Complete two-loop dominant corrections to the mass of the lightest CP-evenHiggs bosom in the minimal supersymmetric standard model

Using an effective potential approach, we compute two-loop radiative correc tions to the MSSM lightest CP-even Higgs boson mass M(h)0 to O(alpha(t)(2)) for arbitrary left-right top-squark mixing and tan beta. We find that thes e corrections can increase Mho by as much as 5 GeV; assuming a SUSY scale o f 1 TeV, the upper bound on the Higgs boson mass is M(h)0 approximate to 12 9 +/- 5 GeV for the top quark pole mass 175 +/- 5 GeV. We also derive an an alytical approximation formula for M(h)0 which approximates our numerical r esults to a precision of less than or similar to 0.5 GeV for most of the pa rameter space and suitable to be further improved by including renormalizat ion group resummation of leading and next-to-leading order logarithmic term s. Our final compact formula admits a dear physical interpretation: radiati ve corrections up to the two-loop level can be well approximated by a one-l oop expression with parameters evaluated at the appropriate scales, plus a smaller finite two-loop threshold correction term. (C) 2000 Elsevier Scienc e B.V. All rights reserved.