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We study chiral partner structure of four light nucleons, $N(939)$,$N(1440)$, $N(1535)$ and $N(1650)$ using an effective chiral model based on theparity doublet structure. In our model we introduce four chiralrepresentations, $({\bf 1},{\bf 2})$, $({\bf 2},{\bf 1})$, $({\bf 2},{\bf 3})$and $({\bf 3},{\bf 2})$ under ${\rm SU}(2)_{\rm L} \otimes {\rm SU}(2)_{\rm R}$symmetry. We determine the model parameters by fitting them to availableexperimental values of masses, widths and the axial charge of $N(939)$ togetherwith the axial charges of $N(1535)$ and $N(1650)$ by lattice analyses. We findfive groups of solutions: In a group the chiral partner to $N(939)$ is$N(1440)$ having small chiral invariant mass. In another group, the chiralpartner is a mixture of $N(1535)$ and $N(1650)$ having a large chiral invariantmass. We claim that off-diagonal elements of axial-charge matrix can be usedfor distinguishing these groups. We also discuss changes of masses associatedwith chiral symmetry restoration, which could emerge in high density matter.

Chiral partner structure of light nucleons in an extended parity doublet model