Soft pion theorems, meson exchange currents, and pion double charge exchange

Meson exchange current (MEC) contributions to pion double charge exchange (DCX) scattering at low energies are studied using soft pion theorems. A general form for the scattering amplitude on any scalar-isovector target is derived and applied as a constraint on the pion-nucleus DCX scattering amplitude. One result is an estimate that for a nuclear target with two excess valence neutrons, the MEC amplitude is less than ∝10-2 fm. Second, a consistency condition on different MEC contributions is obtained, which exhibits the requirement of cancellations among these, and leads to a sum rule for evaluating all contributions of one type of exchanged meson. Consequences for vector meson exchange are discussed. (orig.)