Pion--nucleus optical potential containing effects of genuine pion absorption and emission

We derive in two ways a lowest-order optical potential V/sub piA/ using the projectile and a pair of target nucleons as the basic interacting unit. The first deviation: a heuristic one: uses a multiple-scattering theory, while the second employs a field theory where pions can be absorbed and emitted as well as scattered. The lowest-order terms of V/sub piA/ contain the two-particle ground state density and the πNN→πNN scattering matrix. In the same field-theoretical model one analyzes the latter in standard multiple-scattering contributions and genuine absorption corrections. The form of these corrections and their relation to often ad hoc assumed forms is discussed