Quark-Model Classification of Low-Mass,I=12Nucleon Isobars

Abstract

A comparison is made of recent data on $\eta$-meson photoproduction with photoexcitation amplitudes calculated in the nonrelativistic quark model. This leads to definite multiplet assignments for all low-mass $I=\frac{1}{2}$ nucleon isobars. In particular, we find no evidence for a radial sequence beyond $n=2\mathrm{}$. The assignment of $P_{11}\mathrm{}\left(1750\right)\mathrm{}$ to an $n=3\mathrm{}$ second radially excited [56, $0^{+}$] appears ruled out; instead, we argue that consistency of results from photoproduction and diffraction scattering makes the configuration [70, $0^{+}$] the best choice for this state.