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

Abstract

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 $B(\tau\to3\mu)$ is found to be $8.7\times 10^{-8}$ ($7.5\times 10^{-8}$) at $90\%$ CL.