Search for $B^{+}\to K^{+}\nu \bar{\nu }$ decays with an inclusive tagging method at the Belle II experiment

This contribution illustrates a new search for the flavor-changing neutral-current decay $B^{+}\to K^{+}\nu \bar{\nu }$ performed by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. In this study, a sample corresponding to an integrated luminosity of $63, \rm{fb}^{-1}$collectedatthe$\rm \Upsilon(4S)$ resonance and an additional sample of $9{\mathrm{f}\mathrm{b}}^{-1}$ collected at an energy $60\mathrm{M}\mathrm{e}\mathrm{V}$ below the resonance are used. A novel technique, based on an inclusive tagging method and exploiting the topological features of the $B^{+}\to K^{+}\nu \bar{\nu }$ decay, is employed and it provides a higher signal efficiency with respect to the methods used in the previous searches. No significant signal is observed. An upper limit of $4.1\times {10}^{-5}$ is set on the $B^{+}\to K^{+}\nu \bar{\nu }$ branching fraction at the $90\mathrm{\%}$ confidence level.