Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement
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2026
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| author | Gärtner, Lorenz Krug, Nikolai Kuhr, Thomas Schmidt, Michael A. Stefkova, Slavomira Yabsley, Bruce |
| author_facet | Gärtner, Lorenz Krug, Nikolai Kuhr, Thomas Schmidt, Michael A. Stefkova, Slavomira Yabsley, Bruce |
| contents | Belle II measurement of the branching fraction for $B^{+} \to K^{+} ν\barν$ shows a $2.7σ$ excess over the Standard Model prediction and motivates new-physics explanations such as axion-like particles, Higgs-like scalars, or beyond Standard Model gauge bosons. A two-body decay \BKX 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 $m_X = 2.1^{+0.2}_{-0.1}$ GeV and the product $\mathcal{B}(B^+\to K^+ X) \cdot \mathcal{P}_{X,\rm inv} = 9.2^{+1.8}_{-3.4} \cdot 10^{-6}$, where $\mathcal{P}_{X,\rm inv}$ is the probability that $X$ (and its decay products) are undetected. Bayes 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 $3.0σ$. A light invisible resonance plus the Standard Model therefore provides a compelling description of the Belle II data. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2602_09666 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement Gärtner, Lorenz Krug, Nikolai Kuhr, Thomas Schmidt, Michael A. Stefkova, Slavomira Yabsley, Bruce High Energy Physics - Phenomenology High Energy Physics - Experiment Belle II measurement of the branching fraction for $B^{+} \to K^{+} ν\barν$ shows a $2.7σ$ excess over the Standard Model prediction and motivates new-physics explanations such as axion-like particles, Higgs-like scalars, or beyond Standard Model gauge bosons. A two-body decay \BKX 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 $m_X = 2.1^{+0.2}_{-0.1}$ GeV and the product $\mathcal{B}(B^+\to K^+ X) \cdot \mathcal{P}_{X,\rm inv} = 9.2^{+1.8}_{-3.4} \cdot 10^{-6}$, where $\mathcal{P}_{X,\rm inv}$ is the probability that $X$ (and its decay products) are undetected. Bayes 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 $3.0σ$. A light invisible resonance plus the Standard Model therefore provides a compelling description of the Belle II data. |
| title | Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement |
| topic | High Energy Physics - Phenomenology High Energy Physics - Experiment |
| url | https://arxiv.org/abs/2602.09666 |