The Nucleon and Roper Resonance in a Chiral Quark Diquark Model
  A description of the nucleon and Roper resonance in a quark-diquark approach is presented. We show that two states with the quantum number of the nucleon can appear in the ground state of the spatial configuration, when there are two types of diquarks: scalar-isoscalar and axial-vector-isovector diquarks. The mass difference between the two states is generated by the mass difference between the two diquarks, which is due to the spin-spin interaction between the two quarks in the diquarks. The two states are then identified with the nucleon and Roper resonance. 
Keitaro Nagata and Atsushi Hosaka