PubDB | BELLE2-PUB-PH-2025-005

Abstract	We search for the flavor-changing neutral-current decays $B\to X_{s}\nu\bar{\nu}$ , where $X_{s}$ is a hadronic system with strangeness equal to 1, in data collected by the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}e^{−}$ collider. The data sample has an integrated luminosity of 365 fb$^{−1}$ collected at the \Upsilon(4S) resonance and 43 fb$^{−1}$ collected with a 60 MeV lower center-of-mass energy for estimation of continuum background. One of the $B$ mesons from the $Upsilon(4S)\to B\bar{B}$ decay is fully reconstructed in hadronic decay modes, and the signal $B\to X_{s}\nu\bar{\nu}$ decay is reconstructed in 30 exclusive decay modes. We do not observe a significant signal and set 90% confidence level upper limits on the branching fractions. The search is performed in three regions of the $X_{s}$ mass, chosen to separate contributions from prominent resonances. We set upper limits for the regions 0.0 < $M(X_{s})$ < 0.6 GeV/c$^{2}$ , 0.6 < $M(X_{s})$ < 1.0 GeV/c$^{2}$ , and 1.0 GeV/c$^{2}$ < $M(X_{s})$ at $2.2\times 10^{−5}$ , $9.5\times 10^{−5}$ , and $31.2 \times 10^{−5}$ , respectively. Combining the three mass regions, we obtain the upper limit for the branching fraction of $\mathcal{B}(B \to X_{s}\nu\bar{\nu}) < 3.2 \times 10^{−4} $. This is the most stringent limit to date.
Tags	Run 1
Working group	Radiative and Electroweak Penguin
Principal authors	@ Akimasa Ishikawa, Youngjoon Kwon, Junewoo Park, Yutaka Ushiroda
Reference	submitted to PRL
Links	arXiv:2511.10980,

Abstract

We search for the flavor-changing neutral-current decays $B\to X_{s}\nu\bar{\nu}$ , where $X_{s}$ is a hadronic system with strangeness equal to 1, in data collected by the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}e^{−}$ collider. The data sample has an integrated luminosity of 365 fb$^{−1}$ collected at the \Upsilon(4S) resonance and 43 fb$^{−1}$ collected with a 60 MeV lower center-of-mass energy for estimation of continuum background. One of the $B$ mesons from the $Upsilon(4S)\to B\bar{B}$ decay is fully reconstructed in hadronic decay modes, and the signal $B\to X_{s}\nu\bar{\nu}$ decay is reconstructed in 30 exclusive decay modes. We do not observe a significant signal and set 90% confidence level upper limits on the branching fractions. The search is performed in three regions of the $X_{s}$ mass, chosen to separate contributions from prominent resonances. We set upper limits for the regions 0.0 < $M(X_{s})$ < 0.6 GeV/c$^{2}$ , 0.6 < $M(X_{s})$ < 1.0 GeV/c$^{2}$ , and 1.0 GeV/c$^{2}$ < $M(X_{s})$ at $2.2\times 10^{−5}$ , $9.5\times 10^{−5}$ , and $31.2 \times 10^{−5}$ , respectively. Combining the three mass regions, we obtain the upper limit for the branching fraction of $\mathcal{B}(B \to X_{s}\nu\bar{\nu}) < 3.2 \times 10^{−4} $. This is the most stringent limit to date.

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