Measurement of R at the $Z_{0}$ peak using neural networks

In this thesis a determination of the quotient of th hadronic and leptoni c widths of the Z_0 is presented, through the measurement of the ratio of cross sections sigma (e+e- --> hadrons)/sigma (e+e- --> leptons) where leptons are The data used were collected by the ALEPH detector at CERN's LEP accelerator during the 1991 running period. muons or taus. The classification of the different Z_0 decay modes is based on the Neural Network technique. An algorithm of "learn and grow" has been developed to train the Neural Network.