Mesure de la masse du boson W au seuil

The mass of the W boson is a fundamental parameter of the Standard Model of electroweak interactions. Its direct measurement is one of the major goals of the second phase (LEP2) of the experimental program for the LEP collider at CERN: this phase started in June 1996, when the LEP centre-of-mass energy was raised to 161 GeV to allow the production of pairs of W bosons. Close to threshold, the W mass can be determined directly by the measurement of the W pair cross section. This thesis describes the measurement of the cross section at threshold and subsequent extraction of the W mass, using data collected during summer 1996 with the ALEPH detector. In chapter 1, the importance of the measurement of the W mass as a test of the Standard Model and as an indirect measurement of the Higgs boson mass is reviewed. In chapter 2, the methods used for the measurement of the W mass at LEP2 and elsewhere are summarized; in particular, the general principles of the threshold method and the choice of the optimal energy are discussed. In chapter 3, the ALEPH detector and some of the tools used for the analysis of experimental data are described. At LEP2, W pair events can be classified into three channels, leptonic, semileptonic and hadronic, according to the decay modes of the two W bosons. Each channel has characteristic features and requires a specific analysis procedure. Chapters 4, 5 and 6 describe the details of the measurement of the cross section for the three channels. In chapter 7, the inclusive WW cross section at threshold is derived and the W mass is determined from it. The result obtained by ALEPH is then combined to those of the other three LEP Collaborations, and the LEP average is compared to previous measurements of the W mass.