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Scalar flavour-changing neutral currents in the large-tan(beta) limit

We analyse scalar flavour-changing neutral currents of down-type quarks in models with two Higgs doublets, coupled separately to up- and down-type quarks, in the limit where the ratio of the two expectation values ($\tan β= v_u/v_d$) is large. We clarify the origin of this phenomenon, both in $ΔF=1$ and $ΔF=2$ processes, analysing differences and analogies between supersymmetric and non-supersymmetric models. We confirm previous findings of a sizeable enhancement at large $\tanβ$ of specific $ΔF=1$ and $ΔF=2$ 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 $B_{s,d} \to τ^+ τ^-$ and $B_{s,d} \to μ^+ μ^-$ in probing this scenario.