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 − → HZ . The analysis is performed assuming the future lepton collider CEPC reference detector operating at $$\sqrt s $$ = 240 GeV with statistics of 5.6 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 effects are taken into account in WHIZARD. We develop a novel data-analysis method that exploits the initial state radiation induced shift in the reconstructed event energy E RECO as an additional source of information on the Higgs CP structure. The method combines the initial state radiation sensitive event-energy information with three angular observables in a multidimensional likelihood analysis. The expected upper limit on the CP -odd Higgs admixture is improved by 15% compared with the standard analysis based on three angular variables.