Measurements of Sigma+ and Sigma- Time-like Electromagnetic Form Factors for center-of-mass energies from 2.3864 to 3.0200 GeV

Abstract

The Born cross sections of the $e^{+}{e}^{-}\to \hat{I}{\pounds }^{+}\bar{\hat{I}}{\pounds }^{-}$ and $e^{+}{e}^{-}\to \hat{I}{\pounds }^{-}\bar{\hat{I}}{\pounds }^{+}$ processes are determined with high precision for center-of-mass energy from 2.3864 to 3.0200 GeV with the BESIII detector. Nonzero cross sections near threshold are observed. The resulting ratio of effective form factors for the $\hat{I}{\pounds }^{+}$ and $\hat{I}{\pounds }^{-}$ is consistent with 3, agreeing with the ratio of the incoherent sum of the squared charges of the $\hat{I}{\pounds }^{+}$ and $\hat{I}{\pounds }^{-}$ valence quarks, but disagreeing with various theoretical predictions. In addition, ratios of the $\hat{I}{\pounds }^{+}$ electric and magnetic form factors, $|G_E/G_M|$, are obtained at three center-of-mass energies through an analysis of the angular distributions. These measurements, which are studied for the first time in the off-resonance region, provide precision experimental input for understanding baryonic structure. The observed novel features of the $\hat{I}{\pounds }^{\pm }$ form factors require a new theoretical description for the hyperons.