A Unified Dark Matter Explanation for $\boldsymbol{B^+ \!\to K^+ν\barν}$ and the Super-Kamiokande Antineutrino Excess

Abstract

Recent results from Super-Kamiokande and Belle II have revealed intriguing excesses over Standard Model expectations. Super-Kamiokande observes a mild excess of $\barν_e^{}$-like events near $20\,\,\mathrm{MeV}$, while Belle II reports a branching fraction for $B^+ \!\to K^+ν\barν$ that exceeds the Standard Model prediction by approximately $2.7σ$. In this work, we study the simplest UV-complete complex scalar dark matter model with a gauged $\text{U}(1)_{\textsf{L}_μ- \textsf{L}_τ}^{}$ symmetry. We demonstrate that a light dark sector can simultaneously reproduce the observed dark matter relic density and accommodate both excesses within a unified framework.