The Belle II measurement of the branching fraction for $B^{+}\to K^{+}\nu \overline{\nu }$ shows a $2.7\sigma$ excess over the Standard Model prediction and motivates new-physics explanations such as axionlike particles, Higgs-like scalars, or beyond Standard Model gauge bosons. A two-body decay $B^{+}\to K^{+}X$ with an invisible $X$ provides a natural candidate explanation. This work provides a comprehensive test of this hypothesis using Belle II’s public model-agnostic likelihood. Posterior distributions are derived for the resonance mass $m_X$ and the branching fraction, and a modified frequentist upper-limit mass scan is performed. The data favor a resonance with mass $m_X={2.1}_{-0.1}^{+0.2}\mathrm{GeV}$ and the product $\mathcal{B}(B^{+}\to K^{+}X)·{\mathcal{P}}_{X,\mathrm{inv}}={9.2}_{-3.4}^{+1.8}\times {10}^{-6}$, where ${\mathcal{P}}_{X,\mathrm{inv}}$ is the probability that $X$ (and its decay products) are undetected. Bayes’s factors indicate a very strong preference for the Standard Model plus resonance over the Standard Model-only hypothesis. A frequentist likelihood-ratio test favors the Standard Model plus resonance hypothesis by $3.0\sigma$. A light invisible resonance plus the Standard Model therefore provides a compelling description of the Belle II data.

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• Electroweak interaction
• Flavor changing neutral currents
• Particle dark matter
• Particle interactions
• Phenomenology
• Signatures with missing energy
• Axion-like particles
• Axions
• Lepton colliders
• Particle data analysis
• Probability theory