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

Abstract

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 $63\, \rm{fb}^{-1}$ collected at the $\rm \Upsilon(4S)$ resonance and an additional sample of $9\, \rm{fb}^{-1}$ collected at an energy $60\, \rm MeV$ 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 $90\, \%$ confidence level.