Determination of the ${\Sigma }^{+}$ Timelike Electromagnetic Form Factors

Based on data samples collected with the BESIII detector at the BEPCII collider, the process $e^{+}{e}^{-}\to {\Sigma }^{+}{\overline{\Sigma }}^{-}$ is studied at center-of-mass energies $\sqrt{s}=2.3960$, 2.6454, and 2.9000 GeV. Using a fully differential angular description of the final state particles, both the relative magnitude and phase information of the ${\Sigma }^{+}$ electromagnetic form factors in the timelike region are extracted. The relative phase between the electric and magnetic form factors is determined to be $\sin \Delta \Phi =-0.67\pm 0.29\left(\mathrm{stat}\right)\pm 0.18\left(\mathrm{syst}\right)$ at $\sqrt{s}=2.3960\mathrm{GeV}$, $\Delta \Phi =55°\pm 19°\left(\mathrm{stat}\right)\pm 14°\left(\mathrm{syst}\right)$ at $\sqrt{s}=2.6454\mathrm{GeV}$, and $78°\pm 22°\left(\mathrm{stat}\right)\pm 9°\left(\mathrm{syst}\right)$ at $\sqrt{s}=2.9000\mathrm{GeV}$. For the first time, the phase of the hyperon electromagnetic form factors is explored in a wide range of four-momentum transfer. The evolution of the phase along with four-momentum transfer is an important input for understanding its asymptotic behavior and the dynamics of baryons.