Composite-particle structure of pion-nucleus amplitudes

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Abstract
The structural properties of scattering amplitudes which involve composite particles are applied to pion absorption on and elastic scattering from nuclei. The reactive parts, as opposed to the initial and final state parts, of the amplitudes for these processes are identified and the constraints upon their structure are investigated. These constraints are used to determine the modifications, in various models for pionic interactions, which are necessary to avoid double-counting dynamics already accounted for in the nuclear states. The previously known decomposition of the elastic pion-nucleus scattering amplitude into the contributions from scattering and true absorption is derived in a general context. We go on to point out the structural features of both of these parts of the elastic amplitude which are required in order not to duplicate nuclear dynamics.