Search for lepton-flavor-violating tau decays to $\ell \hat{I}\pm$ at Belle

Abstract

We report a search for the lepton-flavor-violating decays $\ddot{I}{„}^{\pm }\to {\ell }^{\pm }\hat{I}\pm$~($\ell =e,\hat{I}¼$), where $\hat{I}\pm$ is an undetected spin-0 particle, such as an axion-like particle using $736\times 10^6$ tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95\% confidence level: $\mathcal{B}(\ddot{I}{„}^{\pm }\to e^{\pm }\hat{I}\pm )$ $<$ $(0.4$--$6.4)\times 10^{-4}$ and $\mathcal{B}(\ddot{I}{„}^{\pm }\to \hat{I}{¼}^{\pm }\hat{I}\pm )$ $<$ $(0.2$--$3.5)\times 10^{-4}$ at 95\% confidence level for an $\hat{I}\pm$ mass in the range $0.0\le m_{\hat{I}}\pm \le 1.6$~GeV/$c^2$.