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

Using an effective potential approach, we compute two-loop radiative corrections 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 these corrections can increase Mho by as much as 5 GeV; assuming a SUSY scale of 1 TeV, the upper bound on the Higgs boson mass is M(h)0 approximate to 129 +/- 5 GeV for the top quark pole mass 175 +/- 5 GeV. We also derive an analytical approximation formula for M(h)0 which approximates our numerical results to a precision of less than or similar to 0.5 GeV for most of the parameter space and suitable to be further improved by including renormalization group resummation of leading and next-to-leading order logarithmic terms. Our final compact formula admits a dear physical interpretation: radiative corrections up to the two-loop level can be well approximated by a one-loop expression with parameters evaluated at the appropriate scales, plus a smaller finite two-loop threshold correction term. (C) 2000 Elsevier Science B.V. All rights reserved.