$B_{s}^0\to K^0\overline{K}{}^0$ beyond the Standard Model

Within the Standard Model, the branching fraction of the rare decay $B_s\to K^0\bar K^0$ is related to other decay rates and CP asymmetries through the approximate $SU(3)$ flavor symmetry of the strong interactions and the heavy-quark limit. Three such relations were shown to be violated at a level of about $3\sigma$ each. By means of a systematic search for new-physics explanations of these puzzles, we find that possible solutions are highly fine-tuned and either excluded by other data or rather implausible. The tight correlation between $B_s\to K\bar K$ and $B\to\pi K$ decays, which is maintained even in the presence of flavor-specific new-physics operators, plays a central role in our analysis.