Search for $B^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }$ Decays at the Belle Experiment

We report the result of a search for the decay $B^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }$. The signal events are selected based on the presence of a high momentum muon and the topology of the rest of the event showing properties of a generic $B$-meson decay, as well as the missing energy and momentum being consistent with the hypothesis of a neutrino from the signal decay. We find a 2.4 standard deviation excess above background including systematic uncertainties, which corresponds to a branching fraction of ${\cal B}(B^{-}\to\mu^{-}\bar\nu_\mu) =(6.46 \pm 2.22 \pm 1.60 )\times10^{-7}$ or a frequentist 90% confidence level interval on the $B^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }$ branching fraction of $[2.9, 10.7]\times 10^{-7}$\mathrm{.}Thisresultisobtainedfroma$711\ $\text{fb}$^{-1}$ data sample that contains $772\times {10}^6$ $B\bar{B}$ pairs, collected near the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.