Radiative Corrections to the Neutral Higgs Spectrum in Supersymmetry with a Gauge Singlet

The radiative corrections to the $3\times3$scalar and the $2\times2$ pseudoscalar neutral Higgs boson massmatrices are calculated in the supersymmetric extension of the standard model including a gauge singlet superfield in the effective potential approach. The full $t$ and $b$ quark/squark contributions including the nonlogarithmic terms are taken into account, which are seen to affect the result significantly. An analytic formula for the upper bound on the mass of the lightest neutral Higgs scalar including these corrections is given. (Figure available upon request)