Summary
The description of π-nucleus interaction in the energy region of the Δ-resonance in terms of pion optics is reviewed. From the phenomenological π-optical potential analysis of Masutani and Yazaki it is inferred that the pion dispersion equation cannot be solved perturbatively at the effective nuclear-surface density at which the interaction occurs. As a consequence, it is not allowed, in this energy region, to speak of a pion in the medium as a quasi-particle with well-defined energy-momentum relation and width. This proves a complete mixing between the pion and the renormalized Δ-hole states in nuclear matter. The most striking evidence of such strong coupling is provided by the enhancement, due to the contribution of the peripheral partial waves, of the reaction cross-section with respect to the elastic one.
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Dillon, G. Pion in the medium and pion optics. Nuov Cim A 106, 823–834 (1993). https://doi.org/10.1007/BF02771496
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DOI: https://doi.org/10.1007/BF02771496