Search for lepton-flavor-violating decays of the $Z$ boson into a $\tau$ 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 a muon and a hadronically decaying $\tau$ 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 $\sqrt{s}=13$ TeV. No 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: $\mathcal{B}(Z\to e\tau )<5.8\times {10}^{-5}$ and $\mathcal{B}(Z\to \mu \tau )<2.4\times {10}^{-5}$. This is the first limit on $\mathcal{B}(Z\to e\tau )$ with ATLAS data. The upper limit on $$\mathcal{B}$ (Z\to \mu\tau)$ 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 $\sqrt{s}=8$ TeV and the combined upper limit at 95% confidence level is $\mathcal{B}(Z\to \mu \tau )<1.3\times {10}^{-5}$.