Abstract: This Letter presents direct searches for lepton flavour violation in Higgs boson decays, and , performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy TeV, corresponding to an integrated luminosity of . No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% () and 0.28% () for and , respectively

The first direct search for lepton-flavour-violating decays of the recently discovered Higgs boson (H) is described. The search is performed in the H 2![mu][tau]e and H 2![mu][tau]h channels, where [tau]e and [tau]h are tau leptons reconstructed in the electronic and hadronic decay channels, respectively. The data sample used in this search was collected in pp collisions at a centre-of-mass energy of [arrow]" = 8 TeV with the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 19.7 fb-1. Furthermore, the sensitivity of the search is an order of magnitude better than the existing indirect limits. A slight excess of signal events with a significance of 2.4 standard deviations is observed. The p-value of this excess at MH = 125 GeV is 0.010. We found that the best fit branching fraction is B(H 2![mu][tau]) = (0.84+0.39 -0.37)%. A constraint on the branching fraction, B(H 2![mu][tau])