Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed
| Summary:
We search for the rare decay $B^{+}rightarrow K^{+}nubar{nu}$ in a $362
rm{fb}^{-1}$ sample of electron-positron collisions at the $Upsilon(4S)$
resonance collected with the Belle II detector at the SuperKEKB collider. We
use the inclusive properties of the accompanying $B$ meson in $Upsilon(4S) to
Bkern 0.18emoverline{kern -0.18em B}{}$ 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 $B^{+}rightarrow K^{+}nubar{nu}$
branching fraction of $left[2.7pm 0.5(mathrm{stat})pm
0.5(mathrm{syst})right] times 10^{-5}$ and
$left[1.1^{+0.9}_{-0.8}(mathrm{stat}){}^{+0.8}_{-0.5}(mathrm{syst})right]
times 10^{-5}$, respectively. Combining the results, we determine the
branching fraction of the decay $B^{+}rightarrow K^{+}nubar{nu}$ to be
$left[2.3 pm 0.5(mathrm{stat})^{+0.5}_{-0.4}(mathrm{syst})right]times
10^{-5}$, 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.
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