Testing Higgs $CP$ properties at the CEPC with an additional ISR parameter

TL;DR

The paper investigates how to constrain CP-odd admixtures in the Higgs–Z boson coupling at CEPC by studying $e^+e^- \to ZH$ with $Z\to\mu^+\mu^-$ at $\sqrt{s}=240$ GeV and $5.6~\text{ab}^{-1}$. It combines three angular observables with a new ISR-sensitive variable, the reconstructed energy $E_{\text{RECO}}$, and uses the Higgs Characterisation framework within a WHIZARD+DELPHES workflow to generate CP-even, CP-odd, and mixed scenarios across $\tilde{c}_{ZZ}$ values. The analysis employs a four-bin, four-interval likelihood in $(\phi,\cos\theta_1,\cos\theta_2)$ augmented by $E_{\text{RECO}}$ to extract a CP-odd sensitivity, explicitly modeling $|M|^2 = |M^+|^2 + 2\tilde{c}_{ZZ}\Re(M^+(M^-)^{\dag}) + \tilde{c}_{ZZ}^2|M^-|^2$. The main result is an upper limit $\tilde{c}_{ZZ} = 0.058$ at 68% CL, a roughly 20% improvement over angular observables alone, demonstrating that ISR-based observables can meaningfully enhance CP tests at future Higgs factories and can be extended to other production/decay channels.

Abstract

We evaluate the experimental sensitivity to the $CP$-odd admixture in the standard Higgs boson for the process $e^+e^-\to HZ$. The analysis is performed assuming the future lepton collider CEPC reference detector operating at $\sqrt{s}=240~\text{GeV}$ with statistics of $5.6~\text{ab}^{-1}$. Using the WHIZARD generator with the Higgs Characterisation model and the DELPHES detector simulation framework we obtain data samples for different $CP$-odd Higgs admixture parameters $\tilde{c}_{ZZ}$. The initial state radiation (ISR) effects are taken into account in WHIZARD. Angular and ISR energy shift variables are used to distinguish the $CP$-odd and $CP$-even Higgs components. The upper limit on the $CP$-odd Higgs admixture is improved by 20\% with this method compared to the standard analysis with three angular variables.

Figures (7)

• Figure 1: Kinematics for the $e^+e^-\to HZ,~Z \to \mu^+\mu^-$ process.
• Figure 2: Distributions of the observables for the different Higgs boson $CP$ properties in the $e^+e^-\to HZ,~Z \to \mu^+\mu^-$ process at $\sqrt{s}=240~\text{GeV}$. The $\phi$ (top-left picture), $E_{\text{RECO}}$ (top-right picture), $\cos \theta_1$ (bottom-left), $\cos \theta_2$ (bottom-right) distributions are shown. The samples with different Higgs boson $CP$ properties are shown: red circles for $0^+$, blue squares for $0^-$ and purple triangles for $1/\sqrt{2}\left( \ket{0^+} + \ket{0^-}\right)$ quantum numbers.
• Figure 3: Dependence of the cross section $\sigma(\sqrt{s})$ on the center of mass energy for different Higgs boson $CP$ properties in the $e^+e^-\to HZ$ process. The red circles correspond to $0^+$ sample, blue squares to $0^-$, purple triangles to $1/\sqrt{2}\left( \ket{0^+} + \ket{0^-}\right)$ admixture. The equal cross section are assumed at $\sqrt{s}=240~\text{GeV}$.
• Figure 4: $E_{\text{true}}$ vs $E_{\text{RECO}}$ for the signal process $e^+e^-\to HZ$ after applying preselection on $M_{\mu^+\mu^-}$.
• Figure 5: $\Delta = E_{\text{true}}-E_{\text{RECO}}$ distribution before (red circles) and after (blue squares) applying preselection on $M_{\mu^+\mu^-}$.