First search for $B\to X_s\nu \bar{\nu }$ decays

Abstract

We report the first 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 with the Belle~II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The data sample corresponds to an integrated luminosity of $365{\text{fb}}^{-1}$ collected at the ${\rm Y}(4S)$ resonance and $43{\text{fb}}^{-1}$ collected at a center-of-mass energy $60\text{MeV}$ below resonance for estimation of $e^{+}{e}^{-}\to q\bar{q}$ continuum background. One of the $B$ mesons from the ${\rm Y}(4S)\to B\bar{B}$ 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 $0.0<M_{X_s}<0.6\text{GeV}/c^2$, $0.6<M_{X_s}<1.0\text{GeV}/c^2$, and $1.0\text{GeV}/c^2<M_{X_s}$ of $2.2\times 10^{-5}$, $9.3\times 10^{-5}$, and $30.9\times 10^{-5}$, respectively. Combining the three mass regions, we obtain the upper limit on the branching fraction, $B(B\to X_s\nu \bar{\nu })<3.3\times 10^{-4}$.

arXiv comment8 pages, 2 figures + supplemental material

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