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

TL;DR

This paper reports the observation of an unexpected enhancement in the cross section near the mass threshold in the process $e^{+}e^{-} ightarrow ext{Lambda}ar{ ext{Lambda}}$, challenging existing theoretical predictions and providing more precise measurements.

Contribution

The study provides the first measurement of the cross section just above the Lambda-antLambda threshold and observes a significant enhancement not predicted by current theories.

Findings

Cross section at 2.2324 GeV is 305±45+66−36 pb, above theoretical expectations.

Measured cross sections at higher energies agree with previous results but with improved accuracy.

Effective electromagnetic form factors of Lambda are derived from the data.

Abstract

The process $e^{+}e^{-}\rightarrow\Lambda\bar{\Lambda}$ 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 $\Lambda\bar{\Lambda}$ mass threshold, to be $305\pm45^{+66}_{-36}$ 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.