Search for lepton-flavor violation in Z-boson decays with τ 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 $139{\mathrm{fb}}^{-1}$ of Run 2 $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ 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 $Z\to \ell \tau$ decays. The $Z\to \ell \tau$ branching fractions are constrained in this analysis to $B(Z\to e\tau )<7.0\times {10}^{-6}$ and $B(Z\to \mu \tau )<7.2\times {10}^{-6}$ at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: $B(Z\to e\tau )<5.0\times {10}^{-6}$ and $B(Z\to \mu \tau )<6.5\times {10}^{-6}$ at 95% confidence level.

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ATLAS Collaboration, Bawa, H. S., & Gao, Y. S. (2021). Search for lepton-flavor violation in Z-boson decays with τ leptons with the ATLAS Detector. Physical Review Letters, 127(27), 271801. https://doi.org/10.1103/PhysRevLett.127.271801