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

We report the results of a search for the rare, purely leptonic decay $B^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }$ performed with a $711{\mathrm{fb}}^{-1}$ data sample that contains $772\times {10}^{6}B\overline{B}$ pairs, collected near the $\mathrm{\Upsilon }(4S)$ 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 $\mathcal{B}(B^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu })=(6.46\pm 2.22\pm 1.60)\times {10}^{-7}$ or a frequentist 90% confidence level interval on the $B^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }$ branching fraction of $[2.9,10.7]\times {10}^{-7}$.

Figure 1

The distributions of the neural network output variable for the signal and major background processes predicted by MC in the signal-enhanced region $2.644\mathrm{GeV}/c<p_{\mu }^{*}<2.812\mathrm{GeV}/c$ (the relative normalization of these distributions is arbitrary).

Figure 2

Projections of the fitted distribution to data onto the histogram axes in the signal-enhanced regions (top panel) $0.84<o_{\mathrm{nn}}$ and (bottom panel) $2.6\mathrm{GeV}/c<p_{\mu }^{*}<2.85\mathrm{GeV}/c$.