QCD axial anomaly and mixing of pseudoscalars

Starting with the QCD axial anomaly we discuss systematically the mixing of light-light and light-heavy pseudoscalar mesons. The π0-η and π0-η' mixing angles are found to be λπη≅0.016 and λsub(πη')≅0.004, which are derived without invoking the diagonalization of the mass matrix. For heavy pseudoscalars we assume that the matrix elements of the quark (e.g. the charm quark) operator anti cγ5c are saturated by the ηc meson and a series of its radial excitations. The π0-ηc and η-ηc mixing angles are then found to be λπηc≅-1x10-4 and ληηc≅-5x10-3. By use of the mixing of η with canti c states the calculated rate for ψ→γη is in agreement with experimental data and the branching ratio for ψ'→γη is predicted to be ≅ 6x10-5. In addition, the η-η' mixing is also discussed. (orig.)