Mixing and decay constants of pseudoscalar mesons: Octet-singlet vs. quark flavor basis

Although η-η' mixing is qualitatively well understood as a consequence of the U(1)A anomaly in QCD together with a broken SU(3)F flavor symmetry, until recently the values of decay and mixing parameters of the η and η' were only approximately known, e.g. values for the octet-singlet mixing angle between -20 deg. and -10 deg. could be found in the literature. New experimental data, especially for the reactions γγ* → η,η' and B → η' K, together with new theoretical results from higher order corrections in chiral perturbation theory stimulated a phenomenological re-analysis of this subject, which led to a coherent qualitative and quantitative picture of η-η' mixing and even of η - η' - ηc mixing