Measurement of Energy-Dependent Pair-Production Cross Section and Electromagnetic Form Factors of a Charmed Baryon

We study the process e⁺e^-→Λc⁺Λc^- at twelve center-of-mass energies from 4.6119 to 4.9509 GeV using data samples collected by the BESIII detector at the BEPCII collider. The Born cross sections and effective form factors (|Geff|) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process Λc⁺→pK^-π⁺. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure $Y(4630)$, as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the |Geff| energy dependence of Λc⁺, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the Λc⁺ baryon at each energy point, the moduli of electric and magnetic form factors (|GE| and |GM|) are extracted and separated. For the first time, the energy dependence of the form factor ratio |GE/GM| is observed, which can be well described by an oscillatory function.