Excitation of Δ-nuclear states in π-4He scattering

Abstract A calculation of π − — 4 He scattering in the resonance region is presented. Starting from the optical-model expansion of the multiple scattering formalism, π-nucleus scattering is shown to proceed through the excitation of Δ-nuclear states. Without using the static approximation these Δ-nuclear states are calculated for the π − — 4 He system. Our calculations thus include recoil and binding corrections to the elementary amplitude which are used to define the Hamiltonian of the intermediate Δ. Furthermore, second-order contributions arising from the Pauli correlations are taken into account and the modifications due to short-range correlations are calculated. It is shown that each partial wave amplitude in π — 4 He scattering is dominated by one “collective” Δ-h (Δ-hole) state which exhausts the transition strength to the π-nucleus states. A pole of the S -matrix can be associated to the corresponding collective Δ-h state. On the basis of this resonance description, a qualitative discussion of π — 4 He scattering is presented. The comparison with the experimental data is used to study the basic couplings of the collective modes to more complicated modes of the π — 4 He system. Special attention is paid to the coupling of the Δ-h states to purely nuclear states, i.e., to the influence of pion absorption in elastic scattering.