A light dimuon resonance in $B$ decays?

The observed deviations from the Standard Model in several $b \to s\mu\mu$ processes can be explained in terms of a new vector boson produced on-shell in B meson decays. A mass of 2.5-3 GeV and a total width of 10-20% allow to hide the associated dimuon bump in the poorly known charmonium region, and the large invisible decay width can be interpreted in terms of Dark Matter. This proposal predicts a contribution to the muon anomalous magnetic moment, that could explain the long-standing tension with the Standard Model. It also predicts sizeable invisible $B$ decays and a peculiar $q^2$-dependence of the lepton flavor universality ratios $R_K$ and $R_{K^{*}}$ , that could be tested at the LHCb and Belle-II. This proceeding is based on arXiv:1704.06188, and slightly extends it with comments about Dark Matter