Isovector correlations and pion single charge exchange reactions in light nuclei

The reactions ⁷Li(π⁺, π⁰)⁷Be(g.s.), ⁷Li(π⁺, π⁰)⁷Be*(429keV) and ¹³C(π⁺, π⁰)¹³N(g.s.) are treated in DWIA, including corrections arising from two-step processes in which an intermediate state is reached in the course of the charge exchange. The distortions are introduced by means of a first-order optical potential; no special assumptions of extreme high energy limits are made. The closure sum over intermediate states in the two-step charge exchange mechanism leads to an isovector two-particle correlation function whose properties for p-shell nuclei are discussed in detail. Consequences of the introduction of effective transition operators for the single-step mechanism, with and without spin-flip, and for the two-step contribution are discussed, as are the allowable modifications in nuclear shell radii consistent with present knowledge of Coulomb energies. Comparison is made with experiment - in the case of the separated ⁷Be states, for the first time - and with other theoretical approaches.