U(1) Problem Revisited
  In the anomaly equation for the singlet axial current the chiral limit of the quark mass term does not vanish but comprises contribution from fermion zero modes whose integral exactly cancels the topological charge arising from the Adler-Bell-Jackiw anomaly. This signals chiral symmetry and opens a window for restoring the status of Goldstone boson for the singlet $\eta^\prime$ without having to invoke the large $N_c$ limit in the underlying QCD. We construct the anomaly term in the effective action that incorporates the chiral symmetry property and yet accounts for the excess mass of $\eta^\prime$ only when chiral symmetry is broken explicitly by the quark masses. The anomaly term in the present scenario thus plays the role of a catalytic agent that enhances the mass of $\eta^\prime$ so that the singlet axial current obeys the popular PCAC condition. 
H. Banerjee and Gautam Bhattacharya