Search for Lepton-Flavor Violation in Z-Boson Decays with τ Leptons with the ATLAS Detector

A search for lepton-flavor-violating Z → eτ and Z → μτ decays with pp collision data recorded by the ATLAS detector at the LHC is presented. This analysis uses 139 fb−1 of Run 2 pp collisions at sqrt(s) = 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the τ lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying τ leptons significantly improves the sensitivity reach for Z → lτ decays. The Z → lτ branching fractions are constrained in this analysis to BðZ → eτÞ < 7.0 × 10−6 and BðZ → μτÞ < 7.2 × 10−6 at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: BðZ → eτÞ < 5.0 × 10−6 and BðZ → μτÞ < 6.5 × 10−6 at 95% confidence level.