Observation of a cross-section enhancement near mass threshold in $e^{+}e^{-}rightarrowLambdabar{Lambda}$

The process $e^{+}e^{-}rightarrowLambdabar{Lambda}$ is studied using data samples at $sqrt{s}=2.2324$, 2.400, 2.800 and 3.080 GeV collected with the BESIII detector operating at the BEPCII collider. The Born cross section is measured at $sqrt{s}$=2.2324 GeV, which is 1.0 MeV above the $Lambdabar{Lambda}$ mass threshold, to be $305pm45^{+66}_{-36}$ pb, where the first uncertainty is statistical and the second systematic. The substantial cross section near threshold is significantly larger than that expected from theory, which predicts the cross section to vanish at threshold. The Born cross sections at $sqrt{s}$=2.400, 2.800 and 3.080 GeV are measured and found to be consistent with previous experimental results, but with improved precision. Finally, the corresponding effective electromagnetic form factors of $Lambda$ are deduced.