Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement

[2602.09666] Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement Skip to main content Search Submit Donate Log in High Energy Physics - Phenomenology arXiv:2602.09666 (hep-ph) [Submitted on 10 Feb 2026 ( v1 ), last revised 28 Sep 2026 (this version, v3)] Title: Constraints on invisible $B^{+}\to K^{+} X$ decays from the Belle II $B^{+} \to K^{+} ν\barν$ measurement Authors: Lorenz Gärtner , Nikolai Krug , Thomas Kuhr , Michael A. Schmidt , Slavomira Stefkova , Bruce Yabsley View PDF HTML (experimental) Abstract: The Belle II measurement of the branching fraction for $B^+\to K^+\nu\bar{\nu}$ 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 $m_X = 2.1^{+0.2}_{-0.1}\,\mathrm{GeV}$ and the product $\mathcal{B}(B^+\to K^+X)\cdot\mathcal{P}_{X,\mathrm{inv}} = 9.2^{+1.8}_{-3.4}\cdot 10^{-6}$, 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. Subjects: High Energy Physics - Phenomenology (hep-ph) ; High Energy Physics - Experiment (hep-ex) Cite as: arXiv:2602.09666 [hep-ph] (or arXiv:2602.09666v3 [hep-ph] for this version) https://doi.org/10.48550/arXiv.2602.09666 Focus to learn more arXiv-issued DOI via DataCite Journal reference: Phys. Rev. D 114, 032003 (2026) Related DOI : https://doi.org/10.1103/ggtk-gkx4 Focus to learn more DOI(s) linking to related resources Submission history From: Lorenz Gärtner [ view email ] [v1] Tue, 10 Feb 2026 11:21:19 UTC (936 KB) [v2] Thu, 27 Aug 2026 08:52:14 UTC (595 KB) [v3] Mon, 28 Sep 2026 08:48:51 UTC (944 KB) Full-text links: Access Paper: View PDF HTML (experimental) TeX Source view license Current browse context: hep-ph < prev | next > new | recent | 2026-02 Change to browse by: hep-ex References & Citations INSPIRE HEP SciX Google Scholar Semantic Scholar export BibTeX citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer ( What is the Explorer? ) Connected Papers Toggle Connected Papers ( What is Connected Papers? ) Litmaps Toggle Litmaps ( What is Litmaps? ) scite.ai Toggle scite Smart Citations ( What are Smart Citations? ) Code, Data, Media Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv ( What is alphaXiv? ) Links to Code Toggle CatalyzeX Code Finder for Papers ( What is CatalyzeX? ) DagsHub Toggle DagsHub ( What is DagsHub? ) GotitPub Toggle Gotit.pub ( What is GotitPub? ) Huggingface Toggle Hugging Face ( What is Huggingface? ) ScienceCast Toggle ScienceCast ( What is ScienceCast? ) Demos Demos Replicate Toggle Replicate ( What is Replicate? ) Spaces Toggle Hugging Face Spaces ( What is Spaces? ) Spaces Toggle TXYZ.AI ( What is TXYZ.AI? ) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower ( What are Influence Flowers? ) Core recommender toggle CORE Recommender ( What is CORE? ) IArxiv recommender toggle IArxiv Recommender ( What is IArxiv? ) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them. Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs . Which authors of this paper are endorsers? | Disable MathJax ( What is MathJax? ) We gratefully acknowledge support from our major funders , member institutions , and all contributors. About · Help · Contact · Subscribe · Copyright · Privacy · Accessibility · Operational Status (opens in new tab)

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