Revisiting Bs→μ+μ- in the two-Higgs doublet models with Z2 symmetry

We revisit the rare leptonic decay Bs → μ+μ− in the two-Higgs doublet models (2HDMs) with a softly broken Z2 symmetry, namely type-I, type-II, type-X and type-Y 2HDMs. We have derived the relevant full one-loop Wilson coefficients of the four 2HDMs from the recent calculation in the aligned 2HDM by Li, Lu and Pich, which could be mapped to all the four 2HDMs for both large and small tan β. It is found that a new term associated with the soft Z2 symmetry breaking parameter M can be enhanced by tan2 β in the type-II 2HDM, which has not been considered in the literature. Imposing both theoretical and experimental constraints, we have renewed the bounds on the parameter spaces of the four 2HDMs. Different from our previous paper, however, we find that all the four 2HDMs give sizable and similar contributions to B(Bs → μ+μ−) within the stringently restricted parameter spaces, but very tiny as regards the mass-eigenstate rate asymmetryA ; this makes it unfeasible to discriminate the four types of 2HDM with the correlations between the observables in Bs → μ+μ− decay.