EFFECT OF PION EXTERNAL DISTORTION ON LOW-ENERGY PION DOUBLE-CHARGE-EXCHANGE

We simplify a treatment of pion distortions using a separable expansio n of optical potential in momentum space. We use this method to gain i nsight into the role of distortions in isoelastic pion charge-exchange scattering within the framework of the isospin-invariant optical mode l. The method provides an analytic procedure for calculating external distortions of single- and double-charge-exchange scattering. This all ows us to analyze separately the role of on- and off-shell pions in di stortions for each partial wave. Results for sequential scattering at 50 MeV for the reaction C-14(pi(+), pi(-))O-14 are presented. It is sh own that the external distortions give a small enhancement in forward directions and reduce the differential cross sections at large angles.