Search for \(B^{+}\to K^{+}\nu\bar{\nu}\) decays using an inclusive tagging method at Belle II

A search for the flavor-changing neutral-current decay \(B^{+}\to K^{+}\nu\bar{\nu}\) is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The results are based on a data sample corresponding to an integrated luminosity of \(63\,\mbox{fb}^{-1}\) collected at the \(\Upsilon{(4S)}\) resonance and a sample of \(9\,\mbox{fb}^{-1}\) collected at an energy \(60\mathrm{\,Me\kern -0.1em V}\) below the resonance. A novel measurement method is employed, which exploits topological properties of the \(B^{+}\to K^{+}\nu\bar{\nu}\) decay that differ from both generic bottom-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 \(B^{+}\to K^{+}\nu\bar{\nu}\) of \(4.1 \times 10^{-5}\) is set at the 90% confidence level.