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4. Observation of ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }{e}^{+}{e}^{-}$ and search for ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }{\mu }^{+}{\mu }^{-}$

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5. Evidence for $B^{+}\to K^{+}\nu \overline{\nu }$ decays

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

   Adachi, I; Adamczyk, K; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Anh_Ky, N; Asner, D M; Atmacan, H; et al (June 2024, Physical Review D)

   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 

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