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

TL;DR

This paper reports a search for lepton-flavor-violating decays of the Z boson into electron or muon and tau leptons using ATLAS data, setting the most stringent limits to date on these rare processes.

Contribution

It combines new analysis of leptonically decaying tau leptons with previous results, significantly improving the sensitivity to Z boson lepton-flavor-violation decays.

Findings

Set upper limits on branching fractions: <7.0×10⁻⁶ for Z→eτ and <7.2×10⁻⁶ for Z→μτ.

Combined analysis with previous data to establish the strongest constraints to date.

Achieved improved sensitivity by including leptonic tau decay channels.

Abstract

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 $Z\to\ell\tau$ decays. The $Z\to\ell\tau$ branching fractions are constrained in this analysis to $\mathcal{B}(Z\to e\tau)<7.0\times10^{-6}$ 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: $\mathcal{B}(Z\to e\tau)<5.0\times10^{-6}$ and $\mathcal{B}(Z\to \mu\tau)<6.5\times10^{-6}$ at 95% confidence level.