Estudio del canal electronico de desintegracion del lepton tau en LEP con el detector DELPHI

Abstract

This thesis presents a study of the decays $\tau\rightarrow e\nu\overline{\nu}$ identified after a selection of $\tau^+\tau^-$ events produced in $e^+e^-$ collisions at LEP. These collisions, predominantly mediated by a $Z^0$ boson, offer a good laboratory for the study of weak current parameters. Moreover, the decays of the tau lepton, which proceed via a $W$ boson, increase the possibilities of our study. The experimental observables, which imply an indirect knowledge of the coupling constants, include the polarization asymmetry and the forward-backward polarization asymmetry. The branching ratio of the electronic decay of the $\tau$ lepton ($18.24\pm0.28\pm0.32)\%$) derives a measurement of the strong coupling constant $\alpha_s$ at the scale of the $Z^0$ boson mass ($0.118^{+0.004}_{-0.006}\pm0.006$) and a test of lepton universality in weak currents ($\frac{g_{\tau}}{g_{\mu}}=1.000\pm0.014$). The spectra of momenta and electromagnetic energy depositions used as estimates of the energy of the electron allow to measure the polarization asymmetries ($P_{\tau}=-0.16\pm0.09\pm0.05$ and $A^{FB}_{pol}=-0.19\pm0.09\pm0.01$) giving subsequent values for the effective $sin^2\theta_W$ ($0.230\pm0.013$ and $0.218\pm0.015$).