Higgs boson mass bounds in three and four generation scenarios - art. no. 017701

In light of recent experimental results, we present updated bounds on the l ightest Higgs boson mass in the standard model (SM) and in the minimal supe rsymmetric extension of the standard model (MSSM). The vacuum stability low er bound on the pure SM Higgs boson mass when the SM is taken to be valid u p the Planck scale lies above the MSSM lightest Higgs boson mass upper boun d for a large amount of SUSY parameter space. If the lightest Higgs boson i s detected with a mass M-H < 134 GeV (150 GeV) for a top quark mass M-top = 172 GeV (179 GeV), it may indicate the existence of a fourth generation of fermions. The region of inconsistency is removed and the MSSM is salvagabl e for such values of M-H if one postulates the existence of a fourth genera tion of leptons and quarks with isodoublet degenerate masses M-L and M-Q su ch that 60 GeV < M-L < 110 GeV and M-Q > 170 GeV. [S0556-2821(99)04513-0].