M1Photo-ExcitationN→N*andSU(6)Symmetry

Abstract

A direct evaluation of the $M1\mathrm{}$ amplitude for the photo-excitation $p\to N^{*+}$ is made from the ${\pi }^0$ photo-production cross section at resonance, with result (1.28±0.02) times the $\mathrm{SU}\left(6\right)\mathrm{}$ prediction $\frac{2\sqrt{2}{\mu }_p}{3}$. It is shown that the $N^{*}$ electro-excitation data also lead to the same value, within larger errors. There is pointed out a strong kinematic form-factor correction to this $\mathrm{SU}\left(6\right)\mathrm{}$ prediction, which occurs in consequence of the $N-N^{*}$ mass difference arising from $\mathrm{SU}\left(6\right)\mathrm{}$ symmetry-breaking interactions. The most direct phenomenological evaluation (based on the three-quark model for the baryonic states) for this correction would reduce the predicted $M1\mathrm{}$ amplitude to $0.79\times \left(\frac{2\sqrt{2}{\mu }_p}{3}\right)$.