Search for lepton-flavor-violating ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{\mu }^{-}$ decays at Belle II

We present the result of a search for the charged-lepton-flavor violating decay ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{\mu }^{-}$ using a $424f{b}^{-1}$ sample of data recorded by the Belle II experiment at the SuperKEKB $e^{-}{e}^{+}$ collider. The selection of $e^{-}{e}^{+}\to {\tau }^{+}{\tau }^{-}$ 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 $90\mathrm{\%}$ 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.