Is Dark Matter the origin of the $B\to K \nu\bar\nu$ excess at Belle II?

Abstract

We present two models of dark matter (DM) that can provide a natural explanation of the excess of $B^+\to K^+ +\,\text{invisible}$ events with respect to the Standard Model (SM) prediction for $B^+\to K^+ \nu\bar\nu$, which has been reported by the Belle II collaboration. Interactions between the dark and the visible sector are mediated by an axion-like particle (ALP) in one case, by the kinetic mixing between a dark photon and the SM photon in the second case. Both models encompass a light fermion singlet as the DM candidate and can account for the observed DM relic abundance through, respectively, the freeze-in and the freeze-out production mechanism, while simultaneously explaining the Belle II excess.