ISOSPIN VIOLATING PION EMISSION FROM HEAVY QUARKONIUM

In the absence of a satisfactory quantitative QCD description of the e mission of light-quark pseudoscalar mesons from charmonium and the ano malously large psi' --> psi eta decay, we use this decay as input in a phenomenological treatment to provide a universal framework for inter preting currently accumulating data for all these decays. The decay is described by a model in which the initial <c(c)over bar> enters a ''b lack box'' and the final <c(c)over bar> emerges together with a flavor -singlet <q(q)over bar> pair which hadronizes into a physical pseudosc alar meson. Symmetry breaking is treated on the same footing in both p i and eta decays and arises from quark-mass differences which break th e symmetry of the charmonium pseudoscalar couplings and from the flavo r mixing in the physical pseudoscalar mesons. All processes in the bla ck box are described by phenomenological parameters determined by fitt ing experimental data.