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

A search is made for the lepton avour violating decay τ− → μ+μ−μ− using 3.0 fb−1 of data collected by LHCb at √ s = 7 TeV and √ s = 8 TeV in 2011 and 2012 respectively. In LHCb, τ− leptons are copiously produced, almost exclusively from the decays of B and D− s mesons. In the analysis of the data, the τ − production rate is normalised to the control channel D− s → φ(μ+μ−)π−. The observed yield is consistent with the background expectation and a world's best upper limit of B(τ− → μ+μ−μ−) < X.X(X.X)× 10−X is set at 95% (90%) con dence level.