Effective Lagrangian for the (t)over-bar-bH+ interaction in the MSSM and charged Higgs phenomenology

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 tan beta 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 --> bH(+), H+ --> t (b) over bar decay rates. In the large tan beta 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 MH+-tan beta plane excluded by the Tevatron searches for a supersymmetric charged Higgs boson in top quark decays, as a function of the MSSM parameter space. (C) 2000 Elsevier Science B.V. All rights reserved.