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→qq¯qq¯ and WW→lνqq¯ events, supplemented by measurements using the kinematic properties of the leptons in the \mathrm{W}\mathrm{W}\rightarrow\mbox{\ell\nu }\mbox{$\ell\nu $} 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 mW=80.432±0.072(stat.)±0.041(syst.)±0.019(FSI)±0.017(LEP)GeV/c2, where FSI represents the possible effects of final state interactions in the qq¯qq¯ channel. In a second two-parameter fit to the qq¯qq¯ , \mathrmeνqq¯ and μνqq¯ 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/ c2 }, 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 mW=80.418±0.061(stat.)±0.040(syst.)±0.019(FSI)±0.017(LEP)GeV/c2.