Vacuum phenomenology of the chiral partner of the nucleon in a linear sigma model with vector mesons

We investigate a linear sigma model with global chiral U(2)R×U(2)L symmetry. The mesonic degrees of freedom are the standard scalar and pseudoscalar mesons and the vector and axial-vector mesons. The baryonic degrees of freedom are the nucleon, N, and its chiral partner, N*, which is usually identified with $N(1535)$. The chiral partner is incorporated in the so-called mirror assignment, where the nucleon mass is not solely generated by the chiral condensate but also by a chirally invariant mass term, m₀. The presence of (axial-) vector fields modifies the expressions for the axial-coupling constants of the nucleon, gAN, and its partner, gA^N*. Using experimental data for the decays N*→Nπ and a₁→πγ, as well as lattice results for gA^N* we infer that in our model m₀~500 MeV, i.e., an appreciable amount of the nucleon mass originates from sources other than the chiral condensate. We test our model by evaluating the decay N*→Nη and the s-wave nucleon-pion scattering lengths a₀^(±).