Nucleon-nucleon correlations and the pion-nucleus interaction

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Abstract
The scattering of pions from a correlated pair of nucleons is studied in a recently proposed covariant theory of pion-nucleus scattering. In this theory the off-shell effects arising from the correlations can be treated unambiguously. In the covariant theory, "off-shell effects" refers to both the usual off-shell effects known from conventional nonrelativistic multiple-scattering theories and to off-mass-shell features particular to the relativistic theories. It is shown that these effects, absent in a "fixed-scatterer approximation," require knowledge of the fundamental pion-nucleon scattering amplitudes in regions which are not readily parametrized using on-shell scattering data. In a simplified model we illustrate this point by comparing our dynamical approach to the corresponding fixed-scatterer analysis. We conclude that any reliable estimate of correlation effects in pion-nucleus interaction requires that serious consideration be given to the off-shell aspects of the dynamics.