Radiative proton-antiproton annihilation and isospin mixing in protonium

A detailed analysis of the radiative p (p) over bar annihilation is made in the framework of a two-step formalism; the p (p) over bar annihilates into meson channels containing a vector meson with a subsequent conversion into a photon via the vector dominance model (VDM). Both steps are derived from the underlying quark model. First, branching ratios for radiative protoniu m annihilation are calculated and compared with data. Then, details of the isospin interference are studied for different models of the initial proton ium state and also for different kinematical form factors. The isospin inte rference is shown to be uniquely connected to the p (p) over bar-n (n) over bar mixing in the protonium state. Values of the interference terms direct ly deduced from data are consistent with theoretical expectations, indicati ng a dominant p (p) over bar component for the S-1(0) and a sizable n (n) o ver bar component for the S-3(1) protonium state. The analysis is extended to the p (p) over bar-->gamma Phi transition, where the large observed bran ching ratio remains unexplained in the VDM approach. [S0556-2813(99)00402-1 ].