w physics at lep
DESCRIPTION
W Physics at LEP. Paolo Azzurri Pisa (ALEPH-CMS ) Hanoi August 7, 2004 5th Rencontres du Vietnam New Views in Particle Physics. Outline. W-pair production at LEP2 W Gauge Couplings W Decay Couplings Other four fermion processes W mass. Introduction. - PowerPoint PPT PresentationTRANSCRIPT
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W Physics at LEP
Paolo Azzurri
Pisa (ALEPH-CMS)
Hanoi August 7, 2004
5th Rencontres du Vietnam New Views in Particle Physics
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Outline
W-pair production at LEP2
W Gauge Couplings
W Decay Couplings
¤ Other four fermion processes
W mass
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Introduction
a clear evidence of the presence of Standard Model
triple gauge couplings and of their SU(2)U(1) structure
the first direct measurements of the W decay branching ratios, and from these:☆ a direct test of the W lepton family and lepton-quark universality to 1% and 0.5%
a test of CKM unitarity and best current determination of the Vcs amplitude
an independent W mass measurement at production threshold (200 MeV accuracy)☼ a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy
The study of W-pair production at LEP2 was a nice and exciting work that provided:
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W-pair production at LEP2
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W-pair production
WWee
CC03 diagrams (not gauge invariant)
LEP2 produced about 10000 WW events/exp. with ~700pb-1/exp. Expect:
qqqqWW4500~
qqWW4500~
WW1000~
per experiment.
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W-pair event selection
Clear event decay ...
… sophisticated selections
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
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W-pair cross sections
2GeV/ 21.040.80 cmW Test of the SM radiative corrections to the CC03 diagrams
Clear proof of SU(2)xU(1) gauge couplings !
Precisionbetter than 1%
s (GeV)
WW (pb)
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W Gauge Couplings
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Triple Gauge Couplings
Standard electroweak theory
U(1)
SU(2)
triple and quartic SU(2) gauge boson self couplings
The signature of the non-abelian SU(2) electroweak structure !
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Triple Gauge Couplings
The most general Lorentz Invariant WWV (V=,Z) vertex has 7 complex couplings
0000011SM
~~ggg 451
P
CVV
VVVV
V
1W
W
1W
g2
e dipole magnetic
eg charge electric
m
Q
2W
W
WW
e quadrupole electric
~~2
e dipole electric
mq
md
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W-pair angular distributions
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W Decay Couplings
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W leptonic couplings
0.0141.030
0.0141.028
0.0100.998
e
e
gg
gg
gg
Direct test of W lepton universality at the 1% precision level
0.0281.061 ))/Br(WBr(W
0.0291.058 )e)/Br(WBr(W
0.0210.997 )e)/Br(WBr(W
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W hadronic couplings
025.0026.2
,,,
2
bsdjcui
ijV
007.0048.122222 cbcdubusud VVVVV
014.0989.0 csV
bsdjcui
ijWS Vm
,,,
2)(1
hadr)Br(W-1
hadr)Br(W
002.0121.0)( WS m
0.006994.0q gg
Direct test of W quark-lepton universality at the 0.6% precision level
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
¤ Other four fermion processes
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Single W and Z
In agreement with SM within 8% precision In agreement with SM within 7% precision
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W mass
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
W mass and width
W 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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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Standard Model Fit
All this and more in Roberto Tenchini’s talk tomorrow !
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5th Rencontres du Vietnam P.Azzurri - W Physics at LEP
Conclusions = Introduction
a clear evidence of the presence of Standard Model
triple gauge couplings and of their SU(2)U(1) structure
the first direct measurements of the W decay branching ratios, and from these:☆ a direct test of the W lepton family and lepton-quark universality to 1% and 0.5%
a test of CKM unitarity and best current determination of the Vcs amplitude
an independent W mass measurement at production threshold (200 MeV accuracy)☼ a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy
The study of W-pair production at LEP2 was a nice and exciting work that provided: