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.

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