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

Although eta-eta' mixing is qualitatively well understood as a consequence of the U(1)(A) anomaly in QCD together with a broken SU(3)(F) flavor symmet ry, until recently the values of decay and mixing parameters of the eta and eta' were only approximately known, e.g. values for the octet-singlet mixi ng angle between -20 degrees and -10 degrees could be found in the literatu re. New experimental data, especially for the reactions gamma gamma* --> et a,eta' and B --> eta'K, together with new theoretical results from higher o rder corrections in chiral perturbation theory stimulated a phenomenologica l re-analysis of this subject, which led to a coherent qualitative and quan titative picture of eta-eta' mixing and even of eta-eta'-eta(c) mixing.