Stringent test of $CP$ symmetry in ${\Sigma }^{+}$ hyperon decays

Abstract

The non-leptonic two-body weak decays ${\Sigma }^{+}\to p{\pi }^0$ and ${\bar{\Sigma }}^{-}\to \bar{p}{\pi }^0$ are investigated, utilizing $(1.0087\pm 0.0044)\times 10^{10}$ $J/\psi$ events and $(2.7124\pm 0.0143)\times 10^9$ $\psi (3686)$ events collected by BESIII experiment. The precision of the weak-decay parameters for the decays ${\Sigma }^{+}\to p{\pi }^0$ (${\alpha }_0$) and ${\bar{\Sigma }}^{-}\to \bar{p}{\pi }^0$ (${\bar{\alpha }}_0$) is improved by a factor of three compared to the previous world average. Furthermore, the quantum-entangled ${\Sigma }^{+}{\bar{\Sigma }}^{-}$ system enables the most precise test of $CP$ symmetry for the decay ${\Sigma }^{+}\to p{\pi }^0$, through the asymmetry observable $A_{CP}=({\alpha }_0+{\bar{\alpha }}_0)/({\alpha }_0-{\bar{\alpha }}_0)$ that is measured to be $-0.0118\pm 0.0083_{\mathrm{stat}}\pm 0.0028_{\mathrm{syst}}$. Assuming $CP$ conservation, the average decay parameter is determined to be $⟨{\alpha }_0⟩=({\alpha }_0-{\bar{\alpha }}_0)/2=-0.9869\pm 0.0011_{\mathrm{stat}}\pm 0.0016_{\mathrm{syst}}$, which is the most precise measurement of the asymmetry decay parameters in baryon sectors. The angular dependence of the ratio of the polarization of the ${\Sigma }^{+}$ in both $J/\psi$ and $\psi (3686)$ decays is studied for the first time.

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