Bs to K0 and K0bar, Three Puzzles

Several recent experimental results by ATLAS, CMS, LHCb and Belle II provide new tests of the Standard Model and probe new physics. I will focus on the branching fraction of the rare decay $B_s\to K^0\bar K^0$. Within the Standard Model, it 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 are 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.