Search for ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{\mu }^{-}$ decays at the LHCb experiment with Run 2 data

A search for the lepton-flavour-violating decay ${\tau }^{-}\to {\mu }^{-}{\mu }^{+}{\mu }^{-}$ is carried out using data collected by the LHCb experiment between 2016 and 2018 in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb${}^{-1}$. An upper limit of $1.9(2.3)\times {10}^{-8}$ is set at the 90% (95%) confidence level on the branching fraction of the decay.