Search for lepton-flavor-violating τ⁻ → μ⁻μ⁺μ⁻ decays at Belle II

We present the result of a search for the charged-lepton-flavor violating decay τ⁻ → μ⁻μ⁺μ⁻ using a 424fb⁻¹ sample of data recorded by the Belle II experiment at the SuperKEKB e⁻e⁺ collider. The selection of e⁻e⁺→τ⁺τ⁻ 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.