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

A search for the lepton flavour violating decay - - + -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( - - + -) < 4.6 10 -8 .