Search for the lepton flavour violating decay τ − → μ − μ + μ −

A search for the lepton flavour violating decay τ − → μ − μ + μ − is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of 1.0 fb −1 of proton-proton collisions at a centre-of-mass energy of 7 TeV and 2.0 fb −1 at 8 TeV. No evidence is found for a signal, and a limit is set at 90% confidence level on the branching fraction, ℬ τ − → μ − μ + μ − < 4.6 × 10 − 8 .