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

Abstract

We study the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}^{+}{\overline{\mathrm{\ensuremath{\Lambda}}}}_{c}^{\ensuremath{-}}$ 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 ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$. 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 $|{G}_{\mathrm{eff}}|$ energy dependence of ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}$, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}$ 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.