ATLAS collaboration (2868 authors)Aguilar-Saavedra, Juan AntonioAmor Dos Santos, Susana PatriciaAnjos, NunoAraque, Juan PedroCantrill, RobertCarvalho, JoãoCastro, Nuno FilipeConde Muiño, PatriciaDa Cunha Sargedas De Sousa, Mario JoseFiolhais, MiguelGalhardo, BrunoGomes, AgostinhoGonçalo, RicardoJorge, PedroLopes, LourencoMachado Miguens, JoanaMaio, AméliaManeira, JoséOnofre, AntónioPalma, AlbertoPedro, RutePina, João AntonioSantos, HelenaSaraiva, JoãoSilva, JoséTavares Delgado, AdemarVeloso, FilipeWolters, Helmut2019-02-042019-02-042016-04-26http://dx.doi.org/10.1140/epjc/s10052-016-4041-9http://hdl.handle.net/10400.26/27009This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of $\tau \rightarrow 3\mu $ . A method utilising the production of $\tau $ leptons via $W\rightarrow \tau \nu $ decays is used. This method is applied to the sample of 20.3 fb$^{-1}$ of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the $\tau $ lepton branching fraction into three muons, $\mathrm{Br}(\tau \rightarrow 3\mu )$ , is $3.76\times 10^{-7}$ ( $3.94\times 10^{-7}$ ) at 90 % confidence level.engjournal article2019-02-04