Measurements of four fermion cross-sections at LEP

The production of four fermions in e+e− collisions at LEP allows the verification of the Standard Model of the Electroweak Interactions in the Charged and Neutral Current Sectors. Among the four-fermion final states, the highest purity and the clearest four-fermion events are characterized by the presence of leptons in the final state. The identification of such final states in the full data sample collected by the L3 experiment at LEP in the years from 1997 through 2000 is described. The total amount of data analyzed in this thesis corresponds to the total integrated luminosity 675.5 pb−1. ^ This thesis presents the results of the selection of the Z boson pair production together with the measurement of the cross-section for leptonic four-fermion final states, and the first measurement of cross-sections in the four-lepton and two-lepton and missing energy channels of the Zg* production with the L3 detector. The cross-section average over the whole data sample was found to be: s&parl0;e+e-→Zg *→l+l-l′ +l′-&parr0; =0.087+0.030-0.026pb, s&parl0;e+e-→Zg *→l+l-n n&d1;&parr0; =0.062+0.032-0.027p b. This thesis also presents the results of the selection of the single W production and the cross-section measurements of the semileptonic decays of the W boson.^ All measurements are consistent with the Standard Model expectations. ^