Mesure de la masse du boson W par la méthode de reconstruction directe dans l'expérience ALEPH au LEP

The mass of the W boson is a fundamental parameter of the Standard Model which describes matter and its interactions. The precise measurement of the W boson mass allows one to test the model and to impose new constraints on some of its parameters like the mass of the Higgs boson. Since 1996, the center of mass energy of the LEP collider at CERN has been gradually increased and is high enough to produce W boson pairs in $e^+e^-$ collisions and to measure its mass, first by the threshold method, then by direct reconstruction of the decay products of the $W^+W^-$ pairs. The data recorded with the ALEPH detector in 1996 and 1997, corresponding to an integrated luminosity of 10.65 and 57.01 $pb^{-1}$, at a mean energy of 172.1 and of 182.7 GeV, respectively, are used to determine the W mass by direct reconstruction in the two relevant channels of decay of the pair : the hadronic and the semi-leptonic channels. The data accumulated lead to the following measurement in accordance with previous measurements and reducing the errors with respect to the threshold method by more than 200 MeV. The W boson mass obtained seems to indicate a low mass for the Higgs boson.