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

TL;DR

This study measures the electromagnetic form factors of Sigma hyperons in electron-positron collisions across a range of energies, revealing unexpected results that challenge existing theories and providing new insights into baryonic structure.

Contribution

First measurement of Sigma hyperon form factors in the off-resonance region, offering high-precision data that challenge current theoretical models.

Findings

Effective form factor ratio for Sigma+ and Sigma- aligns with charge sum expectations.

Nonzero cross sections observed near production threshold.

First determination of electric and magnetic form factor ratios at multiple energies.

Abstract

The Born cross sections of the $e^{+}e^{-}\to\Sigma^{+}\bar{\Sigma}^{-}$ and $e^{+}e^{-}\to\Sigma^{-}\bar{\Sigma}^{+}$ 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 $\Sigma^{+}$ and $\Sigma^{-}$ is consistent with 3, agreeing with the ratio of the incoherent sum of the squared charges of the $\Sigma^{+}$ and $\Sigma^{-}$ valence quarks, but disagreeing with various theoretical predictions. In addition, ratios of the $\Sigma^{+}$ 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 $\Sigma^{\pm}$ form factors require a new theoretical description for the hyperons.