Electromagnetic form factors of singly charmed baryons $\Sigma_c$ and $\Lambda_c$ in a covariant quark-diquark model

Abstract

We present a systematic study of the spacelike electromagnetic form factors of the ground-state singly charmed baryons, $\Sigma_c$ ($\Sigma_c^{++},\Sigma_c^+,\Sigma_c^0$) and $\Lambda_c^+$, within a covariant quark-diquark model. Based on this framework, we obtain their magnetic moments as well as electric charge and magnetic moment radii. Such observables are important to understand the internal structure and the inner dynamics of these heavy baryon states. Our theoretical calculations are in qualitative agreement with available lattice QCD results of $\Sigma_c^{++}$ and $\Sigma_c^0$ baryons. We also discuss the mechanism behind the dependence of the numerical results on the charm quark and light diquark. A key finding is that the electric form factors of the singly charmed baryons fall off much more slowly with momentum transfer $Q^2$ than that of the proton, indicating a more compact electric charge distribution, which is attributed to the heavy charm quark acting as a localized core. More importantly, we observe a striking difference in the magnetic structure: while the magnetic form factors of $\Sigma_c$ are dominated by the light axial-vector diquark, those of $\Lambda_c^+$ are unexpectedly governed by the charm quark due to the vanishing contribution of the scalar diquark. This highlights the decisive role of the light-diquark spin configuration in determining the magnetic properties. Finally, using an empirical asymptotic relation connecting the spacelike and timelike regions, we predict the total cross section for $e^+e^-\to\Sigma_c\bar{\Sigma}_c$. Our findings can be tested at existing facilities including the BESIII, Belle II and LHCb, as well as the proposed Super Tau-Charm Facility.