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 ($\tan \beta = v_u/v_d$) is large. We clarify the origin of this phenomenon, both in $\Delta F=1$ and $\Delta F=2$ processes, analysing differences and analogies between supersymmetric and non-supersymmetric models. We confirm previous findings of a sizeable enhancement at large $\tan\beta$ of specific $\Delta F=1$ and $\Delta F=2$ amplitudes in the MSSM and, in these cases, we discuss how large-$\tan\beta$ corrections can be controlled beyond lowest order. Finally, we emphasize the unique role of the rare processes $B_{s,d} \to \tau^+ \tau^-$ and $B_{s,d} \to \mu^+ \mu^-$ in probing this scenario.