When Energy Goes Missing: New Physics in $b\to s\nu \nu$ with Sterile Neutrinos

Belle II recently reported the first measurement of $B^{+}\to K^{+}+\mathrm{i}\mathrm{n}\mathrm{v}$, which is $2.8\sigma$ above the Standard Model prediction. We explore the available parameter space of new physics within Standard Model effective field theory extended by sterile neutrinos ($\nu$SMEFT) and provide predictions for the other $B\to K^{(\star )}+\mathrm{i}\mathrm{n}\mathrm{v}$ decay modes and invisible $B_s$ decays. We also briefly comment on charged current decays $B\to D^{(\star )}\mathrm{\ell }\bar{\nu }$ and possible ultraviolet completions of the relevant $\nu$SMEFT operators.