Measurement of the semi-leptonic and hadronic branching ratios and mass of Bs0 with the ALEPH detector at LEP

Since the discovery of the b quark in 1977 the physics of the beauty has known an important development. Among the main aspects of this field, one can find the measurement of the branching ratios and the mass of the beauty mesons. This study allows a confrontation between the experimental results on one hand and predictions given by phenomenological models on the other hand. This thesis is dedicated to the study of the Bs0 meson, which contains both a beauty quark and a strange quark. This analysis uses 4 millions hadronic events taken by the ALEPH experiment between 1991 and 1995 on the electron-positron collider LEP located at CERN. From these events the Bs0 meson is searched in two types of decays: semi-leptonic and hadronic decays. This study allows to extract various branching ratios of Bs0. From the fully reconstructed candidates in the hadronic channels a measurement of the Bs0 is carried out. The measurements are in agreement with the predictions of the phenomenological models available at present. This thesis is also on the tracking and identification upgrade of the charged particles in the ALEPH detector, especially the Kalman Filter (tracking part) at a complementary measurement of the specific ionization, dE/dx (identification part). Tests carried out in the field of Bd0 semi-leptonic decays shows that these improvements will be rewarding in the future. (author)