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

TL;DR

This paper reports the first observation of hyperon polarization and entanglement in $J/\psi$ decays, measuring decay parameters, phase differences, and CP asymmetry with high precision using BESIII data.

Contribution

It presents the first measurement of hyperon polarization, phase between form factors, and CP asymmetry in $J/\psi$ decays, providing new insights into baryon-antibaryon production.

Findings

First observation of hyperon polarization in $J/\psi$ decays.

Precise measurement of decay parameters and phase difference.

Improved CP asymmetry measurement with reduced uncertainty.

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 $\Delta\Phi=(42.4\pm0.6\pm0.5)^\circ$. 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 $\alpha_-=0.750\pm0.009\pm0.004$, $\alpha_+=-0.758\pm0.010\pm0.007$ and $\bar\alpha_0=-0.692\pm0.016\pm0.006$, respectively. The obtained value of $\alpha_-$ is higher by $(17\pm 3)\%$ than the current world average. In addition, the $CP$ asymmetry of $-0.006\pm0.012\pm0.007$ is extracted with substantially improved precision. The ratio $\bar{\alpha}_0/\alpha_{+} = 0.913\pm 0.028 \pm 0.012$ is also measured.