Measurement of the tau lepton polarization in the ALEPH experiment using the tau direction

The work presented in this thesis has been performed within the ALEPH experiment located on the LEP electron-positron collider. It is a part of the precision measurements with the goal of determining the fundamental parameters of the standard model of the electroweak interactions and testing its internal coherence. The measurement of the τ polarization allows a precise determination of the electroweak mixing angle by measuring the coupling constants of the Z0 boson to the τ and to the electron and a test of the electron-tau universality predicted by the standard model. The τ direction is used for the first time in a polarization measurement in this analysis. The use of the τ direction increases the sensibility of the polarization measurement in the hadronic τ decay channels ρ and a1. Both decay modes of the a1, a1 -> 3τ+- and a1 -> τ+-2τ0, were considered. The τ channel was analyzed as well. Data recorded by the ALEPH detector between 1990 and 1994 were analyzed, corresponding to 136 000 τ+ τ- events. The value of the measured τ polarization is Aeτ = - 0.1530 ± 0.0047stat ± 0.0030syst, yielding the value of the sinus squared of the electroweak mixing angle sin2 θWeff = 0.23076 ± 0.00070. It is one of the most precise measurements among those done at LEP. (author). 68 refs., 103 figs., 41 tabs., 1 append