Revisiting models that enhance $B^{+}\to K^{+}\nu \bar{\nu }$ in light of the new Belle II measurement

Abstract

Belle II has recently reported the new measurement $\mathcal{B}(B^{+}\to K^{+}\nu \bar{\nu })=(2.3\pm 0.7)\times 10^{-5}$ \cite{Belle-II:2023esi} which is two times larger than their previous result (although consistent within errors) and about $2.7\sigma$ higher than the SM prediction. We re-examine new physics scenarios we have discussed previously which can enhance this rate to determine if they can accommodate the higher value reported in the new measurement. We use consistency with existing bounds on $B\to K^{\ast }\nu \bar{\nu }$, $b\to s{\ell }^{+}{\ell }^{-}$, $B\to D^{(\ast )}\ell \bar{\nu }$ and $B_s$ mixing to limit possible explanations for the excess. For the case of LFV neutrino couplings, we find that only two leptoquarks remain viable requiring a large $C_{9^{\prime }}^{\tau \tau }=-C_{10^{\prime }}^{\tau \tau }$. For models with different types of light dark matter particle pairs (scalar, fermion, or vector), the preliminary $q^2$ distribution from Belle II, which shows that the excess appears mostly for bins with $3\le q^2\le 7$ GeV${}^2$ \cite{Belle-II:2023esi}, implies only the vector current operators with scalar or vector dark matter particles with masses in the hundreds of MeV can match the anomaly.

Comments: Correct two typos