Search for lepton-flavor-violating $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ decays at Belle II

Abstract

We present the result of a search for the charged-lepton-flavor violating decay $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ using a $424f{b}^{-1}$ sample of data recorded by the Belle II experiment at the SuperKEKB $e^{-}{e}^{+}$ collider. The selection of $e^{-}{e}^{+}\to \ddot{I}{„}^{+}\ddot{I}{„}^{-}$ events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a $90\%$ confidence level upper limit of $1.9\times 10^{-8}$ on the branching fraction of the \taumu decay, which is the most stringent bound to date.