Upper limit on mh in the MSSM and M-SUGRA vs. prospective reach of LEP

The upper limit on the lightest CP-even Higgs boson mass, mh, is analyzed within the MSSM as a function of tan β for fixed mt and MSUSY. The impact of recent diagrammatic two-loop results on this limit is investigated. We compare the MSSM theoretical upper bound on mh with the lower bound obtained from experimental searches at LEP. We estimate that with the LEP data taken until the end of 1999, the region mh < 108.2 GeV can be excluded at the 95% confidence level. This corresponds to an excluded region 0.6 <or∼ tan β <or∼ 1.9 within the MSSM for mt=174.3 GeV and MSUSY ≤ 1 TeV. The final exclusion sensitivity after the end of LEP, in the year 2000, is also briefly discussed. Finally, we determine the upper limit on mh within the minimal supergravity (M-SUGRA) scenario up to the two-loop level, consistent with radiative electroweak symmetry breaking. We find an upper bound of mh∼127 GeV for mt=174.3 GeV in this scenario, which is slightly below the bound in the unconstrained MSSM. (orig.)