Measurement of the Cross Section for $e^{+}e^{-}rightarrowLambdabarLambda$ and Observation of the Decay $psi(3770)rightarrowLambdabarLambda$

The Born cross section of the process $e^{+}e^{-}rightarrowLambdabarLambda$ 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 fb$^{-1}$ collected with the BESIII detector at the BEPCII collider. Describing the energy dependence of the cross section requires a justification from the $psi(3770)rightarrowLambdabarLambda$ decay, which is fitted with a significance of 4.9$sigma$ with the systematic uncertainty included. The lower bound on its branching fraction is $2.4times10^{-6}$ at the 90% confidence level (C.L.), at least five times larger than expected from predictions using a simple scaling approach. This result indicates the importance of effects from vector charmonium(-like) when interpreting data in terms of {it e.g.}, electromagnetic structure observables. There are no definite conclusions about the interplay with other vector charmonium(-like), and we set 90% C.L.upper limits for the products of the electronic widths and the branching fractions.