Test of CP Symmetry in the Neutral Decays of Λ via J/ψ → Λ Λ ̄

Using (10 087±44)×10⁶ J/ψ events collected with the BESIII detector, a full angular analysis is carried out on the process J/ψ→ΛΛ¯→nπ⁰p¯π⁺+c.c. The decay parameters α₀ for Λ→nπ⁰ and α¯₀ for Λ¯→n¯π⁰ are measured to be 0.668±0.007±0.002 and -0.677±0.007±0.003, respectively, yielding the most precise test for CP symmetry of neutral decays of Λ, A_CP⁰=(α₀+α¯₀)/(α₀-α¯₀), to be -0.006±0.007±0.002. The ratios α₀/α_- and α¯₀/α₊ are determined to be 0.884±0.013±0.006 and 0.885±0.013±0.004, where α_- and α₊ are the decay parameters of Λ→pπ^- and Λ¯→p¯π⁺, respectively. The ratios, found to be smaller than unity by more than 5σ, confirm the presence of the ΔI=3/2 transition in the Λ and Λ¯ decays, which is expected to improve the theoretical calculations for isospin amplitudes, and A_CP, in hyperon decays. In all results, the first and second uncertainties are statistical and systematic, respectively.