Bell inequality is violated in charmonium decays

The experimental data on the helicity amplitudes of charmonium decays allow us to measure entanglement in final state spin correlations and test possible violations of the Bell inequality. We find that the Bell inequality is violated with a significance of 5σ or more in the decays ηc,χc0,J/ψ→Λ+Λ¯, J/ψ→Ξ−+Ξ¯+,Ξ0+Ξ¯0,Σ−+Σ¯+,Σ0+Σ¯0, ψ(3686)→Ξ−+Ξ¯+,Σ−+Σ¯+,Σ0+Σ¯0, χc0,χc1→ϕ+ϕ. The decays ψ(3686)→Λ+Λ¯ and Ξ0+Ξ¯0 show the same violation but with less significance. The decay ψ(3686)→Ω−+Ω¯+ displays entanglement. These results firmly establish the presence of entanglement and quantum nonseparability at high energies, in a setting with particles of different spins and interacting through electroweak and strong interactions. In addition, the relatively long lifetime of some of the strange baryons produced in the decays provides a natural probe to test whether quantum spin correlations remain after the particles have interacted with the beam pipe and the first few layers of the detector. Published by the American Physical Society 2024