Mixing and decay constants of pseudoscalar mesons: Octet-singlet vs.   quark flavor 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) flavor symmetry, 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 mixing angle between -20 degrees and -10 degrees could be found in the literature. New experimental data, especially for the reactions gamma gamma* -> eta, eta' and B -> eta' 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 eta-eta' mixing and even of eta-eta'-eta_c mixing. 
Thorsten Feldmann (Univ. Wuppertal)