The verification of quantum non-local correlation in ΛΛ- system

The non-locality of quantum correlations is a fundamental feature of quantum theory. The Bell inequality serves as a benchmark for distinguishing between predictions made by quantum theory and local hidden variable theory (LHVT). Recent advancements in photon-entanglement experiments have addressed potential loopholes and have observed significant violations of variants of Bell inequality. However, these violations do not completely disprove LHVT. To further rule out LHVT, experiments involving massive-entangled particles are necessary, in addition to those conducted with massless photons, which interact electromagnetically. In this study, we utilize (10.087±0.044)×10 ⁹ J/ψ events collected with the BESIII detector at the BEPCII collider. We present the non-local correlation test of spin entanglement using a massive ΛΛbar bipartite system, which is formed and decays through strong and weak interactions, respectively. Through measurements of the angular distribution of p pbar in J/ψ→ γη _c and subsequent η _c →Λ(pπ ^- )Λbar(pbar π ⁺ ) cascade decays, a significant violation of LHVT predictions is observed in a loophole-free test of a variant of the Bell inequality, which is given by the Clauser-Horne inequality. The exclusion of LHVT is determined to be significant at a level of 5.2σ.