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

A search for lepton-flavor-violating $Z\to e\tau$ and $Z\to \mu \tau$ 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 $\tau$-lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying $\tau$-leptons significantly improves the sensitivity reach for $Z\to \mathrm{\ell }\tau$ decays. The $Z\to \mathrm{\ell }\tau$ branching fractions are constrained in this analysis to $\mathcal{B}(Z\to e\tau )<7.0\times {10}^{-6}$ and $$\mathcal{B}$(Z\to \mu\tau)<7.2\times10^{-6}$ at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: $\mathcal{B}(Z\to e\tau )<5.0\times {10}^{-6}$ and $$\mathcal{B}$(Z\to \mu\tau)<6.5\times10^{-6}$ at 95% confidence level.