A two-loop contribution to B_s --> mu^+ mu^- at large tan(beta) in the MSSM

We consider a two-loop contribution of Higgs-mediated penguin diagram to $B(B_s \to \mu^+\mu^-)$ at large $\tan\beta$ in the MSSM, motivated by a recently proposed two-loop magnetic penguin diagrams by Chen and Geng [1]. Typically the two-loop diagram is a $\alpha_s$ correction to the one-loop contributions. However the new contribution can dominate one-loop contributions in a region of parameter space where $\mu$, $A_t$ and $A_b$ are very large and scalar top, scalar bottom and charged Higgs are light. Both constructive and destructive interferences are possible. The total branching ratio can be drastically changed from the one-loop result.