Adachi, I.Aggarwal, L.Aihara, H.Akopov, N.Aloisio, A.Althubiti, N.Anh Ky, N.Asner, D.M.Atmacan, H.Aushev, V.Aversano, M.Ayad, R.Babu, V.Bae, H.Bahinipati, S.Bambade, P.Banerjee, S.Bansal, S.Barrett, M.Baudot, J.et al.2025-03-242025-03-242024The Journal of High Energy Physics, 2024; 2024(9):62-1-62-221029-84791029-8479https://hdl.handle.net/2440/144003We 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.en© The Authors. Article funded by SCOAP3. Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.e +-e − Experiments, Flavour Physics, Tau PhysicsJournal article10.1007/jhep09(2024)062710369De La Motte, S.A. [0000-0003-3905-6805]Harris, C. [0000-0003-0448-4244]