Pion double charge exchange reaction mechanisms

Abstract

The energy dependence of the forward-angle pion double charge exchange cross section on nuclei is studied in the energy range from 0 to 300 MeV. The most striking feature seen in the data is a peak around 50 MeV. It has been claimed that this peak provides evidence for the existence of a dibaryon at this energy [1]. In this work we show that such a peak is a natural consequence of the distorted waves representing the pion propagation in the sequential process. In the present calculations we use realistic pion elastic scattering wave functions obtained by adding a correction for the pion true absorption to the potential, increasing the number of independent partial wave ranges from 2 (which had been usually used in the past) to 6 and fitting the remaining parameters to measured elastic scattering and reaction cross sections. Since the short-range behavior of the double charge exchange operator can influence the dependence of the cross section on energy, the inclusion of a crude approximation to a one-gluon-exchange interaction is made in order to estimate the size of such corrections. A considerable sensitivity to the strength of the strong coupling constant is found. The overall predictions from this theory are in good agreement with the existing data at low energies both for elastic scattering for 12C, 160 and 40Ca and double charge exchange for carbon and calcium nuclei.