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

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 $\mathcal{B}(Z\to e\tau )<7.0\times 10^{-6}$ and $\mathcal{B}(Z\to \mu \tau )<7.2\times 10^{-6}$ at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: $\mathcal{B}(Z\to e\tau )<5.0\times 10^{-6}$ and $\mathcal{B}(Z\to \mu \tau )<6.5\times 10^{-6}$ at 95% confidence level.

28 pages in total, author list starting page 13, 1 figure, 3 tables, submitted to PRL. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2020-28/

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• Flavor Physics at the CEPC: a General Perspective
• Exploring Optimal Sensitivity of Lepton Flavor Violating Effective Couplings at the $e^{+}{e}^{-}$ Colliders
• Flavor physics in SU(5) GUT with scalar fields in the 45 representation
• SMEFT analysis of charged lepton flavor violating $B$-meson decays
• $Z$ boson decays $Z\to {l_i}^{\pm }{l_j}^{\mp }$ and Higgs boson decays $h\to {l_i}^{\pm }{l_j}^{\mp }$ with lepton flavor violation in a $U(1)$ extension of the MSSM
• Lepton Flavor Violation: From Muon Decays to Muon Colliders
• Lepton Flavor Violation at FCC-ee and CEPC
• Search for charged lepton flavor violating Z and Z' boson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV
• Decays $Z\to e_a{e}_b$ in a 3-3-1 model with neutral leptons
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