precision electroweak measurements at lep

Precision Electroweak Measurements at LEP

Paolo Azzurri – INFN Pisa

WIN 05Weak Interactions and Neutrinos 2005

Delphi - June 8, 2005

Working Group 1: Electroweak Symmetry Breaking

WIN'05 Delphi P.Azzurri - Precision EW @ LEP 2

LEP1 & LEP2

• LEP1 (1989-1995): 4200pb-1 @ √s=mZ

→ 45 Million Z decays

• LEP2 (1996-2000): 4800pb-1 @ √s=161-209 GeV → 410,000 W pairs

Precision on Z and W mass

ΔmZ(1986)=1.7 GeV [SPS] → ΔmZ(1996)=2.1MeV [LEP]

ΔmW(1986)=1.5 GeV [SPS] → ΔmW(2002)=39 MeV [LEP+TEV]


EW Tree Level

Standard deviations

-2

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9

Years

Nu

mb

er

of

sig

mas

W Mass vs Tree Level

77

78

79

80

81

82

83

84

1 2 3 4 5 6 7 8 9

Years

GeV

/c2

Tree Level

F

Z

Z

W

W

G

M

M

MM

1

2

)(1 2

22

More and more evidence for EW radiative

corrections !

1986 2002

1986 2002

2

2

(R.Tenchini)


One loop EW radiative corrections

sHiggstopZFZii MmMGMfO ,,,,,

Need to introduce three additional parameters

mtop mhiggs αS

Observables Oi are

rGM

MM

FZ

WW

1

1

21

2

22

Contribution of radiative corrections


Z pole

feFB AAfA 43

Forward-Backward Asymmetries

WfA

fV

fA

fV

fgg

ggsA 2

2 sin 1


B Asymmetry

NEW FINAL RESULT

Error dominated by statistics

Χ2/n=0.58


Sin2ΘW from Asymmetries

Prob=3.7%

Historical difference betweenALR(l) SLD and AFB(b) LEP

FINAL RESULT

now 3.2σ


Hadronic Vacuum Polarization and α(MZ)

')'('

)'(

3)(

)()()(1

)0()(

24

)5(

)5(

dssss

sRss

ssss

m

hadhad

tophadl

046.0936.128

1)M( Z s

)61(0359895.137)0(1


Small angle Bhabha scattering

Evidence for αQED running and for Δαhad in the t-channel


EW at LEP2


Photons @ LEP2

08.084.2 Nee

ee

Constraints on: Coulomb Deviations, QED cutoffs, SUSY Neutralinos & Gravitinos,

extra-dimensions Gravitons, excited electrons, …. (Λ>1TeV)

Clean environment for new physics !


Fermion Pairs @ LEP2

More constraints on: extra-dim Gravitons, Contact Interactions, Z’ bosons, squarks,

leptoquarks, …

(Λ>1TeV)


Single W and Z

In agreement with SM within 8% precision In agreement with SM within 7% precision

Weee Zeeee


Z-pairs

ZZee

In agreement with SM within 5% precision

No gauge self couplings involved in Standard Model Z-pair production


W-pair events

qq

qq,We

qq

qq,Zee

,,ll

bkg

75-80%80-90%qqqq

80-85%50-80%qq

~95%75-90%qq

~90%75-90%eqq

80-90%50-80%ll

purityefficiencyChannel

WWee


W-pair cross sectionsClear proof of SU(2)xU(1) gauge couplings !

s (GeV)

WW (pb)

2GeV/ 21.040.80 cmW Test of the SM radiative corrections to the CC03 diagrams

Precisionbetter than 1%

Without O(α) R=0.974±0.009


Triple Gauge CouplingsStandard electroweak theory

U(1)

SU(2)

triple and quartic SU(2) gauge boson self couplings are the signature of the non-abelian SU(2) electroweak structure !

