Search for $B^+ \to e^+ \nu$ and $B^+ \to \mu^+ \nu$ decays using hadronic tagging

TL;DR

This study searches for rare leptonic decays of B+ mesons into electron and muon neutrinos using the full Belle dataset, finding no evidence and setting upper limits on their branching fractions.

Contribution

It provides the first comprehensive search for these decays with hadronic tagging at Belle, establishing new upper limits on their branching fractions.

Findings

No evidence of the decays was observed.

Upper limits on branching fractions are set at 3.5×10⁻⁶ for B+→e+ν_e.

Upper limit for B+→μ+ν_μ is 2.7×10⁻⁶.

Abstract

We present a search for the rare leptonic decays $B^+ \to e^+ \nu_e$ and $B^+ \to \mu^+ \nu_\mu$, using the full $\Upsilon(4S)$ data sample of $772\times 10^6$ $B\bar{B}$ pairs collected with the Belle detector at the $\small\textbf{KEKB}$ asymmetric-energy $e^+ e^-$ collider. One of the $B$ mesons from the $\Upsilon(4S) \to B\bar{B}$ decay is fully reconstructed in a hadronic mode, while the recoiling side is analyzed for the signal decay. We find no evidence of a signal in any of the decay modes. Upper limits of the corresponding branching fractions are determined as ${\cal B}(B^+ \to e^+ \nu_e)<3.5\times 10^{-6}$ and ${\cal B}(B^+ \to \mu^+ \nu_\mu)<2.7 \times 10^{-6}$ at 90$\%$ confidence level.