Evidence for $B^{+}\to K^{+}\nu \bar{\nu }$ decays

Abstract

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

arXiv comment29 pages, 23 figures

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