Excitation spectra of heavy baryons in diquark models

The excitation energy spectra of heavy baryons consisting of a heavy quark and two light quarks are investigated by using diquark models in order to examine the nature of the diquark as a constituent of single heavy baryons. We consider two diquark models; in model A the diquark is treated as a point-like particle, while it has a spatial size in model B. We determine the masses of scalar and axial vector diquarks by the mass difference of the ground state charmed baryons to the $\Lambda_{c}$ baryon, while the mass of the $ud$ scalar diquark in the $\Lambda_{c}$ baryon is assumed to be 500 MeV as a reference. The parameters of these models are fixed by the $1p$ excitation energy of $\Lambda_{c}$. We find that model A reproduces well the excitation energy spectra of the charmed and bottomed baryons, although the string tension of the confinement potential in model A should be a half of that of the charmonium, while Model B suggests degeneracy of the $2s$ and $1d$ states, which is not seen in the $\Lambda_{c}$ spectrum.