We calculate the axial couplings g 8 A (0) and g 0 A (0) related to the spin of the nucleon in a constituent quark model. In addition to the standard one-body axial currents, the model includes two-body axial exchange currents. The latter are necessary to satisfy the Partial Conservation of Axial Current (PCAC) condition. For both axial couplings we find significant corrections to the standard quark model prediction. Exchange currents reduce the valence quark contribution to the nucleon spin and afford an interpretation of the missing nucleon spin as orbital angular momentum carried by nonvalence quark degrees of freedom.
We discuss the axial form factors of the nucleon within the context of the nonrelativistic chiral quark model. Partial conservation of the axial current (PCAC) imposed at the quark operator level enforces an axial coupling for the constituent quarks which is smaller than unity. This leads to an axial coupling constant of the nucleon g A in good agreement with experiment. PCAC also requires the inclusion of axial exchange currents. Their effects on the axial form factors are analyzed. We find only small exchange current contributions to g A , which is dominated by the one-body axial current. On the other hand, axial exchange currents give sizeable contributions to the axial radius of the nucleon r 2 A , and to the non-pole part of the induced pseudoscalar form factor g P . For the latter, the confinement exchange current is the dominant term.
We calculate the axial N → (1232) and N → N * (1440) transition form factors in a chiral constituent quark model. As required by the partial conservation of axial current (PCAC) condition, we include one-and two-body axial exchange currents. We compare the axial N → (1232) form factors with previous quark model calculations that use only one-body axial currents, and with experimental analyses. The paper provides the first calculation of all weak axial N → N * (1440) form factors. Our main result is that exchange currents are very important for certain axial transition form factors. In addition to improving our understanding of nucleon structure, the present results are relevant to the neutrino-nucleus scattering cross section predictions needed in the analysis of neutrino mixing experiments.
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