Search for lepton-flavor-violating $\tau$ decays into a lepton and a vector meson using the full Belle data sample

TL;DR

This study searches for lepton-flavor-violating tau decays into a lepton and a vector meson using Belle data, setting new upper limits that improve previous constraints, with no significant signal observed.

Contribution

First comprehensive search for tau to lepton plus vector meson decays using the full Belle dataset, establishing more stringent upper limits on branching fractions.

Findings

No significant excess observed in tau decays.

Upper limits on branching fractions are improved by 30%.

Constraints on new physics models are strengthened.

Abstract

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of $\tau$ lepton decays into a light charged lepton ($\ell$ = $e^{\pm}$ or $\mu^{\pm}$) and a vector meson ($V^0$ = $\rho^0$, $\phi$, $\omega$, $K^{\ast0}$, or $\overline{K}{}^{\ast0}$) using 980 fb$^{-1}$ of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the $\tau \rightarrow \ell V^0$ branching fractions in the range of (1.7--$4.3) \times 10^{-8}$. These limits are improved by 30% on average from the previous results.