Isospin violating pion emission from heavy quarkonium

In the absence of a satisfactory quantitative QCD description of the emission of light-quark pseudoscalar mesons from charmonium and the anomalously large ψ′ → ψη decay, we use this decay as input in a phenomenological treatment to provide a universal framework for interpreting currently accumulating data for all these decays. The decay is described by a model in which the initial cc enters a “black box” and the final cc emerges together with a flavor-singlet qq pair which hadronizes into a physical pseudoscalar meson. Symmetry breaking is treated on the same footing in both π and η decays and arises from quark-mass differences which break the symmetry of the charmonium pseudoscalar couplings and from the flavor mixing in the physical pseudoscalar mesons. All processes in the black box are described by phenomenological parameters determined by fitting experimental data.