First Search for B→X_{s}νν[over ¯] Decays

We report the first search for the flavor-changing neutral-current decays B→X_{s}νν[over ¯], 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  fb^{-1} collected at the ϒ(4S) resonance and 43  fb^{-1} collected at a center-of-mass energy 60 MeV below resonance for estimation of e^{+}e^{-}→qq[over ¯] continuum background. One of the B mesons from the ϒ(4S)→BB[over ¯] decay is fully reconstructed in a hadronic decay mode. The B→X_{s}νν[over ¯] 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, M_{X_{s}}^{true}, chosen to separate contributions from K, K^{*}(892), and heavier strange final states. 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