CP violation angles from H$\to\tau\tau$ decays at FCC-ee

TL;DR

This paper assesses FCC-ee's potential to measure CP violation in Higgs decays to tau pairs, achieving high precision in determining the CP state and constraining new physics operators.

Contribution

It provides the first detailed sensitivity study of FCC-ee to Higgs CP violation via $H o\tau\tau$ decays, including EFT interpretation and comparison with other measurements.

Findings

Projected CP phase measurement precision of ±2.5° at 68% CL.

Dominant sensitivity from one-prong hadronic tau decays.

Derived limits on CP-odd operators within EFT framework.

Abstract

Measuring charge-parity (CP) violation in Higgs-fermion interactions is a key target of future precision Higgs programs. The decay $H\to\tau\tau$ is particularly sensitive to the CP structure of the Higgs Yukawa coupling via $\tau$ spin correlation, while the clean environment of $e^+e^-$ collisions at the Future Circular Collider (FCC) enables accurate reconstruction of CP-sensitive observables. In this letter, we study the sensitivity of FCC-ee to the Higgs CP state using $H\to\tau\tau$ events produced in associated $ZH$ production at $\sqrt{s}=240$ GeV. In the anomalous-coupling parametrization, we project a precision of $\Delta\phi_{\tau\tau}=\pm 2.5^\circ$ at 68 % CL, with the dominant contribution arising from one-prong hadronic $\tau$ decays. We further interpret the analysis in the Effective Field Theory framework, deriving expected limits on the CP-odd operators, and compare them with electron and magnetic dipole moment measurements and the anomalous-coupling approach.