Measurement of the mass and width of the W boson

The mass and width of the W boson are measured using e(+)e(-) -> W+W- events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV. The mass (m(W)) and width (Gamma(W)) are determined using direct reconstruction of the kinematics of W+W- -> q (q) over bar lfv and W+W- -> q (q) over barq (q) over bar events. When combined with previous OPAL measurements using W+W- -> lvlv events and the dependence on m(W) of the WW production cross-section at threshold, the results are determined to be m(W) = 80.415 +/- 0.042 +/- 0.030 +/- 0.009 GeV Gamma(W) = 1.996 +/- 0.096 +/- 0.102 +/- 0.003 GeV where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam energy. By measuring m(W) in the q (q) over barq (q) over bar channel using several different determinations of the direction of jets with differing sensitivities to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects in W+W- -> q (q) over barq (q) over bar events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides an important test of the Standard Model of electroweak interactions.