Search for B+ -> K+ nunubar Decays Using an Inclusive Tagging Method at Belle II

A search for the flavor-changing neutral-current decay is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of collected at the resonance and a sample of collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the decay, but also the inclusive properties of the other meson in the event, to suppress the background from other meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of of is set at the 90% confidence level.