Measurement of the W mass and width in e(+)e(-) collisions at 189 GeV

The mass of the W boson is determined in e(+)e(-) collisions at LEP by the direct reconstruction of W decays in WW --> q (q) over barq (q) over bar and WW --> lvq (q) over bar events, supplemented by measurements using the kinematic properties of the leptons in the WW --> lvlv decay channel. The main sample of W pairs is selected from an integrated luminosity of 174 pb(-1) collected with the ALEPH detector in 1998 at a centre-of-mass energy of 188.63 GeV. The combined result from all channels is m(W) = 80.432 +/- 0.072(stat.) +/- 0.041(syst.) +/- 0.019(FSI) +/- 0.017(LEP) GeV/c(2), where FSI represents the possible effects of final state interactions in the q (q) over barq (q) over bar channel. In a second two-parameter fit to the q (q) over barq (q) over bar, evq (q) over bar and mu vq (q) over bar channels, where the W mass and width are decoupled, the average W width is found to be 2.24 +/- 0.20(stat.) +/- 0.13(syst.) GeV/c(2), consistent with the Standard Model prediction. The combination of the mass measurement presented in this paper together with those derived previously from the W pair cross section at 161 and 172 GeV and direct reconstruction at 172 and 183 GeV gives m(W) = 80.418 +/- 0.061(stat.) +/- 0.040(syst.) +/- 0.019(FSI) +/- 0.017(LEP) GeV/c(2).