Two-Loop Radiative Corrections to the Lightest Higgs Boson Mass in the Minimal Supersymmetric Model

In the minimal supersymmetric model (MSSM), the upper limit of the lightest Higgs boson mass, $\mhl$, depends strongly on the top quark mass, $m_t$. We have computed the dominant two-loop radiative corrections to this upper limit of $\mhl$ in order to eliminate large uncertainties due to QCD and $m_t^6$ corrections. It is shown that the QCD corrections significantly reduce the one-loop corrections. As a result, the SUSY parameter space accessible to LEP experiments is significantly increased.