Instantons and the Spin of the Nucleon

Motivated by measurements of the flavor singlet axial coupling constant of the nucleon in polarized deep inelastic scattering we study the contribution of instantons to OZI violation in the axial-vector channel. We consider, in particular, the f_1-a_1 meson splitting, the flavor singlet and triplet axial coupling of a constituent quark, and the axial coupling constant of the nucleon. We show that instantons provide a short distance contribution to OZI violating correlation functions which is repulsive in the f_1 meson channel and adds to the flavor singlet three-point function of a constituent quark. We also show that the sign of this contribution is determined by positivity arguments. We compute long distance contributions using numerical simulations of the instanton liquid. We find that the iso-vector axial coupling constant of a constituent quark is (g_A^3)_Q=0.9 and that of a nucleon is g_A^3=1.28, in good agreement with experiment. The flavor singlet coupling of a quark is close to one, while that of a nucleon is suppressed, g_A^0=0.77. However, this number is larger than the experimental value g_A^0=(0.28-0.41).