Photoproduction of Etas and N${}^{\ast }$(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${}^{\ast }$(1535) excitation. These parameters are, in units of $10^{-4}Me{V}^{-1}$, $(2.20\pm 0.15)$ and $(-1.86\pm 0.20)$ respectively. They yield the ratio of the helicity amplitudes for the excitation of N${}^{\ast }$(1535) by photons off neutrons and protons to be $A_{1/2}^n/A_{1/2}^p=-0.84\pm 0.15$, a quantity of fundamental importance in the baryon structure. Best estimates in the quark model are close to this ratio.

Comments: 1 fig. in ps.. It is an invited talk in Baryon'95, Santa Fe, Oct. 1995