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### W Physics at LEP

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W-pair event selection

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

P.Azzurri - W Physics at LEP

Introduction

The study of W-pair production at LEP2

was a nice and exciting work that provided:

- 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

P.Azzurri - W Physics at LEP

W-pair production at LEP2

P.Azzurri - W Physics at LEP

LEP2 produced about 10000 WW events/exp. with ~700pb-1/exp.

Expect:

per experiment.

W-pair productionCC03 diagrams (not gauge invariant)

P.Azzurri - W Physics at LEP

Channel

efficiency

purity

bkg

ll

50-80%

80-90%

,,ll

eqq

75-90%

~90%

qq,Zee

75-90%

~95%

50-80%

80-85%

qq,We

qqqq

80-90%

75-80%

Clear event decay ...

P.Azzurri - W Physics at LEP

W-pair cross sections

Clear proof of SU(2)xU(1) gauge couplings !

WW (pb)

s (GeV)

Precision

better than 1%

Test of the SM radiative corrections to the CC03 diagrams

P.Azzurri - W Physics at LEP

W Gauge Couplings

P.Azzurri - W Physics at LEP

triple and quartic SU(2) gauge boson self couplings

The signature of the non-abelian SU(2) electroweak structure !

Triple Gauge CouplingsStandard electroweak theory

U(1)

SU(2)

P.Azzurri - W Physics at LEP

Triple Gauge Couplings

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

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.

P.Azzurri - W Physics at LEP

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

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%) !

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

one dimensional fit results (LEP):

P.Azzurri - W Physics at LEP

W Decay Couplings and WW

P.Azzurri - W Physics at LEP

Direct test of W lepton universality and WW

at the 1% precision level

W leptonic couplingsP.Azzurri - W Physics at LEP

Direct test of W quark-lepton universality and WW

at the 0.6% precision level

W hadronic couplingsP.Azzurri - W Physics at LEP

Other four fermion processes and WW

P.Azzurri - W Physics at LEP

Z-pairs and WW

No gauge self couplings involved

in Standard Model Z-pair production

In agreement with SM within 5% precision

P.Azzurri - W Physics at LEP

Single W and Z and WW

In agreement with SM within 8% precision

In agreement with SM within 7% precision

P.Azzurri - W Physics at LEP

W mass and WW

P.Azzurri - W Physics at LEP

W mass and WW

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

Improve the W invariant mass resolution:

kinematic fit

energy-momentum conservation

(equal mass constraints)

- 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

P.Azzurri - W Physics at LEP

m and WWW(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

W mass systematicsThe fully hadronic channel is de-weighted to 0.10

for possible final state interconnection effects !

P.Azzurri - W Physics at LEP

Colour Reconnection effects: and WW

measure particle flow in regions

between W’s / inside W’s

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

ΔmW=90 MeV

Hadronic final state interactionsBose-Einstein correlations:

measure two particle correlations

between W’s / inside W’s

full effect ΔmW=35 MeV

measured fraction ΔmW=15 MeV

P.Azzurri - W Physics at LEP

CR insensitive jet reconstructions ? and WW

Prospects

Can reduce the CR mass shifts by a factor 2-3

deteriorating the statistical precision by 20%

CR systematics : ΔmW=90 40 MeV

statistical 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

- Ways to reduce the CR bias:
- Remove low energy particles (pcut)
- Hybrid cone algorithm (R)

P.Azzurri - W Physics at LEP

W mass and width and WW

W measured with the same methods used for the mW extraction

mW= 80.412±0.042 GeV/c2

W= 2.150±0.91 GeV/c2

P.Azzurri - W Physics at LEP

Standard Model Fit and WW

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

P.Azzurri - W Physics at LEP

Conclusions = Introduction and WW

The study of W-pair production at LEP2

was a nice and exciting work that provided:

- 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

P.Azzurri - W Physics at LEP

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