Search for lepton-flavor-violating decays of the Z boson into a τ lepton and a light lepton with the ATLAS detector

Direct searches for lepton flavor violation in decays of the Z boson with the ATLAS detector at the LHC are presented. Decays of the Z boson into an electron or muon and a hadronically decaying τ lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1     fb − 1 at a center-of-mass energy of √ s = 13     TeV . No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: B ( Z → e τ ) < 5.8 × 10 − 5 and B ( Z → μ τ ) < 2.4 × 10 − 5 . This is the first limit on B ( Z → e τ ) with ATLAS data. The upper limit on B ( Z → μ τ ) is combined with a previous ATLAS result based on 20.3     fb − 1 of proton-proton collision data at a center-of-mass energy of √ s = 8     TeV and the combined upper limit at 95% confidence level is B ( Z → μ τ ) < 1.3 × 10 − 5 .