We study the process $e^{ +}e^{ -}\to\Lambda_{ c}^{ +}\bar{ \Lambda}_c^{ -}$ at twelve center-of-mass energies from $4.6119$ to $4.9509~\mathrm{ GeV}$ 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 $\Lambda_{ c}^{ +}\to pK^{ -}\pi^{ +}$. Flat cross sections around $4.63~\mathrm{ GeV}$ 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 $\Lambda_{ c}^{ +}$, 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.
Categories: hep-ex