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 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 $36.1\text{}\text{}{\mathrm{fb}}^{-1}$ at a center-of-mass energy of $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$. No statistically 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: $B\left(Z\to e\tau \right)<5.8×{10}^{-5}$ and $B\left(Z\to \mu \tau \right)<2.4×{10}^{-5}$. This is the first limit on $B\left(Z\to e\tau \right)$ with ATLAS data. The upper limit on $B\left(Z\to \mu \tau \right)$ is combined with a previous ATLAS result based on $20.3\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=8\text{}\text{}\mathrm{TeV}$ and the combined upper limit at 95% confidence level is $B\left(Z\to \mu \tau \right)<1.3×{10}^{-5}$.