Search for charged-lepton-flavour violation in $Z$-boson decays with the ATLAS detector

Abstract

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 for 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 $8.1\times 10^{-6}$ ($e\tau$) and $9.5\times 10^{-6}$ ($\mu \tau$) at 95% confidence level using 139 fb${}^{-1}$ of proton-proton collision data at a centre-of-mass energy $\sqrt{s}=13$ TeV and 20.3 fb${}^{-1}$ at $\sqrt{s}=8$ TeV. These results supersede the limits from the Large Electron-Positron Collider experiments conducted more than two decades ago.

36 pages in total, author list starting page 20, 4 figures, 4 tables, published in Nature Physics. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2018-36/

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• Magnetic Moments of Leptons, Charged Lepton Flavor Violations and Dark Matter Phenomenology of a Minimal Radiative Dirac Neutrino Mass Model
• Flavor Physics at the CEPC: a General Perspective
• Constraining Lepton Flavor Violating Higgs Couplings at the HL-LHC in the Vector Boson Fusion Channel
• Renormalization group improved implications of semileptonic operators in SMEFT
• Snowmass 2021 White Paper: Charged lepton flavor violation in the tau sector
• Exploring Optimal Sensitivity of Lepton Flavor Violating Effective Couplings at the $e^{+}{e}^{-}$ Colliders
• SMEFT analysis of charged lepton flavor violating $B$-meson decays
• RGE effects on the LFV scale from meson decays
• Z lepton flavour violation as a probe for new physics at future $e^{+}{e}^{-}$ 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
• The \texttau{} challenge at FCC-ee
• Lepton flavor violating top quark FCNC processes at the $\mu$TRISTAN