Measurements of the mass and width of the W boson from $e^+ e^- \to W^+ W^- \to (e/\mu)\nu q\overline{q}$ events with the ALEPH detector

In 1998 at CERN (Geneva), 174 pb^-1 of data was collected with the ALEPH detector, at an e+e- centre-of-mass energy of 188.6 GeV. For this dataset, the selection of semi-leptonic decays of W boson pairs (WW->(e/m)vqq) is improved. The resulting efficiencies for selecting evqq and mvqq events are 82.0% and 88.1% respectively, with purities of 92.4% and 93.3%. For the first time in ALEPH, the width of the W boson is measured using a two parameter (mass Mw and width Gw) fit to the reconstructed W mass distribution, yielding: Mw = 80.313 +- 0.119 (stat.) +- 0.052 (syst.) GeV/c^2 Gw = 2.17 +0.29 -0.26 (stat.) +- 0.11 (syst.) GeV/c^2, for evqq and mvqq decays combined. The mass value is in good agreement with the world average mass and the width value is in good agreement with Standard Model calculations. The systematic error includes detector uncertainties as well as uncertainties in the background, the beam energy and the theoretical model. Additional studies are done with respect to previous years to take into account errors in the angular and energy resolutions of the ALEPH detector. The stability of the data fit results is checked as a function of various selection cuts. A preliminary measurement is performed at centre-of-mass energies ranging from 191.6 to 201.6 GeV. These results are combined with the result above to obtain the preliminary result for a total integrated luminosity of 411 pb^-1: Mw = 80.537 +- 0.079 +- 0.052 GeV/c^2 Gw = 2.11 +0.20 -0.18 +- 0.11 GeV/c^2.