New physics upper bound on the branching ratio of B_s --> l+ l-

We consider the most general new physics effective Lagrangian for b --> s l+ l-. We derive the upper limit on the branching ratio for the processes B_s --> l+ l- where l=e, mu, subject to the current experimental bounds on related processes, B --> K l+ l- and B --> K* l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B --> (K,K*) l+ l- constrain B(B_s --> l+ l-) to be of the same order of magnitude as their respective Standard Model predictions. On the other hand, if the new physics interactions are of scalar/pseudo-scalar form, B --> (K,K*) l+ l- rates do not impose any constraint on B_s --> l+ l- and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B(B_s --> l+ l-) to be > 10^{-8} then the new physics giving rise to these decays has to be of the scalar/pseudo-scalar form.