Search for lepton-flavor-violating ▫$\tau^- \to \mu^- \mu^+ \mu^-$▫ decays at Belle IIAdachi, Ichiro (Avtor) Borah, Jyotirmoi (Avtor) Bračko, Marko (Avtor) Feichtinger, Paul (Avtor) Ganiev, Eldar (Avtor) Korpar, Samo (Avtor) Križan, Peter (Avtor) Mrvar, Manca (Avtor) Pestotnik, Rok (Avtor) Podobnik, Tomaž (Avtor) Starič, Marko (Avtor) Šantelj, Luka (Avtor) Zhukova, Valentina (Avtor) Žlebčík, Radek (Avtor) flavour physicstau physicse+-e− ExperimentsWe 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.SISSA20242026-04-22 14:37:14Neznano29169Italija sl© The Authors.