Searches for the Higgs boson in Minimal Supersymmetric CP-conserving and CP-violating Standard Model scenarios at LEP

It is important to study extended models containing more than one physical Higgs boson in the spectrum. In particular, Two Higgs Doublet Models (2HDMs) are attractive extensions of the SM, predicting new phenomena with the fewest new parameters. The Higgs sector in the Minimal Supersymmetric extension of the SM (MSSM) is a 2HDM itself. The neutral Higgs searches performed at LEP are showing no evidence of the presence of a signal and have therefore been interpreted in the context of 2HDMs. Depending on the model considered exclusion of large regions of the parameter space can be obtained, but the existence of the lightest Higgs boson with masses lower than 90 GeV is not ruled out in all models by LEP. In the MSSM at least one of the neutral Higgs bosons is predicted to have its mass close to the electroweak energy scale; when radiative corrections are included, this mass should be less than about 140 GeV. This prediction provides a strong motivation for searches at present and future colliders.