Measurement of the cross section for ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ and evidence of the decay $\psi \left(3770\right)\to \Lambda \overline{\Lambda }$

The Born cross section of the process ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }$ is measured at 33 center-of-mass energies between 3.51 and 4.60 GeV using data corresponding to the total integrated luminosity of $20.0\text{}\text{}{\mathrm{fb}}^{-1}$ collected with the BESIII detector at the BEPCII collider. Describing the energy dependence of the cross section requires a contribution from the $\psi \left(3770\right)\to \Lambda \overline{\Lambda }$ decay, which is fitted with a significance of $4.6-4.9\sigma $ including the systematic uncertainty. The lower bound on its branching fraction is $2.4×{10}^{-6}$ at the 90% confidence level (C.L.), at least an order of magnitude larger than expected from predictions using a scaling based on observed electronic widths. This result indicates the importance of effects from vector charmonium(like) states when interpreting data in terms of e.g., electromagnetic structure observables. The data do not allow for definite conclusions on the interplay with other vector charmonium(like) states, and we set 90% C.L. upper limits for the products of their electronic widths and the branching fractions.