Effective lagrangian for the $\overline{t}bH^{+}$ interaction in the MSSM and charged Higgs phenomenology

In the framework of a 2HDM effective lagrangian for the MSSM, we analyse some phenomenological implications of the 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 \tb 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->tb decay rates, and we derive simple formulae embodying all the dominant contributions for large \tb values. We also compute the corresponding branching ratios. We show, as an example, the effect of these new results on the region of the \mH-\tb plane excluded by the D0 searches for a supersymmetric charged Higgs boson, as a function of the MSSM parameter space.