Squark Contributions to Higgs Boson Masses in the Next--to--Minimal Supersymmetric Standard Model

Within the context of an effective potential formalism we calculate the contribution to Higgs boson masses in the next--to--minimal supersymmetric standard model from squark loops. We then supplement a previously performed renormalisation group analysis of the Higgs sector of this model with these results in order to determine the shift in the bound on the lightest CP-even Higgs boson mass as a result of squark effects. The improved bound on the lightest neutral CP-even Higgs boson mass, including squark contributions, is $m_h \leq 146 \ \ (139, 149)$ GeV for $m_t = 90 \ \ (140, 190)$ GeV. For $m_t = 190$ GeV squark effects contribute 23 GeV to the bound, with smaller contributions for small $m_t$ values.