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From 1310.6 × 10 6 J=ψ and 448.1 × 10 6 ψð3686Þ events collected with the BESIII experiment, we report the first observation of Σ þ andΣ − spin polarization in e þ e − → J=ψ½ψð3686Þ → Σ þΣ− decays. The relative phases of the form factors ΔΦ have been measured to be ð−15.5 AE 0.7 AE 0.5Þ°and ð21.7 AE 4.0 AE 0.8Þ°with J=ψ and ψð3686Þ data, respectively. The nonzero value of ΔΦ allows for a direct and simultaneous measurement of the decay asymmetry parameters of Σ þ → pπ 0 ðα 0 ¼ −0.998 AE 0.037 AE 0.009Þ andΣ − →pπ 0 ðᾱ 0 ¼ 0.990 AE 0.037 AE 0.011Þ, the latter value being determined for the first time. The average decay asymmetry, ðα 0 −ᾱ 0 Þ=2, is calculated to be −0.994 AE 0.004 AE 0.002. The CP asymmetry A CP;Σ ¼ ðα 0 þᾱ 0 Þ=ðα 0 −ᾱ 0 Þ ¼ −0.004 AE 0.037 AE 0.010 is extracted for the first time, and is found to be consistent with CP conservation.