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

A search for charged lepton flavour violation in τ→ 3μ 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 focuses on the electroweak W → τν production channel. 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→3μ) is found to be 8.7× 10⁻⁸ (7.5× 10⁻⁸) at $90%$ CL.