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

Using a sample of $1.31times 10^9$ $J/psi$ events collected with the BESIII detector, we report the first observation of spin polarization of $Lambda$ and $barLambda$ hyperons from the coherent production in the $J/psitoLambdabarLambda$ decay. We measure the phase between the hadronic form factors to be $DeltaPhi=(42.4pm0.6pm0.5)^circ$. The decay parameters for $Lambdato ppi^-$ ($alpha_-$), $barLambdatobar ppi^+$ ($alpha_+$) and $barLambdatobar npi^0$ ($baralpha_0$) are measured to be $alpha_-=0.750pm0.009pm0.004$, $alpha_+=-0.758pm0.010pm0.007$ and $baralpha_0=-0.692pm0.016pm0.006$, respectively. The obtained value of $alpha_-$ is higher by $(17pm 3)%$ than the current world average. In addition, the $CP$ asymmetry of $-0.006pm0.012pm0.007$ is extracted with substantially improved precision. The ratio $bar{alpha}_0/alpha_{+} = 0.913pm 0.028 pm 0.012$ is also measured.