Measurement of the Mass and Width of the W Boson in e+e- Collisions at   189 GeV
  The mass and width of the W boson are determined in e+e- collisions at LEP using 183 pb^-1 of data recorded at a centre-of-mass energy roots=189 GeV with the OPAL detector. The invariant mass distributions from 970 WW->qqqq and 1118 WW->qqln candidate events are used to measure the mass of the W boson,   Mw = 80.451 +- 0.076(stat.) +- 0.049(syst.) GeV.   A direct measurement of the width of the W boson gives Gw=2.09 +- 0.18(stat.) +- 0.09(syst.) GeV. The results are combined with previous OPAL results from 78 pb^-1 of data recorded with roots from 161 to 183 GeV, to obtain:   Mw = 80.432 +- 0.066(stat.) +- 0.045(syst.) GeV,   Gw = 2.04 +- 0.16(stat.) +- 0.09(syst.) GeV.   The consistency of the direct measurement of Mw with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions. 
The OPAL collaboration, G. Abbiendi, et al