Contribution to the search for the non-minimal standard Higgs boson through the H0 e+e- and H0 μ+μ- channels in the L3 experiment at LEP

This works concerns the search for Higgs boson with the detector L3 which is one of the four experimental facilities at LEP. The preferential coupling of Higgs boson to high mass particles allows the Higgs-strahlung process to be a interesting way to produce Higgs bosons. The high detection capacity for electrons and muons of L3 allows the author to search for Higgs boson through the decay channels Z0 → H ee and Z0 → H μ μ. The first chapter of the thesis is devoted to the description of the detector L3 and especially to its detection performances for leptons. The second chapter concerns the characteristics of the standard Higgs sector. The most probable values for the Higgs boson mass are presented and the effects of the supersymmetric extension of the standard model considered. The four-fermion final state process is presented. The analytical part of the work is presented in the third chapter. It includes the identification of muons and leptons, the sequential selections in two mass domains (2, 30 GeV and 30, 70 GeV). A special analysis of the invisible decay of Higgs boson (through the h ee channel) is finally presented as well as an interpretation of the results in the frame of the MSSM. The end of the thesis is devoted to the search for Higgs boson at LEP2. (N.T.)