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

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 ${\tau }^{\pm }\to {\ell }^{\pm }\gamma$ ($\ell =e,\mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via ${\tau }^{\pm }\to {\ell }^{\pm }\gamma$ 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, $\mathcal{B}({\tau }^{\pm }\to {\mu }^{\pm }\gamma )$ $\le$ $4.2\times 10^{-8}$ and $\mathcal{B}({\tau }^{\pm }\to e^{\pm }\gamma )$ $\le$ $5.6\times 10^{-8}$, are set at 90\% confidence level.

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