This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of τ→3μ . A method utilising the production of τ leptons via W→τν 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 ττ lepton branching fraction into three muons, Br(τ→3μ)Br(τ→3μ) , is 3.76×10−73.76×10−7 ( 3.94×10−73.94×10−7 ) at 90 % confidence level.
Bibliographical note21 pages plus author list + cover pages (39 pages total), 6 figures, 2 tables, submitted to EPJC, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2014-14/