Measurement of $\Lambda $ baryon polarization in ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ at $\sqrt{s}=3.773\text{}\text{}\mathrm{GeV}$

Using a data sample of $\psi \left(3770\right)$ events collected with the BESIII detector at BEPCII corresponding to an integrated luminosity of $2.9\text{}\text{}{\mathrm{fb}}^{-1}$, we report a measurement of $\Lambda $ spin polarization in ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ at $\sqrt{s}=3.773\text{}\text{}\mathrm{GeV}$. The significance of polarization is found to be $2\sigma $ including the systematic uncertainty, which implies a zero phase between the transition amplitudes of the $\Lambda \overline{\Lambda }$ helicity states. This phase can be interpreted in terms of psionic form factors, and is determined to be $\Delta {\Phi }^{\Psi }={\Phi }_{E}^{\Psi }-{\Phi }_{M}^{\Psi }=\left(7{1}_{-46}^{+66}±5\right)°$. Similarly, the ratio between the form factors is found to be ${R}^{\psi }=|{G}_{E}^{\Psi }/{G}_{M}^{\Psi }|=0.4{8}_{-0.35}^{+0.21}±0.03$. The first uncertainties are statistical and the second systematic.