the

heavy particles (W,top)

Both sets of data prefer a lightHiggs in the SM framework


Fit to all Electroweak Data

Measurement Pull (Omeas−Ofit)/σmeas

-3 -2 -1 0 1 2 3

-3 -2 -1 0 1 2 3

∆αhad(mZ)∆α(5) 0.02761 ± 0.00036 -0.24

mZ [GeV]mZ [GeV] 91.1875 ± 0.0021 0.00

ΓZ [GeV]ΓZ [GeV] 2.4952 ± 0.0023 -0.41

σhad [nb]σ0 41.540 ± 0.037 1.63

RlRl 20.767 ± 0.025 1.04

AfbA0,l 0.01714 ± 0.00095 0.68

Al(Pτ)Al(Pτ) 0.1465 ± 0.0032 -0.55

RbRb 0.21644 ± 0.00065 1.01

RcRc 0.1718 ± 0.0031 -0.15

AfbA0,b 0.0995 ± 0.0017 -2.62

AfbA0,c 0.0713 ± 0.0036 -0.84

AbAb 0.922 ± 0.020 -0.64

AcAc 0.670 ± 0.026 0.06

Al(SLD)Al(SLD) 0.1513 ± 0.0021 1.46

sin2θeffsin2θlept(Qfb) 0.2324 ± 0.0012 0.87

mW [GeV]mW [GeV] 80.449 ± 0.034 1.62

ΓW [GeV]ΓW [GeV] 2.136 ± 0.069 0.62

mt [GeV]mt [GeV] 174.3 ± 5.1 0.00

sin2θW(νN)sin2θW(νN) 0.2277 ± 0.0016 3.00

QW(Cs)QW(Cs) -72.18 ± 0.46 1.52

Summer 2002

Full electroweak fit of all results,including � and �

Overall consistency �� /dof is

29.7/15 (1.3% probability)

Large �� contribution from NuTeV,

without it fit probability is 11%

Standard Model parameters ( �

etc) little affected by NuTeV

Go on to see what the SM fit saysabout �


Constraining the SM Higgs

0

2

4

6

10020 400

mH [GeV]

∆χ2

Excluded Preliminary

∆αhad =∆α(5)

0.02761±0.00036

0.02747±0.00012

Without NuTeV

theory uncertaintyFit to all electroweak data inStandard Model framework

Theory uncertainty includesZFITTER/TOPAZ0 options,

partial two-loop calculations

NuTeV has little impact on �

results, but may affect whetherto believe the SM fit...

From the fit, obtain

� ' � ��

� � � &* GeV at 95% CL


A Hint of the Higgs?

DALI

Run=56065 Evt=3253 ALEPH

4 Gev EC10 Gev HC

(φ−43 )*SIN(θ)

θ=180 θ=0

−xx

x

x

−

x

x

−

−x

−

x

x

−−

x −x xx

x

−

x

x

x−

−− x−

−

x

−

−

x

−x

x

−

−

−x

−

− −

−

x o

ooooooo

oo

oo

o

o

oo

o

oooo

o

oooo o

o

o

o

o

oo

o

ooo

oo

o

oo

22. GeV

P>2 Z0<5 D0<2 YX

0 −0.8cm 0.7cmX

0 −0.8cm

0.7cm

Y

e+

e− Z(∗) Z(∗)

H At LEP-2, main process is� � � � � ZH � � �

Rely on good b tagging and mass reconstruction

In September 2000, ALEPHreported an excess (3

events) in the � � channelconsistent in mass with a

115 GeV H

LEP run extended for 1month...


LEP Higgs Search: Final Results

10-5

10-4

10-3

10-2

10-1

1

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

1-C

Lb

3σ

2σ

4σ

LEP

ObservedExpected for signal+backgroundExpected for background

10-6

10-5

10-4

10-3

10-2

10-1

1

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

CL

s

114.4 115.3

LEP

ObservedExpected forbackground

Final data, analyses and calibrations:

� no confirmation of ALEPH � �

excess

� no significant excess in final com-bined sample ( � 8%)

but extra statistics too limited to excludea 115 GeV SM Higgs

Sophisticated statistical combination ofchannels/experiments

Final direct search result:

� � 114.4 GeV (95% CL)(expected limit 115.3 GeV)


HighlightsWealth of precise electroweak measurements from LEP and SLD

(and the Tevatron)

Amongst hundreds of other results, LEP/SLD have:

� shown there are three light neutrino species

� demonstrated radiative loop corrections

� predicted the top quark mass

� verified SM triple gauge couplings

� put many strong constraints on physics beyond the SM

� indicated where to look for the SM Higgs (and ”nearly” found it)...

LEP and SLD have provided a huge step forward for the Standard Model– but the Higgs sector waits for another day


LEP, SLC and the Standard Model · 2003. 11. 17. · LEP, SLC and the Standard Model Final - and...

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Transcript of LEP, SLC and the Standard Model · 2003. 11. 17. · LEP, SLC and the Standard Model Final - and...