Supersymmetric Higgs singlet effects on FCNC observables

Higgs singlet superfields, usually present in extensions of the Minimal Supersymmetric Standard Model (MSSM) which address the $\mu$-problem, such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM) and the Minimal Nonminimal Supersymmetric Standard Model (mnSSM), can have significant contributions to $B$-meson flavour-changing neutral current observables for large values of $\tan \beta \text{gsim}50$. Illustrative results are presented including effects on the $B_s$ and on the rare decay $B_s\to {\mu }^{+}{\mu }^{-}$. In particular, we find that in the NMSSM, the branching ratio for $B_s\to {\mu }^{+}{\mu }^{-}$ can be enhanced or even suppressed with respect to the Standard Model prediction by more than one order of magnitude.