Search for lepton-flavor violation in Z-boson decays with τ Leptons with the ATLAS detector

Abstract

A search for lepton-flavor-violating Z → eꞇ and Z → μꞇ 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 √s = 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the ꞇ lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying ꞇ leptons significantly improves the sensitivity reach for Z → ℓꞇ decays. The Z → ℓ branching fractions are constrained in this analysis to B(Z → eꞇ) < 7.0 × 10-6 and B(Z → μꞇ) < 7.2 × 10-6 at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: B(Z → eꞇ) < 5.0 × 10-6 and B(Z → μꞇ) < 6.5 × 10-6 at 95% confidence level