Transition form factors of theN*(1535)as a dynamically generated resonance

Abstract

We discuss how electromagnetic properties provide useful tests of the nature of resonances, and we study these properties for the ${N}^{*}(1535)$ that appears dynamically generated from the strong interaction of mesons and baryons. Within this coupled-channels chiral unitary approach, we evaluate the ${A}_{1/2}$ and ${S}_{1/2}$ helicity amplitudes as a function of ${Q}^{2}$ for the electromagnetic ${N}^{*}(1535)\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}N$ transition. Within the same formalism we evaluate the cross section for the reactions $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\eta}N$. We find a fair agreement for the absolute values of the transition amplitudes, as well as for the ${Q}^{2}$ dependence of the amplitudes, within theoretical and experimental uncertainties discussed in the article. The ratios obtained between the ${S}_{1/2}$ and ${A}_{1/2}$ for the neutron or proton states of the ${N}^{*}(1535)$ are in qualitative agreement with experiment and there is agreement on the signs. The same occurs for the ratio of cross sections for the $\ensuremath{\eta}$ photoproduction on neutron and proton targets in the vicinity of the ${N}^{*}(1535)$ energy. The global results support the idea of this resonance as being dynamically generated, hence, largely built up from meson baryon components. However, the details of the model indicate that an admixture with a genuine quark state is also demanded that could help obtain a better agreement with experimental data.