Polarization and Entanglement in Baryon-Antibaryon Pair Production in Electron-Positron Annihilation

Abstract

Using a sample of $1.31\times 10^9$ $J/\ddot{I}ˆ$ events collected with the BESIII detector, we report the first observation of spin polarization of $\hat{I}›$ and $\bar{\hat{I}}›$ hyperons from the coherent production in the $J/\ddot{I}ˆ\to \hat{I}›\bar{\hat{I}}›$ decay. We measure the phase between the hadronic form factors to be $\hat{I}”\hat{I}¦=(42.4\pm 0.6\pm 0.5{)}^{\circ }$. The decay parameters for $\hat{I}›\to p\ddot{I}{€}^{-}$ ($\hat{I}{\pm }_{-}$), $\bar{\hat{I}}›\to \bar{p}\ddot{I}{€}^{+}$ ($\hat{I}{\pm }_{+}$) and $\bar{\hat{I}}›\to \bar{n}\ddot{I}{€}^0$ ($\bar{\hat{I}}{\pm }_0$) are measured to be $\hat{I}{\pm }_{-}=0.750\pm 0.009\pm 0.004$, $\hat{I}{\pm }_{+}=-0.758\pm 0.010\pm 0.007$ and $\bar{\hat{I}}{\pm }_0=-0.692\pm 0.016\pm 0.006$, respectively. The obtained value of $\hat{I}{\pm }_{-}$ is higher by $(17\pm 3)\%$ than the current world average. In addition, the $CP$ asymmetry of $-0.006\pm 0.012\pm 0.007$ is extracted with substantially improved precision. The ratio $\bar{\hat{I}}{\pm }_0/\hat{I}{\pm }_{+}=0.913\pm 0.028\pm 0.012$ is also measured.

12 pages, 6 figures