Search for B−→μ−¯νμ Decays at the Belle Experiment

We report the results of a search for the rare, purely leptonic decay B$^{-}$ → μ− ¯ν$_{μ}$ performed with a 711 fb$^{-1}$ data sample that contains 772 × 10$^{6}$ BB¯ pairs, collected near the ϒ(4S) 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(B$^{-}$ → μ$^{-}$ ¯ν$_{μ}$) = (6.46 $\pm$ 2.22 $\pm$ 1.60) × 10$^{-7}$ or a frequentist 90% confidence level interval on the B$^{-}$ → μ$^{-}$ ¯ν$_{μ}$ branching fraction of [2.9,10.7] × 10$^{-7}$.