Search for lepton-flavor-violating tau-lepton decays to $\ell\gamma$ at Belle

TL;DR

This paper reports a search for lepton-flavor-violating tau decays to a lepton and photon using Belle data, setting new upper limits on the branching fractions due to no observed excess.

Contribution

It provides updated upper limits on tau to lepton gamma decay branching fractions using the full Belle dataset, improving constraints on new physics models.

Findings

No significant excess observed over Standard Model background.

Upper limits on branching fractions are set at 4.2×10⁻⁸ for tau to mu gamma.

Upper limits on branching fractions are set at 5.6×10⁻⁸ for tau to e gamma.

Abstract

Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $\tau^{\pm}\rightarrow\ell^{\pm}\gamma$ ($\ell=e,\mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $\tau^{\pm}\rightarrow\ell^{\pm}\gamma$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $\mathcal{B}(\tau^{\pm}\rightarrow \mu^{\pm}\gamma)$ $\leq$ $4.2\times10^{-8}$ and $\mathcal{B}(\tau^{\pm}\rightarrow e^{\pm}\gamma)$ $\leq$ $5.6\times10^{-8}$, are set at 90\% confidence level.