A search for lepton-flavour violating t ? 3µ decays with the ATLAS detector

A search for charged lepton flavour violation in t ? 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 ? t? 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 (t ? 3 µ) is found to be 8.7×10^-8 (7.5×10^-8) at 90% CL