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

14 August 2018

Abstract: We report the results of a search for the rare, purely leptonic decay ${B}^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }$ performed with a $711\text{}\text{}{\mathrm{fb}}^{-1}$ data sample that contains $772×{10}^{6}\text{}\text{}B\overline{B}$ pairs, collected near the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{-}$ collider. 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 $B\left({B}^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }\right)=\left(6.46±2.22±1.60\right)×{10}^{-7}$ or a frequentist 90% confidence level interval on the ${B}^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }$ branching fraction of $\left[2.9,10.7\right]×{10}^{-7}$.

Published in: Physical Review Letters 121 (2018)