Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam

9 April 2009 Nijmegen Colloquium 1

Precision electroweak measurements at the LEP e+e– collider

Jan Timmermans / NIKHEF Amsterdam

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• First study B. Richter 1976: C~43 km, E=2*100 GeV, L=1032 cm-2s-1, 8 ip’s

•Les Houches study 1979:

study of Z prod. and decay

study of WW production

Higgs search

searches for new leptons, quarks

3 and 4 jet structures, scaling

violations

• 1983: C=27 km; start construction

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Particles in the Standard Model• Fermions (matter particles)

b

t

s

c

d

u

ee

,,

,,

leptons

quarks

• Gauge bosons

, Z, W± (electroweak interaction)

gluons g (strong interaction)

Higgs boson H (resulting from EW symmetry breaking)

E.g. decays: Z +- , Z bb , W+ e+e , W+ cs

WZW mm cos

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Some events at LEP1

e+ e– 2-fermions

Z + – Z e+ e–

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Z + –

Z q q

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3 jets: Z q q g (2 quarks and gluon)

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And some events at LEP2e+ e– 4-fermions

The first WW event!

1996: s = 161 GeV

e+ e– W+W– q1q2q3q4

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e+ e– ZZ +– qqe+ e– Z** +– e+e–

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events/experiment:

4.5 M Z events

10000 WW events 600 ZZ events 250 single-W events

LEP1 (1989-1995): ~200 pb-1/exp LEP2 (1996-2000): ~700 pb-1/exp

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Z line shape and asymmetriese+

e- f

f

Z

e+

e- f

f

int22222

20

/)()(

ZZZ

Zffff msms

ss

220 12

Z

ffee

Zff m

pole cross-section:

)( 22AfVfffgg partial width:

invqqeeZ total width:

vector and axial-vector couplings

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Forward-backward asymmetry:

cos)cos1(8

3

cos

1 2FBAd

d

At the Z pole:2222

,0 22

4

3

4

3

AfVf

AfVf

AeVe

AeVefe

fFB gg

gg

gg

ggAAA

• Measure lepton partial width ll and F/B asymmetry AFB gives the couplings gVl and gAl (for l=e, , )

• Effective weak mixing angle:

Effective couplings contain radiative corrections, which depend on the top quark mass and Higgs boson mass

)1(4

1sin 2

Al

Vllepteff g

g

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Some examples of radiative corrections:

e+

e- b

b

/Z

t

tW

e+

e- f

fZ

H

e+

e- f

f

t

t

Higgs loop

top quark loop

W loop

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Z line shape

Moriond 1990:

mZ = 91171 ± 12 ± 32 (incl. ELEP) MeV

Z = 2538 ± 26 ± 28 MeV

N = 3.04 ± 0.12

first evidence 3 generations

Now:

mZ = 91187.5 ± 2.1 MeV

Z = 2495.2 ± 2.3 MeV

N = 2.9841 ± 0.0083

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Beam energy precision 0.2 MeV from resonant depolarisation

But corrections needed due to:

TIDES LEVEL LAKE TGV

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Axial and Vector couplings for leptons

Measured from partial widths ll,, forward-backward asymmetries AFB

0,l and τ polarisation

gVf / gAf = 1 – 4 | Qf | sin2eff f

Couplings and lepton-universality established at

1 per-mille (gAl) and few % (gVl)

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Vertex Detectors:

Precise -lepton & b-quark studies

Here DELPHI LEP2 version

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Rb = b / had and top quark mass

Summer 1992:

b = 3739 MeV I3b = -1/2

(370 MeV; for I3b =0: 24 MeV)

Top quark must exist

Rb is also sensitive to top quark mass,

but much less than sin2eff !lept

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Top mass: predicted by LEP

Prediction mtop from EW fit

EPS93 Marseille:

mtop = 166 –19 -22 GeV

ICHEP94 Glasgow:

mtop = 178 -11 -19 GeV

and CDF saw excess due to top at

mtop = 174 -10 -23 GeV

EPS95 Brussels: both CDF and D0 observed top at predicted mass.

