Measurement of the Energy-Dependent Electromagnetic Form Factors of a Charmed Baryon

Abstract

We study the process $e^{+}{e}^{-}\to \hat{I}{›}_c^{+}\bar{\hat{I}}{›}_c^{-}$ 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 $\hat{I}{›}_c^{+}\to p{K}^{-}\ddot{I}{€}^{+}$. 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 $\hat{I}{›}_c^{+}$, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the $\hat{I}{›}_c^{+}$ 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.

10 pages, 3 figures