We propose, for the computation of the Higgs mass spectrum and couplings, a renormalization-group improved leading-log approximation, where the renormalization scale is fixed to the top-quark pole mass. For the case m A ∼ M SUSY , our leading-log approximation differs by less than 2 GeV from previous results on the Higgs mass computed using a nearly scale independent renormalization-group improved effective potential up to nextto-leading order. Moreover, for the general case m A < ∼ M SUSY , we provide analytical formulae (including two-loop leading-log corrections) for all the masses and couplings in the Higgs sector. For M SUSY < ∼ 1.5 TeV and arbitrary values of m A , tan β and the stop mixing parameters, they reproduce the numerical renormalization-group improved leading-log result for the Higgs masses with an error of less than 3 GeV. For the Higgs couplings, our analytical formulae reproduce the numerical results equally well. Comparison with other methods is also performed.

We perform an analysis of the behaviour of the electroweak phase transition in the Minimal Supersymmetric Standard Model, in the presence of light stops. We show that, in previously unexplored regions of parameter space, the order parameter v(T c )/T c can become significantly larger than one, for values of the Higgs and supersymmetric particle masses consistent with the present experimental bounds. This implies that baryon number can be efficiently generated at the electroweak phase transition. As a by-product of this study, we present an analysis of the problem of colour breaking minima at zero and finite temperature and we use it to investigate the region of parameter space preferred by the best fit to the present precision electroweak measurement data, in which the left-handed stops are much heavier than the right-handed ones.

We generalize the analytical expressions for the two-loop leading-log neutral Higgs boson masses and mixing angles to the case of general left-and righthanded soft supersymmetry breaking stop and sbottom masses and leftright mixing mass parameters (m Q , m U , m D , A t , A b ). This generalization is essential for the computation of Higgs masses and couplings in the presence of light stops. At high scales we use the minimal supersymmetric standard model effective potential, while at low scales we consider the two-Higgs doublet model (renormalization group improved) effective potential, with general matching conditions at the thresholds where the squarks decouple. We define physical (pole) masses for the top-quark, by including QCD selfenergies, and for the neutral Higgs bosons, by including the leading one-loop electroweak self-energies where the top/stop and bottom/sbottom sectors propagate. For m Q = m U = m D and moderate left-right mixing mass parameters, for which the mass expansion in terms of renormalizable Higgs quartic couplings is reliable, we find excellent agreement with previously obtained results.

We re-examine the lower bound on the mass of the Higgs boson, M H , from Standard Model vacuum stability including next-to-leading-log radiative corrections. This amounts to work with the full one-loop effective potential, V (φ), improved by two-loop RGE, and allows to keep control of the scale invariance of V in a wide range of the φ-field. Our results show that the bound is O (10 GeV ) less stringent than in previous estimates. In addition we perform a detailed comparison between the SM lower bounds on M H and the supersymmetric upper bounds on it. It turns out that depending on the actual value of the top mass, M t , the eventually measured Higgs mass can discard the pure SM, the Minimal Supersymmetric Standard Model or both. September 1994IEM-FT-93/94 hep-ph/9409458 *

We present a simple N = 1 five-dimensional model where the fifth dimension is compactified on the orbifold S 1 /Z 2 . Non-chiral matter lives in the bulk of the fifth dimension (five dimensions) while chiral matter lives on the fixed points of the orbifold (four-dimensional boundaries). The massless sector constitutes the Minimal Supersymmetric Standard Model while the massive modes rearrange in N = 2 supermultiplets. After supersymmetry breaking by the Scherk-Schwarz mechanism the zero modes can be reduced to the non-supersymmetric Standard Model.