Σ+ and ¯Σ− Polarization in the J/ψ and ψ(3686) Decays

From 1310.6×106  J/ψ and 448.1×106  ψ(3686) events collected with the BESIII experiment, we report the first observation of Σ+ and ¯Σ− spin polarization in e+e−→J/ψ[ψ(3686)]→Σ+¯Σ− decays. The relative phases of the form factors ΔΦ have been measured to be (−15.5±0.7±0.5)° and (21.7±4.0±0.8)° with J/ψ and ψ(3686) data, respectively. The nonzero value of ΔΦ allows for a direct and simultaneous measurement of the decay asymmetry parameters of Σ+→pπ0(α0=−0.998±0.037±0.009) and ¯Σ−→¯pπ0(¯α0=0.990±0.037±0.011), the latter value being determined for the first time. The average decay asymmetry, (α0−¯α0)/2, is calculated to be −0.994±0.004±0.002. The CP asymmetry ACP,Σ=(α0+¯α0)/(α0−¯α0)=−0.004±0.037±0.010 is extracted for the first time, and is found to be consistent with CP conservation