Search for $B^{0} \to \tau^\pm \ell^\mp$ ($\ell=e,\mu$) with a hadronic tagging method at Belle

TL;DR

This study searches for lepton-flavor-violating decays of B0 mesons into tau and electron or muon, using Belle data, but finds no evidence and sets upper limits on their branching fractions.

Contribution

First search for $B^{0} o au^ ext{ extpm} \, ext{ extpm}$ decays using a hadronic tagging method at Belle, establishing new upper limits.

Findings

No evidence for the decays was observed.

Upper limits on branching fractions were set at $1.5 imes 10^{-5}$ and $1.6 imes 10^{-5}$.

Results constrain new physics models predicting such decays.

Abstract

We present a search for the lepton-flavor-violating decays $B^{0} \to \tau^\pm \ell^\mp$, where $\ell=(e,\,\mu)$, using the full data sample of $772 \times 10^6$ $B \overline{B}$ pairs recorded by the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode. The $\tau^\pm$ lepton is reconstructed indirectly using the momentum of the reconstructed $B$ and that of the $\ell^\mp$ from the signal decay. We find no evidence for $B^{0} \to \tau^\pm \ell^\mp$ decays and set upper limits on their branching fractions at 90% confidence level of $\cal B$($B^{0} \to \tau^\pm \mu^{\mp}$)$ < 1.5 \times 10^{-5}$ and $\cal B$($B^{0} \to \tau^\pm e^{\mp}$)$ < 1.6 \times 10^{-5}$.