Probing $\tau$ lepton dipole moments at FCC-ee

Abstract

We study the sensitivity of future high-luminosity $e^{+}{e}^{-}$ colliders to the electromagnetic and weak dipole moments of the $\tau$ lepton within the Standard Model Effective Field Theory. We analyse several complementary processes across the FCC-ee operating stages and show that the exceptional luminosity and clean environment of future $e^{+}{e}^{-}$ colliders allow for substantial improvements over current limits from LEP and the LHC. The $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ process provides excellent sensitivity to weak dipole operators through resonant $Z$ production and $Z$-pole energy scans, while $\gamma \gamma \to {\tau }^{+}{\tau }^{-}$ yields the strongest constraint on the anomalous magnetic moment owing to the dominance of linear interference effects in the near-threshold region. Higher-energy stages improve the sensitivity to the $\tau$ electric dipole moment. The radiative processes $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}\gamma$ and $H\to {\tau }^{+}{\tau }^{-}\gamma$ provide complementary, though subleading, probes of $\tau$ dipole operators. Combining all channels, we estimate the statistical reach at the 95% CL to be $|\Delta a_{\tau }|<2.8\times 10^{-5}$, $|d_{\tau }|<2.5\times 10^{-18}e\mathrm{cm}$, $|\Delta a_{\tau }^Z|<1.0\times 10^{-5}$, and $|d_{\tau }^Z|<4.4\times 10^{-19}e\mathrm{cm}$, improving upon the current bounds by several orders of magnitude. These projections quantify the potential statistical reach of the FCC-ee program, motivating dedicated studies incorporating detector effects, systematic uncertainties, and higher-order theoretical predictions.

46 pages, 19 figures