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

TL;DR

This paper reports a search for the rare decay $B^{+} o K^{+} uar{ u}$ at Belle II using an innovative inclusive tagging method, setting an upper limit on its branching fraction with no significant signal observed.

Contribution

It introduces a novel inclusive tagging technique exploiting decay topology differences, improving signal efficiency over previous methods.

Findings

No significant $B^{+} o K^{+} uar{ u}$ signal observed.

Upper limit on branching fraction set at $4.1 imes 10^{-5}$.

Method demonstrates higher efficiency than prior searches.

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 $\Upsilon{(4S)}$ 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.