Scalar flavour - changing neutral currents in the large - tan β limit

We analyse scalar flavour-changing neutral currents of down-type quarks in models with two Higgs doublets, coupled separately to upand down-type quarks, in the limit where the ratio of the two expectation values (tanβ = vu/vd) is large. We clarify the origin of this phenomenon, both in ∆F = 1 and ∆F = 2 processes, analysing differences and analogies between supersymmetric and non-supersymmetric models. We confirm previous findings of a sizeable enhancement at large tanβ of specific ∆F = 1 and ∆F = 2 amplitudes in the MSSM and, in these cases, we discuss how large-tanβ corrections can be controlled beyond lowest order. Finally, we emphasize the unique role of the rare processes Bs,d → ττ and Bs,d → μμ in probing this scenario.