Precision Electroweak Measurements at LEP
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Transcript of 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
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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]
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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)
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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
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Z pole
feFB AAfA 43
Forward-Backward Asymmetries
WfA
fV
fA
fV
fgg
ggsA 2
2 sin 1
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B Asymmetry
NEW FINAL RESULT
Error dominated by statistics
Χ2/n=0.58
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Sin2ΘW from Asymmetries
Prob=3.7%
Historical difference betweenALR(l) SLD and AFB(b) LEP
FINAL RESULT
now 3.2σ
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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
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Small angle Bhabha scattering
Evidence for αQED running and for Δαhad in the t-channel
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EW at LEP2
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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 !
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Fermion Pairs @ LEP2
More constraints on: extra-dim Gravitons, Contact Interactions, Z’ bosons, squarks,
leptoquarks, …
(Λ>1TeV)
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Single W and Z
In agreement with SM within 8% precision In agreement with SM within 7% precision
Weee Zeeee
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Z-pairs
ZZee
In agreement with SM within 5% precision
No gauge self couplings involved in Standard Model Z-pair production
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W-pair events
qq,We
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
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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
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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
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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%) !
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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 !
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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
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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
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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
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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
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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
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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
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Standard Model Fit
C.L. @95% GeV/c 280114 2 HmNew Tevatron run2 top mass …
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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