Observation of transverse polarization and determination of psionic form factors of the $ \mathrm{\Lambda }$ hyperon at $ \sqrt{s}=3.773\text{}\text{}\mathrm{GeV}$

Using a data sample of $ {e}^{+}{e}^{-}$ collision data corresponding to an integrated luminosity of $ 20.3\text{}\text{}{\mathrm{fb}}^{-1}$ collected by the BESIII detector at the BEPCII collider, we present an observation of transverse polarization and a complete determination of the psionic form factors of the $ \mathrm{\Lambda }$ hyperon in $ {e}^{+}{e}^{-}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ decay with the entangled $ \mathrm{\Lambda }-\overline{\mathrm{\Lambda }}$ pair at $ \sqrt{s}=3.773\text{}\text{}\mathrm{GeV}$. The relative phase between the psionic form factors is determined to be $ \mathrm{\Delta }\mathrm{\Phi }=(1.53\pm 0.36\pm 0.03)\text{}\text{}\mathrm{rad}$ with a significance of 5.5 $ \sigma $ taking into account systematic uncertainty. This result indicates a nonzero phase between the transition amplitudes of the $ \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ helicity states. Additionally, we measure the angular distribution parameter and the modulus of the ratio between the psionic form factors to be $ \eta =0.86\pm 0.05\pm 0.03$ and $ R(s)=|{G}_{E}(s)/{G}_{M}(s)|=0.47\pm 0.08\pm 0.05$, where the first uncertainty is statistical and the second systematic.