The two Nucleon States in the Chiral Quark-Diquark Model

We study the ground and first excited states of nucleons in a chiral quark-diquark model. We include two quark-diquark channels of the scalar-isoscalar and axialvector-isovector types for the nucleon states. The diquark correlation violate the spin-flavor SU(4) symmetry, generating a nucleon excited state which does not appear in the ordinary quark model in the SU(4) limit. With a reasonable choice of model parameters, the mass of the excited state appears at around 1.5 GeV, which may be identified with the Roper resonance $N(1440)$.