Search for standard and non-minimal Higgs boson at LEP2 with the L3 detector

This thesis worked out in the frame of L3 collaboration - Higgs working group refers to the search for a Higgs signature, for center-of-mass energies between 192 and 209 GeV, one of the main goals of LEP2. It consists of a contribution to the analyses looking for the Standard Model Higgs boson, especially in the so-called 'two jets plus missing energy' channel. The final state of this channel, denoted H_νν-bar, is characterized by the production of a pair of b quarks, from the decay of the Higgs particle, and a neutrino pair from that of the Z particle, for the prevalent Higgs-Strahlung process. The lower observed mass limit, obtained with the H_νν-bar channel alone, is 96 GeV at a 95% confidence level. In addition, searches for neutral scalar particle production are presented, in the context of general two-Higgs-doublet models of type II, by means of a so-called 'flavor independent' analysis. Searches for 'invisible' Higgs bosons are presented as well, with the Z boson decaying into a pair of fermions while the Higgs boson decays into undetectable particles. These results permit constraining the parameters of the minimal non-universal supersymmetric models (without gaugino mass unification). (author)