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

We study the process ${e}^{+}{e}^{-}\to {\Lambda }_{c}^{+}{\overline{\Lambda }}_{c}^{-}$ 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 ($|{G}_{\mathrm{eff}}|$) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process ${\Lambda }_{c}^{+}\to p{K}^{-}{\pi }^{+}$. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure $Y\left(4630\right)$, as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the $|{G}_{\mathrm{eff}}|$ energy dependence of ${\Lambda }_{c}^{+}$, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the ${\Lambda }_{c}^{+}$ baryon at each energy point, the moduli of electric and magnetic form factors ($|{G}_{E}|$ and $|{G}_{M}|$) are extracted and separated. For the first time, the energy dependence of the form factor ratio $|{G}_{E}/{G}_{M}|$ is observed, which can be well described by an oscillatory function.