An understanding of the mass spectrum and structure of the excited nucleons (N*) is a fundamental challenge in the hadron physics. So far, a number of static hadron models such as quark models have been developed to study the N* spectrum and form factors. In such static models the excited states are usually treated as stable particles, and thus the resulting mass spectrum has real values. However, in reality the N* states couple strongly to the meson-baryon continuum states and can exist only as unstable resonances in the pi N and gamma N reactions. Dynamical effects caused by such strong couplings to the continuum states affect significantly the N* properties and cannot be neglected in extracting the N* resonance parameters (pole mass, residues etc) from the data and giving physical interpretations to those parameters. It is thus well recognized nowadays that properly incorporating such dynamical effects are indispensable to the theoretical studies of the N* spectroscopy.
In this situation, we have been exploring the nature of the N* states as unstable resonances through the comprehensive analysis of the world data of pion-, photon-, and electron-induced meson production reactions off a nucleon. The analysis is performed with a reaction model based on the ANL-Osaka Dynamical Coupled-Channels (DCC) approach. The model satisfies the multichannel unitarity of the S-matrix in the channel-space spanned by the pi N, eta N, pi pi N, rho N, pi Delta, sigma N, K Lambda, and K Sigma channels, and successfully treats the reaction dynamics constrained by the unitarity.
In this talk, I will overview our effort on the extraction of nucleon resonances based on the ANL-Osaka DCC approach, particularly focusing on presenting our recent results of the N* resonance extraction through the fully combined analysis of pion- and photon-induced pi N, eta N, K Lambda, and K Sigma production reactions. I will also present a prospect of our future direction.