Z lepton flavour violation as a probe for new physics at future $e^{+}{e}^{-}$ colliders

Abstract

In this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the $Z$ boson, $Z\to {\ell }_i{\ell }_j$, at the proposed circular $e^{+}{e}^{-}$ colliders CEPC and FCC-ee. Both projects plan to run at the $Z$-pole as a Tera Z factory, i.e., collecting $\mathcal{O}\left(10^{12}\right)$ $Z$ decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV $Z$ decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV $\mu$ and $\tau$ decays on the operators that can induce $Z\to {\ell }_i{\ell }_j$. We find that, while the $Z\to \mu e$ rates are beyond the expected sensitivities, a Tera Z factory is promising for $Z\to \tau \ell$ decays, probing New Physics at the same level of future low-energy LFV observables.

31 pages, 5 figures. References added, typos corrected. Matches published version

References in corpus (13)

• Constraints on two-lepton, two quark operators
• Indirect searches for sterile neutrinos at a high-luminosity Z-factory
• Search for the lepton flavor violating decay $Z\to e\mu$ in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector
• The $\mu \to e\gamma$ decay in a systematic effective field theory approach with dimension 6 operators
• Search for lepton-flavor-violating tau-lepton decays to $\ell \gamma$ at Belle
• Charged Lepton Flavor Violation at the EIC
• The model-discriminating power of lepton flavor violating tau decays
• Lepton flavor violating decays of vector quarkonia and of the $Z$ boson
• Global Analysis of Leptophilic Z' Bosons
• What is Leading Order for LFV in SMEFT?
• New bounds on lepton flavor violating decays of vector mesons and the Z0 boson
• Search for charged-lepton-flavour violation in $Z$-boson decays with the ATLAS detector
• Searching for lepton flavor violating interactions at future electron-positron colliders

Cited by in corpus (2)

• Renormalization group improved implications of semileptonic operators in SMEFT
• Two-body lepton-flavour-violating decays in a 2HDM with soft family-lepton-number breaking