Evidence for B⁺→ K⁺νν̄ Decays

This is the authors' abstract. We don't add key points for this paper.

We search for the rare decay B⁺→ K⁺νν̄ in a 362\ fb⁻¹ sample of electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying B meson in Υ(4S) → B 0.18em -0.18em B̄ events to suppress background from other decays of the signal B candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying B meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the B⁺→ K⁺νν̄ branching fraction of [2.7± 0.5(stat)± 0.5(syst)] × 10⁻⁵ and [1.1+0.9-0.8(stat)+0.8-0.5(syst)] × 10⁻⁵, respectively. Combining the results, we determine the branching fraction of the decay B⁺→ K⁺νν̄ to be [2.3 ± 0.5(stat)+0.5-0.4(syst)]× 10⁻⁵, providing the first evidence for this decay at $3.5$ standard deviations. The combined result is $2.7$ standard deviations above the standard model expectation.