A search for lepton-flavour violating $\tau \to 3\mu$ decays with the ATLAS detector

A search for charged lepton flavour violation in $\tau \to 3\mu$ 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 ${\text{fb}}^{-1}$. The search focuses on the electroweak $W\to \tau \nu$ 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 $B(\tau \to 3\mu )$ is found to be $8.7\times {10}^{-8}$ ($7.5\times {10}^{-8}$) at $90\mathrm{\%}$ CL.