Two-loop radiative corrections to the upper limit of 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, mh0, depends strongly on the top quark mass, mt. We have computed the dominant two-loop radiative corrections to this upper limit of mh0 in order to eliminate large uncertainties due to QCD and mt6 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. (orig.)