Chiral anomaly and nucleon spin decomposition in the chiral quark soliton model

Abstract

We investigate how the chiral anomaly influences the decomposition of nucleon spin between quark spin and orbital motion. We extend the chiral quark soliton model by including a flavor-singlet pseudoscalar field whose mass is generated by the chiral anomaly. Within this framework, we calculate the quark and antiquark helicity and orbital angular momentum distributions, accounting for the singlet field induced by the rotation of the soliton. We vary the anomaly-induced mass while keeping the remaining model parameters fixed to examine how the anomaly affects the spin decomposition. The induced field enhances the quark helicity contribution relative to the conventional pion-only model. Increasing the mass suppresses this enhancement and increases the orbital contribution, while the two contributions continue to account for the nucleon spin. This redistribution remains modest over the parameter values considered and does not qualitatively alter the spin decomposition of the conventional model.