Non-linear density dependence of the Δ-self-energy in nuclear matter and its implications in pionic atoms

The Δ(3/2, 3/2) self-energy in a nuclear medium is shown to be highly non-linear in density, when proper care is taken of the virtual meson propagation in the medium and retardation effects. As a consequence the p-wave absorptive potential for pionic atoms diverges appreciably from the standard p2 form. A fit to the existing data on pionic atoms is carried out with the new functional of the density and turns out to be as good as those with the p2 functional. The successful fits with such different density functionals are due to a very narrow range of nuclear effective densities felt by the pion in the observed pionic atoms. The influence of these effects in related problems is discussed along with the suggestion to widen the range of nuclear densities felt by the pions by looking at other nuclear phenomena. (orig.)