Final State Interactions

Precise measurements of the mass and width of the W boson are carried out in e + e − collisions at Lep, by kinematic reconstruction of the invariant mass distributions of W + W − → qq ′ ℓν ℓ and W + W − → qq ′ qq ′ candidate events. The most recent combination of such results from Aleph, Delphi, L3 and Opal uses approximately 82% of the final Lep2 integrated luminosity and is preliminary. The mass of the W boson so determined is MW = 80.446 ± 0.026 ± 0.030 GeV, while the corresponding direct measurement of the W boson width gives ΓW = 2.148±0.071±0.063 GeV. These measurements are subject to sizeable systematic uncertainties from the QCD phenomena of Bose-Einstein correlations and colour reconnection. Recent substantial progress in both of these areas is reported. The Lep e + e − collider at CERN has provided an ideal environment for the study of the properties of the gauge bosons of the Standard Model of electroweak interactions. Since 1996, it operated at centre-of-mass energies above the W + W − production threshold (Lep2), allowing direct measurements of the W boson mass, M W. When combined with the direct measurements of the top quark mass at the Tevatron, these allow further constraints to be set on the mass of the Higgs boson via electroweak radiative corrections. Comparison between the direct measurements of the mass of the W boson and the value determined indirectly from data recorded at √ s ≈ M Z provides an important test of the self-consistency of the Standard Model. The direct measurement of Γ W further tests the consistency of the Standard Model.