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We report the first evidence for the $h_b\left(2\mathrm{P}\right)\to \mathrm{\Upsilon }\left(1\mathrm{S}\right)\eta$transition with a significance of 3.5 standard deviations. The decay branching fraction is measured to be $\mathcal{B}\left[h_b\right(2\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S}\left)\eta \right]=({7.1}_{-3.2}^{+3.7}\pm 0.8)\times {10}^{-3}$, which is noticeably smaller than expected. We also set upper limits on ${\pi }^0$transitions of $\mathcal{B}\left[h_b\right(2\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S}\left){\pi }^0\right]<1.8\times {10}^{-3}$, and $\mathcal{B}\left[h_b\right(1\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S}\left){\pi }^0\right]<1.8\times {10}^{-3}$, at the 90% confidence level. These results are obtained with a $131.4{\mathrm{fb}}^{-1}$data sample collected near the $\mathrm{\Upsilon }\left(5\mathrm{S}\right)$resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$collider. Published by the American Physical Society2024 

We search for the rare decay $B^{+}\to K^{+}\nu \overline{\nu }$in a $362{\mathrm{fb}}^{-1}$sample of electron-positron collisions at the $\mathrm{\Upsilon }\left(4S\right)$resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$meson in $\mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$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 $B^{+}\to K^{+}\nu \overline{\nu }$branching fraction of $[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst}\left)\right]\times {10}^{-5}$and $\left[{1.1}_{-0.8}^{+0.9}\right(\mathrm{stat}{)}_{-0.5}^{+0.8}\left(\mathrm{syst}\right)]\times {10}^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\to K^{+}\nu \overline{\nu }$to be $[2.3\pm 0.5(\mathrm{stat}{)}_{-0.4}^{+0.5}\left(\mathrm{syst}\right)]\times {10}^{-5}$, 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 Society2024