Effects of nuclear structure in double charge exchange reactions on 16O and 40Ca

The energy dependence of forward double charge exchange (DCX) reaction is studied for the doubly closed shell nuclei O-16 and Ca-40. A common characteristic of the two DCX reactions is a resonance-like peak around 50 MeV pion lab energy. While some authors have interpreted this peak as the evidence of a dibaryon resonance state, we show that it arises naturally in a two-step process in the conventional pion-nucleon system with proper handling of nuclear structure. The effects of nuclear structure are demonstrated through a comparison among different nuclear structure models: the symmetry model SU(4) circle times SU(3), the single particle shell model and the "realistic" shell model.