Combined effects of internal quark motion and SU(6) breaking on the properties of the baryon ground state

A sizeable departure from naive SU(6) results has been shown to come, within the quark-model approach, from the relativistic character of the internal quark motion: axial-vector couplings and chiral configuration mixing. On the other hand, an interband mixing ${(56,0^{+})}_{N = 0} + {(70,0^{+})}_{N = 2}$ of the harmonic-oscillator levels correctly describes the large-$x$ behavior of the ratio $\frac{F_2^{\mathrm{en}}}{F_2^{\mathrm{ep}}}$. This paper is devoted to further consequences of both effects which result from their interference. Keeping the same basic parameters already used in the previous calculations, we compute mass differences within the ${\mathrm{½}}^{+}$ octet and axial-vector and magnetic couplings. The most striking results are (i) the survival of the good SU(6) predictions: $\frac{{\mu }_p^{\mathrm{tot}}}{{\mu }_n}\simeq -\frac{3}{2}$, ${\left(\frac{F}{D}\right)}_{\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{a}\mathrm{l}-\mathrm{v}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}}\simeq \frac{2}{3}$; (ii) the explanation in sign and order of magnitude of the $\Sigma$-$\Lambda$ splitting; (iii) the explanation of the sizeable discrepancy between experiment and SU(6) for the various $\Delta$-$N$ couplings, which are systematically underestimated: the magnetic dipole transition $\frac{{\mu }^{*}}{{\mu }_p^{\mathrm{tot}}}\simeq \left(\frac{2\sqrt{2}}{3}\right)(1\mathrm{} + 30\%)$ and the $\Delta \to N\pi$ width. The new parameters introduced are the $\lambda -\mathcal{p}$ quark mass difference [fixed by the $\frac{(\Sigma + \Lambda )}{2}-N$ splitting], the anomalous quark magnetic moment $\kappa$ (fixed by ${\mu }_p^{\mathrm{tot}}$), and ${\left(g_A\right)}_q$, the renormalized quark axial-vector coupling (fixed by $\frac{G_A}{G_V}$). The conclusions are that—in spite of an unwanted problem for the neutron charge form factor—a good description can be obtained for fine details of the baryon ground state, and that, as suspected, the quarks must be given a structure.