CP violation angles from H$ oττ$ decays at FCC-ee

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.

💡 Research Summary

The paper investigates the potential of the Future Circular Collider operating in electron‑positron mode (FCC‑ee) to measure CP‑violation in the Higgs‑tau Yukawa interaction through the decay H → τ⁺τ⁻. The study focuses on the dominant Higgs production mode at √s = 240 GeV, namely associated ZH production, where an integrated luminosity of 10.8 ab⁻¹ would yield roughly two million Higgs bosons.

Two theoretical frameworks are employed. First, an anomalous‑coupling parametrisation modifies the Hττ vertex by introducing a CP‑mixing angle ϕ_{ττ} defined via κ_τ cos ϕ_{ττ}+i γ₅ \tildeκ_τ sin ϕ_{ττ}. In this picture ϕ_{ττ}=0° corresponds to a pure CP‑even Higgs, 90° to a pure CP‑odd Higgs, and intermediate values to mixed states. Second, a Standard Model Effective Field Theory (SMEFT) approach includes dimension‑six operators that affect the Higgs‑tau coupling (O_{eH}) and the Higgs‑gauge boson interactions (O_{HF W}, O_{HF WB}, O_{HeB}). CP‑odd effects arise from the imaginary parts of the corresponding Wilson coefficients, while CP‑even contributions appear only quadratically in the cross‑section.

Monte‑Carlo event samples are generated at leading order. For the anomalous‑coupling case, three benchmark angles (0°, 45°, 90°) are simulated with Pythia 8. SMEFT signal samples are produced using the SMEFTsim 3.0 model interfaced to MadGraph 5.3.1, with the TauDecay package providing helicity‑aware τ decay amplitudes. The dominant backgrounds—ZZ, WW, and e⁺e⁻ → q\bar q via γ/Z—are also generated with Pythia 8 (or Whizard + Pythia 6 for sub‑dominant processes). Initial‑ and final‑state radiation, as well as beam‑strahlung effects, are included consistently.

Detector response is modelled with Delphes 3.5.1pre05 using the IDEA detector concept, which features a large‑volume drift chamber and a dual‑readout calorimeter. Events are classified according to the Z decay (ℓℓ or qq) and the τ decay modes. Hadronic τ decays (τ_h) are reconstructed from jets using the exclusive Durham algorithm; a net charge of ±1 and an invariant mass below 3 GeV are required. Leptonic τ decays (τ_ℓ) are identified via isolated electrons or muons.

The CP‑sensitive observable is the angle ϕ_{CP} between the decay planes of the two τ leptons in the Higgs rest frame. The analysis follows the method of Refs.