Test of CP Symmetry in the Neutral Decays of Λ via J/ψ→ΛΛ[over ¯]

Using (10 087±44)×10^{6}  J/ψ events collected with the BESIII detector, a full angular analysis is carried out on the process J/ψ→ΛΛ[over ¯]→nπ^{0}p[over ¯]π^{+}+c.c. The decay parameters α_{0} for Λ→nπ^{0} and α[over ¯]_{0} for Λ[over ¯]→n[over ¯]π^{0} 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_{CP}^{0}=(α_{0}+α[over ¯]_{0})/(α_{0}-α[over ¯]_{0}), to be -0.006±0.007±0.002. The ratios α_{0}/α_{-} and α[over ¯]_{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[over ¯]→p[over ¯]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[over ¯] 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.