Search for Lepton-Flavor Violation in Z-Boson Decays with τ Leptons with the ATLAS Detector

Abstract

We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; ANID, Chile; CAS, MOST and NSFC, China; Minciencias, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF and Danish Natural Science Research Council, Denmark; IN2P3-CNRS and CEA-DRF/IRFU, France; Shota Rustaveli National Science Foundation of Georgia, Georgia; BMBF, HGF and MPG, Germany; General Secretariat for Research and Innovation, Greece; RGC and Hong Kong SAR, China; ISF and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; Research Council of Norway, Norway; MNiSW and NCN, Poland; FCT, Portugal; MNE/IFA, Romania; JINR; Ministry of Education and Science of the Russian Federation and NRC KI, Russian Federation; Ministry of Education, Science and Technological Development, Serbia; Ministry of Education, Science, Research and Sport, Slovakia; ARRS and Ministry of Education, Science and Sport, Slovenia; DSI/NRF, South Africa; MICINN, Spain; Swedish Research Council and Wallenberg Foundation, Sweden; Secretariat for Education and Research, Switzerland, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, United Kingdom; DOE and NSF, USA In addition, individual groups and members have received support from BCKDF, CANARIE, Compute Canada and CRC, Canada; COST, ERC, ERDF, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union; Investissements d'Avenir Labex, Investissements d'Avenir Idex and ANR, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Greek NSRF, Greece; BSF-NSF and GIF, Israel; Norwegian Financial Mechanism 2014-2021, Norway; La Caixa Banking Foundation, CERCA Programme Generalitat de Catalunya and PROMETEO and GenT Programmes Generalitat Valenciana, Spain; Göran Gustafssons Stiftelse, Sweden; The Royal Society and Leverhulme Trust, United Kingdom. The crucial computing support from all WLCG partners is acknowledged gratefully, in particular from CERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA), the Tier-2 facilities worldwide and large non-WLCG resource providers. Major contributors of computing resources are listed in Ref. ; A 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 s=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 B(Z→eτ)<7.0×10-6 and B(Z→μτ)<7.2×10-6 at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: B(Z→eτ)<5.0×10-6 and B(Z→μτ)<6.5×10-6 at 95% confidence level. ; ANID ; BSF-NSF ; CEA-DRF ; Cantons of Bern and Geneva ; Czech Republic ; EU-ESF ; GIF, Israel ; GenT Programmes Generalitat Valenciana, Spain ; General Secretariat for Research and Innovation ; IRFU ; La Caixa Banking Foundation ; MSMT ; Ministry of Education, Science, Research and Sport ; Norwegian Financial Mechanism 2014-2021 ; PROMETEO ; RGC ; VSC CR ; Wallenberg Foundation ; National Science Foundation ; U.S. Department of Energy ; Alexander von Humboldt-Stiftung ; Canarie ; Natur og Univers, Det Frie Forskningsråd IN2P3-CNRS ; H2020 Marie Skłodowska-Curie Actions ; Arizona-Nevada Academy of Science ; Compute Canada ; Göran Gustafssons Stiftelser ; Natural Sciences and Engineering Research Council of Canada ; National Research Council Canada ; Canada Foundation for Innovation ; Science and Technology Facilities Council ; Leverhulme Trust ; Royal Society ; European Research Council ; European Cooperation in Science and Technology ; Australian Research Council ; Neurosurgical Research Foundation ; Helmholtz-Gemeinschaft ; Deutsche Forschungsgemeinschaft ; Agence Nationale de la Recherche ; Japan Society for the Promotion of Science ; Ministry of Education, Culture, Sports, Science and Technology ; Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung ; Danmarks Grundforskningsfond ; Fundação de Amparo à Pesquisa do Estado de São Paulo ; National Natural Science Foundation of China ; Fundação para a Ciência e a Tecnologia ; Bundesministerium für Bildung und Forschung ; Chinese Academy of Sciences ; Austrian Science Fund ; Generalitat de Catalunya ; Agencia Nacional de Promoción Científica y Tecnológica ; Nederlandse Organisatie voor Wetenschappelijk Onderzoek ; Bundesministerium für Wissenschaft, Forschung und Wirtschaft ; Ministry of Education and Science of the Russian Federation ; Conselho Nacional de Desenvolvimento Científico e Tecnológico ; Joint Institute for Nuclear Research ; Nella and Leon Benoziyo Center for Neurological Diseases, Weizmann Institute of Science ; Israel Science Foundation ; Instituto Nazionale di Fisica Nucleare ; Narodowe Centrum Nauki ; Javna Agencija za Raziskovalno Dejavnost RS ; Vetenskapsrådet ; Ministarstvo Prosvete, Nauke i Tehnološkog Razvoja ; Ministerstwo Edukacji i Nauki ; Ministry of Science and Technology, Taiwan ; Ministerio de Ciencia e Innovación ; Norges Forskningsråd ; Centre National pour la Recherche Scientifique et Technique ; Horizon 2020 ; British Columbia Knowledge Development Fund ; European Regional Development Fund ; Defence Science Institute ; Council on grants of the President of the Russian Federation ; National Research Center "Kurchatov Institute" ; CERN