+17 +19

+11 +18

+10 +13

Great success for the Standard Model

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Two most precise sin2eff values from

SLD ALR and LEP A0,b differ by 2.9

lept

FB

SM: 0.1036

A0,b (and also A0,c ) prefer a high

value for the Higgs boson massFBFB

A puzzle left ….. : comparison sin2efflept

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lepton: a puzzle solved

lifetime related to lifetime, mass, mass and leptonic branching ratio

Up to 1992 a 2-2.3 discrepancy between the coupling constants g and g

In 1992 a new, more precise mass by BES

Now - -e universality at few per-mille, thanks to high-precision Si vertex detectors

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only exchange

No ZWW vertex

WW cross section

Clear evidence for the SU(2) x U(1)Gauge structure

Exp. Errors 1-3% / energy point

CC03 diagrams:

Theory predictions include full O(em) corrections; theory error 0.5%

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Single W WW

DLO

Quartic Gauge Couplings

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W mass measurement

Mw21 – –––– –––– 1 – r Mw

2

MZ2

GF 2WW

t

b

W

WW

H

Mw ln MH

Mw Mt2

W mass defined by relativistic Breit-Wigner

lineshape of the W propagator with

s-dependent width

Mtop MW

Direct 1.3 GeV 25 MeV

Indirect ~10 GeV 32 MeV

• precision MW measurement 0.04%

• gives handle on Higgs mass

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W mass• W masses reconstructed directly from the decay products

•Constrained fits improve mass resolution from 8 GeV to 3 GeV

before 4C fit

after 4C fit

qqqq

qq

qqqqe

W mass: LEP (prel.) combined results•Single measurements are FINAL

•Working on combination (systematics)

Apart from NuTeV result (from rates of CC and NC (anti-) scattering)

very good agreement between direct and indirect measurements

*

*2008 Tevatron average; now D0 alone: 80.401 ± 0.044 GeV

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We searched…..

and searched…..

and searched…..

and searched…..

and searched…..

No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos....

Large part of the MSSM parameter space excluded

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SM Higgs search

Higgs strahlung: dominant WW fusion: small(can go beyond ‘kinematical limit’)

Branching ratios (mH=115 GeV):

• H bb (74%)

• H (8%)

• H WW (8%)

Final states:

• bbqq (4-jet channel)

• bb (missing energy channel)

• bbee, bb (leptonic channel)

• bb, bb (tau channel)

Selections: cut based or NN

• select multi-hadronic events

• look for b-tagged jets

• lepton identification

• apply constraint fits + sometimes W/Z mass constraints

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Mass distributions:

• But not only mass information useful!

• Combine all information in a discriminating variable (event likelihood or NN output)

• 2-dim inputs to statistical analysis:

- invariant mass of bb jet combination

- discriminating variable (containing b-tags, kinematics, jet-properties)

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CLs+b1-CLb

Likelihood test: sig+bkg bkg

)(

)()(

i

iiHi bL

bsLmQ

i

iQQ )ln(2)ln(2

•1-CLb measures incompatibility with bkg

•CLs+b a measure of compatiblity with s+b

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•Small excess around 116 GeV but less than 2 (mainly coming from ALEPH candidates and from four-jet events)

•Excess was 2.9 on 3/11/2000

• CLs = CLs+b / CLb

• when CLs < 0.05 the hypothesis is rejected at 95% CL

MH > 114.4 GeV (115.3 expected)

FINAL 2003

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Global electroweak fitResults of different fits:

• using only Z pole data

• using all data

%9.910/0.16/

111

173

2

19060

1310

probndof

GeVm

GeVm

H

t

%)19(13/2.17/

90

3.12.173

2

3627

probndof

GeVm

GeVm

H

t

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•Very good consistency between direct and indirect measurement of mt and mW

• Both prefer low Higgs boson mass

mH < 163 GeV at 95% CL

(Tevatron excludes 160-170 GeV)

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Conclusions LEP was great!

Full gauge structure of the SM has been measured, and many measurements in good agreement with the SM prediction

Largest discrepancy between AbFB (LEP) and ALR (SLD) at ~3

but much less w.r.t. average in terms of sin2eff !

SM Higgs boson not yet found: MH > 114.4 GeV

(and MH outside 160-170 GeV window from Tevatron)

A task for the Tevatron and Large Hadron Collider experiments

lept

total output: ~ 300 journal papers/experiment

PhD theses in NL: ~50 (L3 + DELPHI) , ~half of which RUN

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Backup slides

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We searched…..

and searched…..

and searched…..

and searched…..

and searched…..But sometimes……

No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos....

Large part of the MSSM parameter space excluded

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•1995: 130-136 GeV

•4-jet events

•Sum of di-jet masses with smallest M

•16 events (8.3 exptd)

•Prob. accumulation in 6.3 GeV bin: 0.01%

ALEPH

Why it is good to have more than one expt.

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QCD: test of gluon selfcouplingFrom fit to angular distributions in 4-jet events.

Rel. strength of couplings qqg, ggg, gqq depends on gauge group through Casimir factors CF , CA , TF

TF /

CF

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QCD: running of s

Results from fits to different event shape variables.

Combined fit at all energies:

s(MZ) = 0.1201 0.0003

0.0009

0.0009

0.0047

(stat)

(exp.syst)

(theor.hadr.)

(theor.ev.shapes)