Search for lepton-flavor-violating $\ddot{I}{„}^{-}\to {\ell }^{-}\ddot{I}•$ decays in 2019-2021 Belle II data

Abstract

We report a search for lepton-flavor-violating decays $\ddot{I}{„}^{-}\to {\ell }^{-}\ddot{I}•$ (${\ell }^{-}=e^{-},\hat{I}{¼}^{-}$) at the Belle II experiment, using a sample of electron-positron data produced at the SuperKEKB collider in 2019-2021 and corresponding to an integrated luminosity of 190 fb${}^{-1}$. We use a new untagged selection for $e^{+}{e}^{-}\to \ddot{I}{„}^{+}\ddot{I}{„}^{-}$ events, where the signal $\ddot{I}„$ is searched for as a neutrinoless final state of a single charged lepton and a $\ddot{I}•$ meson and the other $\ddot{I}„$ is not reconstructed in any specific decay mode, in contrast to previous measurements by the BaBar and Belle experiments. We find no evidence for $\ddot{I}{„}^{-}\to {\ell }^{-}\ddot{I}•$ decays and obtain upper limits on the branching fractions at 90% confidence level of 23 $\times 10^{-8}$ and 9.7$\times 10^{-8}$ for $\ddot{I}{„}^{-}\to e^{-}\ddot{I}•$ and $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\ddot{I}•$, respectively