Roberta Volpe

24 August 2023
KEK IPNS Seminar on B+ -> Knunu

Abstract: We search for the rare decay B+ -> K+ nu nubar in a 362 /fb sample of electron-positron collisions at the Upsilon(4S) resonance collected with the Belle II detector at the SuperKEKB collider. The branching fraction is predicted with high accuracy in the standard model (SM) and is enhanced in several SM extensions. We use the inclusive properties of the other \B meson in the Upsilon(4S) -> B Bbar events to suppress background from other B-meson decays and light-quark pair production. In addition, we validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying B meson. The signal-reconstruction efficiency and background suppression are validated using control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses obtain consistent results. The accuracy is significantly improved compared to the previous Belle II analysis. The results are combined and compared to the background-only hypothesis and to the SM expectations.