W Physics at LEP

W Physics at LEP

Paolo Azzurri

Pisa (ALEPH-CMS)

Hanoi August 7, 2004

5th Rencontres du Vietnam New Views in Particle Physics

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


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:


W-pair production at LEP2


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.


W-pair event selection

Clear event decay ...

… sophisticated selections

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


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)


W Gauge Couplings


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 !



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


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



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 Decay Couplings


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


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


¤ Other four fermion processes


Z-pairs

ZZee

In agreement with SM within 5% precision

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


Single W and Z

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


W mass


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 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


Standard Model Fit

All this and more in Roberto Tenchini’s talk tomorrow !


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:

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W Physics at LEP

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