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

Abstract

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 ${\rm Y}(4S)$ resonance and a sample of $9\,\mbox{fb}^{-1}$ collected at an energy $60\mathrm{Me}\mathrm{V}$ 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.

arXiv comment10 pages, 3 figures + supplemental material

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