Search for $B^{+}to K^{+} ubar{ u}$ decays using an inclusive tagging method at Belle II

A search for the flavor-changing neutral-current decay $B^{+}to K^{+} ubar{ u}$ 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 $60mathrm{,Mekern -0.1em V}$ below the resonance. A novel measurement method is employed, which exploits topological properties of the $B^{+}to K^{+} ubar{ u}$ 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 $B^{+}to K^{+} ubar{ u}$ of $4.1 times 10^{-5}$ is set at the 90% confidence level.