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

Abstract

This thesis presents a study of the decays $\tau \to e\nu \overline{\nu }$ 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 ($18.24\pm 0.28\pm 0.32)\%$) derives a measurement of the strong coupling constant ${\alpha }_s$ at the scale of the $Z^0$ boson mass ($0.118_{-0.006}^{+0.004}\pm 0.006$) and a test of lepton universality in weak currents ($\frac{g_{\tau }}{g_{\mu }}=1.000\pm 0.014$). The spectra of momenta and electromagnetic energy depositions used as estimates of the energy of the electron allow to measure the polarization asymmetries ($P_{\tau }=-0.16\pm 0.09\pm 0.05$ and $A_{pol}^{FB}=-0.19\pm 0.09\pm 0.01$) giving subsequent values for the effective $si{n}^2{\theta }_W$ ($0.230\pm 0.013$ and $0.218\pm 0.015$).

Doctoral thesis in spanish language. 172 pages including the index, the introduction and the bibliography. A postscript version is available at http://evalu0.ific.uv.es/lozano/phd_e.html