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We search for the rare decay B+→K+νν¯ 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)→BB¯ 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+νν¯ branching fraction of 2.7±0.5(stat)±0.5(syst)×10−5 and 1.1−0.8+0.9(stat)−0.5+0.8(syst)×10−5, respectively. Combining the results, we determine the branching fraction of the decay B+→K+νν¯ to be 2.3±0.5(stat)−0.4+0.5(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. Published by the American Physical Society 2024
Adachi et al. (Thu,) studied this question.