Probing Lepton Flavor Violation at FCC-ee

Abstract

We investigate the prospects for probing Lepton Flavor Violation (LFV) at FCC-ee within the Standard Model Effective Field Theory (SMEFT) framework. The approximately $2\times 10^{11}$ $\tau$-lepton pairs expected at the $Z$-pole run of FCC-ee could improve current limits on the branching fractions of $\tau \to 3\ell$ and $\tau \to \ell \gamma$ ($\ell =e,\mu$) 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 $Z\to \tau \ell$ and $h\to \tau \ell$, which have so far received greater attention in studies of LFV at future $e^{+}{e}^{-}$ colliders. We find that $\tau$ decays can surpass both the current and the projected sensitivity of $Z\to \tau \ell$ 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 $h\to \tau \ell$ provide the strongest sensitivity. By contrast, for semileptonic operators that radiatively induce LFV Higgs decays, this hierarchy can be reversed, with $\tau$ decays providing the leading probe.

arXiv comment8 pages, 6 figures