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

TL;DR

This thesis analyzes tau lepton decays into electrons in LEP collisions to measure weak interaction parameters, test lepton universality, and determine the strong coupling constant using experimental observables like polarization asymmetries.

Contribution

It provides new measurements of tau decay properties, polarization asymmetries, and tests of the Standard Model parameters at the Z boson energy scale.

Findings

Branching ratio of tau to electron: 18.24%

Measured strong coupling constant α_s: 0.118

Lepton universality ratio g_τ/g_μ: 1.000

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 $0.218\pm0.015$).