Mixing of Pseudoscalar Mesons and Isospin Symmetry Breaking

Mixing of the pseudoscalar mesons is discussed in the quark-flavor basis with the hypothesis that the basis decay constants follow the pattern of particle state mixing. The divergences of the axial vector currents which embody the axial vector anomaly, combined with this hypothesis provide a calculational scheme for the parameters describing the mixing of the π, η and η mesons. Phenomenological applications of this mixing scheme are presented with particular interest focussed on isospin symmetry breaking in QCD estimated as η and η admixtures to the pion. In contrast to previous work a possible difference in the basis decay constants fu and fd is considered and consequences of this potentially large effect on the strength of isospin symmetry breaking is discussed.