Search for charged-lepton-flavour violation in Z-boson decays with the ATLAS detector
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ATLAS Collaboration. Search for charged-lepton-flavour violation in Z-boson decays with the ATLAS detector. Nat. Phys. 17, 819–825 (2021). [https://doi.org/10.1038/s41567-021-01225-z]
SponsorshipANPCyT; YerPhI, Armenia; Australian Research Council; BMWFW; Austrian Science Fund (FWF); Azerbaijan National Academy of Sciences (ANAS); SSTC, Belarus; Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPQ); Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Natural Sciences and Engineering Research Council of Canada (NSERC); Canada Foundation for Innovation; CERN; ANID, Chile; Chinese Academy of Sciences; MOST; National Natural Science Foundation of China (NSFC); Departamento Administrativo de Ciencia, Tecnologia e Innovacion Colciencias; Ministry of Education, Youth & Sports - Czech Republic; MPO CR; Czech Republic Government; DNRF; Danish Natural Science Research Council; NRC KI, Russia Federation; JINR; Ministry of Education, Science & Technological Development, Serbia; MSSR, Slovakia; Slovenian Research Agency - Slovenia; MIZS, Slovenia; DST/NRF, South Africa; Spanish Government; SRC; Wallenberg Foundation, Sweden; SERI; Swiss National Science Foundation (SNSF); Cantons of Bern and Geneva, Switzerland; Ministry of Science and Technology, Taiwan; Ministry of Energy & Natural Resources - Turkey; UK Research & Innovation (UKRI); Science & Technology Facilities Council (STFC); United States Department of Energy (DOE); National Science Foundation (NSF); BCKDF; CANARIE; Compute Canada and CRC, Canada; European Research Council (ERC); European Commission European Commission Horizon Marie Sklodowska-Curie Actions and COST, EU; French National Research Agency (ANR); German Research Foundation (DFG); European Commission; Alexander von Humboldt Foundation; Thales Group; Aristeia programmes; European Commission; Greek NSRF, Greece; BSF-NSF; German-Israeli Foundation for Scientific Research and Development; La Caixa Foundation; CERCA Programme Generalitat de Catalunya and PROMETEO; GenT Programmes Generalitat Valenciana, Spain; Goran Gustafssons Stiftelse, Sweden; The Royal Society and Leverhulme Trust, UK; WLCG; ATLAS Tier-1 facilities at TRIUMF (Canada); NDGF (Denmark, Norway, Sweden); CC-IN2P3 (France); KIT/GridKA (Germany); INFN-CNAF (Italy); Netherlands Government; PIC (Spain); ASGC (Taiwan); RAL (UK); BNL (USA); Tier-2 facilities worldwide and large non-WLCG; National Research Centre (NRC)
Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign of undiscovered particles or a yet unknown type of interaction. Here the ATLAS experiment at the Large Hadron Collider at CERN reports a constraint on lepton-flavour-violating effects in weak interactions, searching for Z-boson decays into a tau lepton and another lepton of different flavour with opposite electric charge. The branching fractions for these decays are measured to be less than 8.1 x 10(-6) (e tau) and 9.5 x 10(-6) (mu tau) at the 95% confidence level using 139 fb(-1) of proton-proton collision data at a centre-of-mass energy of root s = 13 TeV and 20.3 fb(-1) at root s = 8 TeV. These results supersede the limits from the Large Electron-Positron Collider experiments conducted more than two decades ago.