Determination of the ? plus Timelike Electromagnetic Form Factors

Based on data samples collected with the BESIII detector at the BEPCII collider, the process e+e- -> E+ E over bar - is studied at center -of -mass energies ffis ffi p = 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 E+ electromagnetic form factors in the timelike region are extracted. The relative phase between the electric and magnetic form factors is determined to be sin AO = -0.67 +/- 0.29(stat) +/- 0.18(syst) at ffis ffi p = 2.3960 GeV, AO = 55 degrees +/- 19 degrees(stat) +/- 14 degrees(syst) at pffisffi = 2.6454 GeV, and 78 degrees +/- 22 degrees(stat) +/- 9 degrees(syst) at ffis ffi p = 2.9000 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 fourmomentum transfer is an important input for understanding its asymptotic behavior and the dynamics of baryons.