A Measurement of the W Boson Mass

A measurement of the W boson mass is presented using data taken by the ALEPH detector during the 1997 running of the Large Electron-Positron Collider II (LEPII) at the European Center for Particle Physics (CERN) near Geneva, Switzerland. A high efficiency, high purity analysis which selects and reconstructs semi-leptonic W+W- final states (W+W- --> jjlv) is employed to define the sample. To improve the mass resolution of the detector, a constrained kinematic fit is developed to impose four-momentum conservation on each selected event. An unbinned maximum likelihood fit to the constrained two dimensional WW mass distribution is then constructed to extract the W pole mass. With 56.84 pb^-1 of data taken by the ALEPH detector near a center of mass energy of sqrt{s} = 183 GeV, the W boson is measured to have a mass of: Mw = 80.20 +/- 0.19 (stat) +/- 0.06 (syst) GeV consistent with predictions from the Standard Model.