Search for Lepton Flavor Violating tau^- Decays into \ell^-\eta, \ell^-\eta' and \ell^-\pi^0

TL;DR

This study searched for rare lepton-flavor-violating tau decays into specific mesons using a large data sample, finding no evidence but setting more stringent upper limits on their branching fractions.

Contribution

First search for these tau decays into pseudoscalar mesons with improved upper limits, constraining new physics models of lepton flavor violation.

Findings

No evidence for the decays was observed.

Upper limits on branching fractions were improved by factors of 2.3 to 6.3.

Results provide tighter constraints on lepton flavor violation theories.

Abstract

We have searched for lepton-flavor-violating $\tau$ decays with a pseudoscalar meson ($\eta$, $\eta'$ {and} $\pi^0$) using a data sample of 401 fb$^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No evidence for these decays is found and we set the following upper limits on the branching fractions: ${\cal{B}}(\tau^-\to e^-\eta) < 9.2\times 10^{-8}$, ${\cal{B}}(\tau^-\to \mu^-\eta) < 6.5\times 10^{-8}$, ${\cal{B}}(\tau^-\to e^-\eta') < 1.6\times 10^{-7}$, ${\cal{B}}(\tau^-\to \mu^-\eta') < 1.3\times 10^{-7}$, ${\cal{B}}(\tau^-\to e^-\pi^0) < 8.0\times 10^{-8}$ and ${\cal{B}}(\tau^-\to \mu^-\pi^0) < 1.2\times 10^{-7}$ at the 90% confidence level. These results improve our previously published upper limits by factors from 2.3 to 6.3.