Search for B-→μ-ν¯μ 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 ×10⁶ B B ¯ pairs, collected near the ϒ (4 S ) 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 ±2.22 ±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.