Testing Bell inequalities and probing quantum entanglement at CEPC

We study quantum entanglement and test violation of Bell-type inequality at the Circular Electron Positron Collider (CEPC), which is one of the most attractive future colliders. It's a promising particle collider designed to search new physics, make Standard Model (SM) precision measurements, and serving as a Higgs factory. Our study is based on a fast simulation of the Z boson pair production from Higgs boson decay at sqrt{s} = 250 GeV. The detector effects are also included in the simulation. The spin density matrix of the joint ZZ system is parametrized using irreducible tensor operators and reconstructed from the spherical coordinates of the decay leptons. To test Bell inequalities, we construct observable quantities for the H → ZZ* process in CEPC by using the (Collins-Gisin-Linden-Massar-Popescu) CGLMP inequality, whose value is determined from the density matrix of the Z boson pairs. The sensitivity of the Bell inequality violation is observed with more than 1σ and the presence of the quantum entanglement is probed with more than 2σ confidence level.