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

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.