PHYS. REV. D 109, 112006 (2024)

ISSN: 2470-0010, SCIE

Evidence for B+ → K+ νν¯ decays

Belle II Collaboration (I. Adachi, ..., N. Anh Ky, ...)

We search for the rare decay Bþ → Kþνν¯ in a 362 fb−1 sample of electron-positron collisions at the ϒð4SÞ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying B meson in ϒð4SÞ → 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 Bþ → Kþνν¯ branching fraction of ½2.7 0.5ðstatÞ 0.5ðsystÞ × 10−5 and ½1.1þ0.9 −0.8 ðstatÞ þ0.8 −0.5 ðsystÞ × 10−5, respectively. Combining the results, we determine the branching fraction of the decay Bþ → Kþνν¯ to be ½2.3 0.5ðstatÞ þ0.5 −0.4 ðsystÞ × 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.

DOI: https://doi.org/10.1103/PhysRevD.109.112006