Excitation spectra of heavy baryons in a quark-diquark model with relativistic corrections

The excitation spectra of \Lambda_c and \Lambda_b baryons are investigated by using a quark-diquark model in which a single-heavy baryon is treated as the bound state of a heavy quark and a scalar diquark. We take two types of relativistic corrections into account for the quark-diquark potential. In the first type, we consider the one-gluon exchange between the heavy quark and one of the light quarks in the diquark. In the second, we consider the one-gluon exchange between a scalar particle and a heavy quark. We find that there is a large difference between two types of corrections due to different treatment of the internal color structure of the diquark. The relativistic corrections are important for the solution to the string tension puzzle, particularly, the Darwin term makes a large contribution.