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

Abstract

Measuring charge-parity (CP) violation in Higgs-fermion interactions is a key target of future precision Higgs programs. The decay $H\toττ$ is particularly sensitive to the CP structure of the Higgs Yukawa coupling via $τ$ 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 $H\toττ$ events produced in associated $ZH$ production at $\sqrt{s}=240$ GeV. In the anomalous-coupling parametrization, we project a precision of $Δφ_{ττ}=\pm 2.5^\circ$ at 68 % CL, with the dominant contribution arising from one-prong hadronic $τ$ 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.

Figures (5)

• Figure 1: Feynman diagram for ZH events in $e^+e^-$ collisions, where the Z boson decays to lepton (electrons or muons considered in this study) or quark pairs and the Higgs decays to a $\tau$ pair. An additional source of CPV (represented by a black dot) could modify the $H \tau \tau$ interaction.
• Figure 2: Distributions for the anomalous coupling signal and backgrounds in the CP-sensitive angular variable. On the top figure: events with Z$\to\ell\ell$. On the bottom figure: events with Z$\to qq$. In both plots, only one-prong hadronic $\tau$ decays are selected. The full kinematic event selection for the respective channels is applied.
• Figure 3: Signal distributions for the SMEFT Yukawa operators in the CP-sensitive angular variable. Only one-prong hadronic $\tau$ decays are selected, while both Z decay channels are combined. Only the object definition and jet clustering are applied here.
• Figure 4: Two-dimensional scan of linear operators $Re(C_{eH})$ and $Im(C_{eH})$, which are the most influential on the CP-odd vertex. The plot shows little correlation between the two.
• Figure 5: Summary of one-dimensional limits on both linear, profiled linear, and linear plus quadratic EFT operators that act on the H$\to\tau\tau$ vertex.