Aad, G.Abbott, B.Abbott, D.C.Abud, A.A.Abeling, K.Abhayasinghe, D.K.Abidi, S.H.Zeid, O.S.A.Abraham, N.L.Abramowicz, H.Abreu, H.Abulaiti, Y.Acharya, B.S.Achkar, B.Adam, L.Bourdarios, C.A.Adamczyk, L.Adamek, L.Adelman, J.Adiguzel, A.et al.2022-04-042022-04-042021Physical Review Letters, 2021; 127(17):271801-1-271801-200031-90071079-7114https://hdl.handle.net/2440/134749Published 28 December 2021A search for lepton-flavor-violating Z → eτ and Z → μτ 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 ffiffiffi s p ¼ 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the τ lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying τ leptons significantly improves the sensitivity reach for Z → lτ decays. The Z → lτ branching fractions are constrained in this analysis to BðZ → eτÞ < 7.0 × 10−6 and BðZ → μτÞ < 7.2 × 10−6 at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: BðZ → eτÞ < 5.0 × 10−6 and BðZ → μτÞ < 6.5 × 10−6 at 95% confidence level.en© 2021 CERN, for the ATLAS Collaboration. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3ATLAS CollaborationJournal article10.1103/PhysRevLett.127.2718012022-03-31577596Jackson, P. [0000-0002-0847-402X]Kong, A.X.Y. [0000-0001-8063-8765]McDougall, A.E. [0000-0001-7713-2800]Oliver, J.L. [0000-0002-0713-6627]Potti, H. [0000-0002-0800-9902]Ruggeri, T.A. [0000-0001-9941-1966]Sato, K. [0000-0001-8988-4065]