Search for B⁺→K⁺ν$\bar{ν}$ Decays Using an Inclusive Tagging Method at Belle II

A search for the flavor-changing neutral-current decay B$^{+}$→K$^{+}$ν$\overline{ν}$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of 63 fb$^{-1}$ collected at the Υ(4S) resonance and a sample of 9 fb$^{-1}$ 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 B$^{+}$→K+ν¯ν decay, but also the inclusive properties of the other B meson in the Υ(4S)→B$\overline{B}$ event, to suppress the background from other B 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 B$^{+}$→K$^{+}$ν$\overline{ν}$ of 4.1×10$^{-5}$ is set at the 90% confidence level.