Observation of a cross-section enhancement near mass threshold in e+e−→ΛΛ

Abstract

The process e þ e − → ΛΛ is studied using data samples at ffiffi ffi s p ¼ 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 ffiffi ffi s p ¼ 2.2324 GeV, which is 1.0 MeV above the ΛΛ mass threshold, to be 305 AE 45 þ66 −36 pb, where the first uncertainty is statistical and the second systematic. The cross section near threshold is larger than that expected from theory, which predicts the cross section to vanish at threshold. The Born cross sections at ffiffi ffi s p ¼ 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 Λ are deduced.

Cross section measurements of e+e−→K+KAblikim1, Ачасов2, Adlarson3 et al. 2019Phys. Rev. DSelf Cite27290View full textAdd to dashboardCite

“…energy, an enhancement at ffiffi ffi s p ¼ 2.232 GeV, i.e., near the ΛΛ production threshold, is observed. The cross section of e þ e − → ΛΛ is also found to be anomalously high at the threshold [34]. In the case of charged baryons one would expect a Coulomb enhancement factor, which, however, is absent in the of the electrically neutral Λ.…”

Hyperons - a strange key to the strong interaction

“…So far, only a few measurements of hyperon form factors exist and they are all in the highq 2 region. Cross section measurements of the e + e − → YȲ process were studied for different 027.5 hyperons by BaBar [24], CLEO-c [25,26] and BESIII [27] but the data samples were too small to separate the electric and the magnetic form factor with any conclusive precision. Some information about the structure of Λ + c has been obtained by measurement by Belle [28] and BESIII [29].…”

Report on scipost_201911_00044v1

“…So far, only a few measurements of hyperon form factors exist and they are all in the highq 2 region. Cross section measurements of the e + e − → YȲ process were studied for different hyperons by BaBar [21], CLEO-c [22] and BESIII [23] but the data samples were too small to separate the electric and the magnetic form factor with any conclusive precision. Some information about the structure of Λ + c has been obtained by measurement by Belle [24] and BESIII [25].…”