Theory of the scattering of pions from nuclei

A nonrelativistic quantum field theoretic formulation of pion-nucleus scattering is presented. A nonrelativistic boson-complex-target Low equation is developed, in which the coupling between the incident boson and target constituent particles is completely general. This development is then particularized to the case where the bosons are pions and the target is a nucleus of A nucleons. Special detailed attention is paid to the case where the pion-nucleon coupling is linear. The pion-nucleus Low equation is decomposed into a finite series of A terms, referred to as a spectator expansion, of which the first term involves one active particle and (A-1) spectators and the higher terms involve an increasingly larger number of active target particles. Within the framework of the nonrelativistic pion-nucleus Low equation, a formal definition of the pion-nucleus optical potential is also given.