${\mathrm{\Sigma }}^{+}$ and ${\bar{\mathrm{\Sigma }}}^{-}$ polarization in the $J\mathrm{/}\psi$ and $\psi (3686)$ decays

From $1310.6\times {10}^6$ $J\mathrm{/}\psi$ and $448.1\times {10}^6$ $\psi (3686)$ events collected with the BESIII experiment, we report the first observation of ${\mathrm{\Sigma }}^{+}$ and ${\bar{\mathrm{\Sigma }}}^{-}$ spin polarization in $e^+e^-\rightarrow J/\psi (\psi(3686)) \rightarrow \Sigma^{+} \bar{\Sigma}^{-}$ decays. The relative phases of the form factors $\mathrm{\Delta }\mathrm{\Phi }$ have been measured to be $(-15.5\pm 0.7\pm 0.5{)}^{\circ }$ and $(21.7\pm 4.0\pm 0.8{)}^{\circ }$ with $J\mathrm{/}\psi$ and $\psi (3686)$ data, respectively. The non-zero value of $\mathrm{\Delta }\mathrm{\Phi }$ allows for a direct and simultaneous measurement of the decay asymmetry parameters of ${\mathrm{\Sigma }}^{+}\to p{\pi }^0({\alpha }_0=-0.998\pm 0.037\pm 0.009)$ and $\bar{\Sigma}^{-}\rightarrow \bar{p} \pi^{0}~(\bar{\alpha}0 = 0.990\pm0.037\pm0.011)$, the latter value being determined for the first time. The average decay asymmetry, $({\alpha }_0-{\bar{\alpha }}_0)\mathrm{/}2$, is calculated to be $-0.994\pm 0.004\pm 0.002$. The CP asymmetry $A{\rm CP,\Sigma} = (\alpha_0

• \bar{\alpha}_0)/(\alpha_0 - \bar{\alpha}_0) = -0.004\pm0.037\pm0.010$ is extracted for the first time, and is found to be consistent with CP conservation.