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 .