Photoproduction of Etas and N$^*$(1535) Electrostrong Properties

Abstract

From the new Mainz data of eta photoproduction off protons\cite{ref1} and deuterons\cite{ref2}, we can extract, via the effective Lagrangian approach (ELA)\cite{ref3}, the electrostrong parameters $\xi_{p}$ and $\xi_{n}$ for the N$^*$(1535) excitation. These parameters are, in units of $10^{-4} MeV^{-1}$, $(2.20\pm 0.15)$ and $(-1.86\pm 0.20)$ respectively. They yield the ratio of the helicity amplitudes for the excitation of N$^*$(1535) by photons off neutrons and protons to be $A^{n}_{1/2}/A^{p}_{1/2}=-0.84\pm 0.15$, a quantity of fundamental importance in the baryon structure. Best estimates in the quark model are close to this ratio.