In the framework of a 2HDM effective lagrangian for the MSSM, we analyse important phenomenological aspects associated with quantum soft SUSY-breaking effects that modify the relation between the bottom mass and the bottom Yukawa coupling. We derive a resummation of the dominant supersymmetric corrections for large values of tanβ to all orders in perturbation theory. With the help of the operator product expansion we also perform the resummation of the leading and next-to-leading logarithms of the standard QCD corrections. We use these resummation procedures to compute the radiative corrections to the t → b H + , H + → tb decay rates. In the large tanβ regime, we derive simple formulae embodying all the dominant contributions to these decay rates and we compute the corresponding branching ratios. We show, as an example, the effect of these new results on determining the region of the M H + -tanβ plane excluded by the Tevatron searches for a supersymmetric charged Higgs boson in top quark decays, as a function of the MSSM parameter space.

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.

The condition of unification of gauge couplings in the minimal supersymmetric standard model provides successful predictions for the weak mixing angle as a function of the strong gauge coupling and the supersymmetric threshold scale. In addition, in some scenarios, e.g. in the minimal SO(10) model, the tau lepton and the bottom and top quark Yukawa couplings unify at the grand unification scale. The condition of Yukawa unification leads naturally to large values of tan β, implying a proper top quark-bottom quark mass hierarchy. In this work, we investigate the feasibility of unification of the Yukawa couplings, in the framework of the minimal supersymmetric standard model with (assumed) universal mass parameters at the unification scale and with radiative breaking of the electroweak symmetry. We show that strong correlations between the parameters µ 0 , M 1/2 and δ = B 0 − (6r/7)A 0 appear within this scheme, where r is the ratio of the top quark Yukawa coupling to its infrared fixed point value. These correlations have relevant implications for the sparticle spectrum, which presents several characteristic features. In addition, we show that due to large corrections to the running bottom quark mass induced through the supersymmetry breaking sector of the theory, the predicted top quark mass and tan β values are significantly lower than those previously estimated in the literature. Recently, it has been observed that for the phenomenologically allowed values of the bottom quark mass and moderate values of tan β < 10, large values of the top quark Yukawa coupling are needed in order to contravene the strong gauge coupling renormalization of the bottom Yukawa coupling [7]-[9]. In general, for large enough values of the top quark Yukawa coupling at the grand unification scale, the low energy Yukawa coupling is strongly focussed to a quasi infrared fixed point [10]-[11]. In the minimal supersymmetric standard model, the quasi infared fixed point predictions for the physical top quark mass M t are given by M t ≃ A sin β, with A ≃ 190 − 210 GeV for the strong gauge coupling α 3 (M Z ) = 0.11−0.13. It has been recently shown that for the values of the strong gauge coupling consistent with the condition of gauge coupling unification, with reasonable threshold corrections at the grand unification and supersymmetry breaking scales, the top quark mass should be within 10% of its quasi infrared fixed point values if the condition of bottom-tau Yukawa unification is required [12]. A more predictive scheme is obtained in the framework of the minimal SO(10) unification. In this case top-bottom quark Yukawa unification is also required, implying that, for a given value of the bottom quark mass and the strong gauge coupling value [13], not only the top quark mass but also the value of tan β may be determined. Remarkably, large values of tan β ≥ 40 are obtained in this case, leading to a proper bottom-top mass hierarchy [14]-[16]. For these large values of tan β, the bottom quark Yukawa coupling itself plays a relevant role in the ...

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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.