First Observation of $\Lambda$ Hyperon Transverse Polarization in $\psi(3686)\to\Lambda\bar\Lambda$

Based on $(448.1\pm2.9)\times10^{6}$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, we present the first observation of spin transverse polarization of $\Lambda$ and $\bar\Lambda$ hyperons produced coherently in the decay $\psi(3686)\to\Lambda(\to p\pi^-)\bar\Lambda(\to\bar p\pi^+)$. The relative phase between the electric and magnetic hadronic form factors is measured to be $\Delta\Phi=(21.0\pm3.7_{\rm stat.}\pm0.8_{\rm syst.})^{\circ}$. The angular distribution parameter $\alpha_{\psi}=0.83\pm0.02_{\rm stat.}\pm0.01_{\rm syst.}$ is determined with a precision improved by a factor of 3.7 compared to the previous measurement. The relative phase between the $S$- and $D$-wave amplitudes for $\Lambda\bar\Lambda$ is observed, and the effective interaction radius is determined to be $0.0450\pm0.0026_{\rm stat.}\pm0.0012_{\rm syst.}$ fm. These results provide new insights into the strong interaction mechanisms and the internal structure of baryons.