HIGGS-BOSON MASS BOUNDS SEPARATE MODELS OF ELECTROWEAK SYMMETRY-BREAKING

Vacuum stability and metastability imply lower Limits on the mass of t he Higgs boson in the standard model (SM). In contrast, we present an improved calculation of the lightest Higgs boson mass in supersymmetri c (SUSY) models, by summing to all orders in perturbation theory the l eading and next-to-leading logarithms with a renormalization group equ ation technique, and by including finite two-loop QCD corrections. We believe our result to be the most accurate available in the literature . The mass calculation leads to an upper bound on the Higgs boson mass when the SUSY-breaking scale is sensibly restricted to less than or s imilar to 1 TeV. In particular, our improvements to the SUSY Higgs bos on mass calculation lower the minimal SUSY standard model (MSSM) upper limit by about 10 GeV. We study the possibility that these SM and MSS M bounds do not overlap, in which case a single Higgs boson mass measu rement will distinguish between the two models. We find the following. (i) A gap emerges between the SM Higgs boson acid the lightest MSSM H iggs boson at similar to 120 GeV for m(t) similar to 175 GeV and alpha (s)(M(Z)(2))=0.118, and for m(t) similar to 180 GeV and more generous values similar to(0.130) of alpha(s), and between the SM and the minim al plus singlet SUSY model [(M+1)SSM] Higgs bosons if the independent scalar self-coupling of the latter is perturbatively small or if the t an beta parameter is small; these mass gaps widen with increasing m(t) ; (ii) the mass gap emerges with m(t) 10 GeV lighter if only vacuum st ability and not metastability is imposed; (iii) restricting tan beta t o the values (similar to 1-2) preferred in supersymmetric grand unifie d theories, the lightest MSSM Higgs boson mass upper bound is reduced by at least 10 GeV (which implies no overlap between the SM and the MS SM bounds at even smaller values of m(t)); for m(t) similar to 175 GeV , the bound is m(h) less than or similar to 110 GeV. Thus, a measureme nt of the first Higgs boson mass will serve to exclude either the MSSM or (M+1)SSM Higgs sectors or the SM Higgs sector. In addition, we dis cuss the upper bound on the lightest Higgs boson mass in SUSY models w ith an extended Higgs sector. Finally, we comment on the discovery pot ential for the lightest Higgs bosons in these models.