Measurement of branching ratio of $B \rightarrow X_{s} \nu \bar{\nu}$ in Belle II experiment

Abstract We report the first search for the flavor-changing neutral-current decays $B \rightarrow X_{s} \nu\bar{\nu}$, where $X_{s}$ is a hadronic system with strangeness equal to 1, in data collected with the Belle~II detector at the SuperKEKB asymmetric-energy $e^+e^-$ collider. The data sample corresponds to an integrated luminosity of $365~\textrm{fb}^{-1}$ collected at the $\Upsilon(4S)$ resonance and $43~\textrm{fb}^{-1}$ collected at a center-of-mass energy $60~\textrm{MeV}$ below resonance for estimation of $e^+e^-\to q\bar{q}$ continuum background. One of the $B$ mesons from the $\Upsilon(4S) \to B\bar{B}$ decay is fully reconstructed in a hadronic decay mode. The $B \to X_s \nu\bar{\nu}$ decay is reconstructed with a sum-of-exclusives approach that uses 30 $X_s$ decay modes. This approach provides high sensitivity to the inclusive decay, despite the presence of two undetected neutrinos. The search is performed in three regions of the $X_{s}$ mass, chosen to separate contributions from prominent resonances. We do not observe a significant signal and set upper limits at 90% confidence level on the partial branching fractions for the regions $0.0 < M_{X_{s}} < 0.6~\textrm{GeV}/c^{2}$, $0.6 < M_{X_{s}} < 1.0~\textrm{GeV}/c^{2}$, and $1.0~\textrm{GeV}/c^{2} < M_{X_{s}}$ of $2.2 \times 10^{-5}$, $9.5 \times 10^{-5}$, and $31.2 \times 10^{-5}$, respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, $B(B \to X_s \nu\bar{\nu}) < 3.2 \times 10^{-4}$.
Tags Run 1
Working group Radiative and Electroweak Penguin
Principal authors @  Akimasa Ishikawa, Youngjoon Kwon, Junewoo Park, Yutaka Ushiroda
Reference Phys. Rev. Lett. 136, 231801 (2026), DOI: 10.1103/kf73-hw61
Document BELLE2-PUB-PH-2025-005
Links arXiv:2511.10980, Inspire, PRL
Bibtex 
@article{Belle-II:2025bho,
    author = "Abumusabh, M. and others",
    collaboration = "Belle-II",
    title = "{First search for $B \rightarrow X_{s} ν\barν$ decays}",
    eprint = "2511.10980",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2025-025, KEK Preprint 2025-27",
    month = "11",
    year = "2025"
}