Flavor symmetry breaking in the 1/Ncexpansion

The breaking of flavor SU(3) symmetry in the axial vector couplings and magnetic moments of baryons is analyzed in the 1/${\mathit{N}}_{\mathit{c}}$ expansion. A simple meson loop graph which is known to be of order √${\mathit{m}}_{\mathit{s}}$ and leading order in 1/${\mathit{N}}_{\mathit{c}}$ correctly predicts the pattern of symmetry breaking in the magnetic moments. It is, however, not possible to use this graph to predict the magnitude of the breaking. The situation with the axial vector couplings is less clear. In this case the breakings are relatively small and do not appear to follow an obvious pattern. Nevertheless there is a clear indication that, with symmetry breaking taken into account, the F/D ratio [defined in the presence of SU(3) breaking] is considerably less than the SU(6) value of 2/3. With sizable uncertainty, we find F/D≊0.44. The quantity 3F-D which is relevant for the analysis of spin-dependent deep inelastic scattering is considerably smaller than the SU(6) value of unity. The new value 3F-D=0.28±0.09 is consistent with vanishing strange-quark spin in the nucleon. © 1995 The American Physical Society.