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We present the result of a search for the charged-lepton-flavor violating decay ^- ^-^+^- using a 424fb^-1 sample of data recorded by the Belle II experiment at the SuperKEKB e^-e^+ collider. The selection of e^-e^+^+^- 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 10^-8 on the branching fraction of the decay, which is the most stringent bound to date.
Collaboration et al. (Sun,) studied this question.