Search for B + → K + ν ¯ ν Decays Using an Inclusive Tagging Method at Belle II

Abstract

A search for the flavor-changing neutral-current decay Bþ → Kþνν¯ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of 63 fb−1 collected at the ϒð4SÞ resonance and a sample of 9 fb−1 collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the Bþ → Kþνν¯ decay, but also the inclusive properties of the other B meson in the ϒð4SÞ → BB¯ event, to suppress the background from other B 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 Bþ → Kþνν¯ of 4.1 × 10−5 is set at the 90% confidence level.