U(1) Problem Revisited

TL;DR

This paper revisits the U(1) problem in QCD, proposing a new anomaly term in the effective action that explains the eta-prime mass without relying on the large N_c limit, emphasizing chiral symmetry considerations.

Contribution

It introduces a novel anomaly term in the effective action that accounts for the eta-prime mass while preserving chiral symmetry, challenging the traditional large N_c approach.

Findings

The anomaly term respects chiral symmetry and explains eta-prime mass.

The approach does not require large N_c limit assumptions.

The singlet axial current satisfies PCAC with the new anomaly term.

Abstract

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.