The most general Lorentz Invariant WWV (V=,Z) vertex has 7 complex couplings

VV

VV

VVV

~~gg

g

54

1 The WWZ and WW gauge couplings can be measured in W-pair events, fitting

the W-pair event rates and the W production and decay angular

distributions. *f

WW

W

WW

d

d

d

d


Triple Gauge Couplings

Standard measurements of three TGC couplings that conserve C and P, U(1)em and global SU(2)L U(1)⊗ Y

0TGCother all

tan1g

1g

real are g

W2Z

1Z

1Z

Z1

0.0210.991g 0.022-0.016 0.0450.984 Z1

one dimensional fit results (LEP):

Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings

0.0611.065Re

0.0621.071Re

0.0731.066gRe

0.0911.123gRe

Z

Z1

1

one-dimensional fit results

for the SM non-zero TGC values(ALEPH data only)

… all other 24 couplings are consistent with zero (within 5-20%) !


W leptonic couplings

0.0141.039

0.0151.036

0.0100.997

e

e

gg

gg

gg

Direct test of W lepton universality at the 1% precision level

0.0281.075 ))/Br(WBr(W

0.0291.069 )e)/Br(WBr(W

0.0210.994 )e)/Br(WBr(W

tau BR is three sigmas larger than e/mu !


W hadronic couplings

026.0000.2

,,,

2

bsdjcui

ijV

007.0048.122222 cbcdubusud VVVVV

014.0976.0 csV

bsdjcui

ijWS Vm

,,,

2)(1

hadr)Br(W-1

hadr)Br(W

002.0119.0)( WS m

0.006000.1q gg

Direct test of W quark-lepton universality at the 0.6% precision level


W mass

Improve the W invariant mass resolution:kinematic fit

energy-momentum conservation (equal mass constraints)

Direct kinematic reconstruction of the W mass in qqqq, qql (and l l ) final states

mW value extracted with different methods

• Breit-Wigner fit (with bias correction)• Monte Carlo reweighting (with different observables M1,δM)• Probability Density function P(M1,M2,..)

Statistical power of the LEP2 data: ΔmW(stat)=21 MeV


W mass systematics

The fully hadronic channel is de-weighted to 0.10

for possible final state interconnection effects !

mW(qqqq)= 80.420±107 MeV mW(qql )= 80.411± 44 MeV

(ρ=0.18)

ΔmW(qqqq-qql )= +22±43 MeV without CR and BE errors


Hadronic final state interactionsBose-Einstein correlations:

measure two particle correlationsbetween W’s / inside W’s

full effect ΔmW=35 MeVmeasured fraction ΔmW=15 MeV

Colour Reconnection effects:measure particle flow in regions

between W’s / inside W’s

upper limits: ki<2.13 Prob(CR)<0.65

ΔmW=90 MeV


CR insensitive jet reconstructions ?

Ways to reduce the CR bias:

1. Remove low energy particles (pcut)

2. Hybrid cone algorithm (R)

Prospects

Can reduce the CR mass shifts by a factor 2-3deteriorating the statistical precision by 20%

CR systematics : ΔmW=90 40 MeVstatistical error: ΔmW=35 40 MeV

total mW(qqqq) error:ΔmW=110 60 MeV

mW(qqqq) weight in combination : 0.10 0.30 combined mW error :42 38 MeV


W mass and widthW measured with the same methods used for the mW extraction

W= 2.150±0.91 GeV/c2 mW= 80.412±0.042 GeV/c2


Standard Model Fit

C.L. @95% GeV/c 280114 2 HmNew Tevatron run2 top mass …


Conclusions

LEP1 final results on the Z pole HF asymmetries

Many LEP2 final results on cross sections and couplings

Still waiting for final results on W mass

All results published by 2006?

1989-2000: LEP data provided a large set of new electroweak measurements

precision electroweak measurements at lep

Documents

sigmatree level

ztree level

sigmaz mass central

wtree level error

w mass mz1986

azzurri precision ew

w pairs precision

gev sps mw2002