A search for lepton-flavour violating τ → 3μ decays with the ATLAS detector

A search for charged lepton flavour violation in τ → 3 μ decays is performed in pp collisions at a centre-of-mass energy of 13 TeV using ATLAS data collected between 2016 and 2018, corresponding to an integrated luminosity of 137 fb − 1 . The search is optimised for the electroweak W → τν production channel, due to its higher trigger efficiency, but also considers potential signal events from heavy-flavour production. Data are collected using two-muon and three-muon triggers and a multivariate analysis is used to separate the signal from the background. An unbinned likelihood fit is then performed to the resulting three-muon invariant mass spectrum and the data are found to be compatible with the background-only hypothesis. The observed (expected) limit on the branching ratio B ( τ → 3 μ ) is found to be 8.7 × 10 − 8 ( 7.5 × 10 − 8 ) at 90% CL.