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

Abstract

Using a sample of $1.31\times 10^9$ $J/\psi$ events collected with the BESIII detector, we report the first observation of spin polarization of $\Lambda$ and $\bar{\Lambda }$ hyperons from the coherent production in the $J/\psi \to \Lambda \bar{\Lambda }$ decay. We measure the phase between the hadronic form factors to be $\Delta \Phi =(42.4\pm 0.6\pm 0.5{)}^{\circ }$. The decay parameters for $\Lambda \to p{\pi }^{-}$ (${\alpha }_{-}$), $\bar{\Lambda }\to \bar{p}{\pi }^{+}$ (${\alpha }_{+}$) and $\bar{\Lambda }\to \bar{n}{\pi }^0$ (${\bar{\alpha }}_0$) are measured to be ${\alpha }_{-}=0.750\pm 0.009\pm 0.004$, ${\alpha }_{+}=-0.758\pm 0.010\pm 0.007$ and ${\bar{\alpha }}_0=-0.692\pm 0.016\pm 0.006$, respectively. The obtained value of ${\alpha }_{-}$ 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 ${\bar{\alpha }}_0/{\alpha }_{+}=0.913\pm 0.028\pm 0.012$ is also measured.

arXiv comment12 pages, 6 figures

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