Probing Lepton Flavor Violation at FCC-ee
We investigate the prospects for probing Lepton Flavor Violation (LFV) at FCC-ee within the Standard Model Effective Field Theory (SMEFT) framework. The approximately $2 \times 10^{11}$ $τ$-lepton pairs expected at the $Z$-pole run of FCC-ee could improve current limits on the branching fractions of $τ\to3\ell$ and $τ\to\ellγ$ ($\ell=e,μ$) by factors ranging from 30 to 800, depending on the specific channel. We compare the resulting new-physics reach with that obtained from searches for $Z \to τ\ell$ and $h \to τ\ell$, which have so far received greater attention in studies of LFV at future $e^+e^-$ colliders. We find that $τ$ decays can surpass both the current and the projected sensitivity of $Z\toτ\ell$ searches to the same LFV interactions, while probing a broader range of SMEFT operators. We further show that, for LFV Yukawa-like interactions, direct searches for $h\toτ\ell$ provide the strongest sensitivity. By contrast, for semileptonic operators that radiatively induce LFV Higgs decays, this hierarchy can be reversed, with $τ$ decays providing the leading probe.