Model Independent Upper Bound on the Lightest Higgs Boson Mass in Supersymmetric Standard Models

One of the main features of the Minimal Supersymmetric Standard Model (MSSM) is the existence of an absolute tree-level upper bound $m_h$ on the mass of the $CP=+1$ lightest Higgs boson, equal to $m_Z$, that could affect detectability at future colliders. The above bound is spoiled by {\bf radiative corrections} and by an {\bf enlarged Higgs sector}, as {\em e.g.} a gauge singlet. Radiative corrections in the MSSM can push the upper bound up to $115\ GeV$ for $m_t \simlt 150\ GeV$. The presence of an enlarged Higgs sector changes the previous upper bound to one depending on the electroweak scale, $\tan \beta$ and the gauge and Yukawa couplings of the theory. When radiative corrections are included, the allowed region in the $(m_h,m_t)$ plane depends on the scale $\Lambda$ below which the theory remains perturbative. In particular, for models with arbitrary Higgs sectors and couplings saturating the scale $\Lambda=10^{16}\ GeV$ we find $m_h \simlt 155\ GeV$ and $m_t \simlt 190\ GeV$.