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

TL;DR

This paper reports a search for the rare decay $B^{-}\to\mu^{-}\bar\nu_\mu$ at Belle, observing a slight excess over background with a measured branching fraction around 6.5 x 10^{-7}, using a large data sample.

Contribution

First search for $B^{-}\to\mu^{-}\bar\nu_\mu$ decay at Belle, providing a measurement and confidence interval for its branching fraction.

Findings

Observed a 2.4 sigma excess over background.

Measured branching fraction: (6.46 ± 2.22 ± 1.60) x 10^{-7}.

Set 90% CL interval: [2.9, 10.7] x 10^{-7}.

Abstract

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 $[2.9, 10.7]\times 10^{-7}$. This result is obtained from a $711\ \text{fb}^{-1}$ data sample that contains $772 \times 10^6$ $B\bar{B}$ pairs, collected near the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.