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Release : 1983
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The .delta.33 resonance is strongly excited in pion-nucleon scattering, but there is clearly only a limited amount of information that can be learned in scattering the pion from an isolated nucleon. One learns that there is a resonance of mass 1232 MeV, width 115 MeV, and, if one is willing to introduce a dynamical model, something about the off-shell extension of the amplitude. One stands to learn much more from pion-nucleus scattering because in this case the .delta.33 resonance has an opportunity to scatter from nucleons, and how this occurs is not well understood. What do we know about the .delta.-N interaction for pion-nucleus scattering. The isobar-hole model was invented to deal directly with the .delta.33-nucleus dynamics, and a phenomenological determination of the isobar shell-model potential was attempted. The unknown dynamics deltaU/sub .delta./ is contained in a central isoscalar spreading potential of strength W0 and a spin orbit potential deltaU0 = W0rho + spin-orbit. The real part of W0rho is measured relative to the nucleon-nucleus potential. From a more theoretical point of view, one would like to be able to calculate deltaU/sub .delta./, including its isospin dependence, from an underlying dynamical model which is formulated in terms of the basic effective meson-baryon couplings. Some salient properties of these couplings can be determined from models of quark-bag structure, which raises the exciting possibility of learning about these fundamental issues from pion scattering. Attempts at Los Alamos to build a theoretical framework to deal with these and other issues are described. (WHK).