W mass and W+W- 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-bar lν-barl and W+W- → qq-bar qq-bar 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. (author)