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 424 fb⁻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 τ⁻→ μ⁻μ⁺μ⁻ decay, which is the most stringent bound to date.