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

Abstract

A search for lepton-flavor-violating $Z\to eτ$ and $Z\toμτ$ 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 $Z\to\ellτ$ decays. The $Z\to\ellτ$ branching fractions are constrained in this analysis to $\mathcal{B}(Z\to eτ)<7.0\times10^{-6}$ and $\mathcal{B}(Z\to μτ)<7.2\times10^{-6}$ at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: $\mathcal{B}(Z\to eτ)<5.0\times10^{-6}$ and $\mathcal{B}(Z\to μτ)<6.5\times10^{-6}$ at 95% confidence level.