π- 6Li scattering and the Δ dynamics

Abstract

π- 6Li scattering is investigated on the basis of Watson's multiple-scattering theory. A realistic α np three-body wavefunction is employed for the ground and excited states of 6Li. The first-order optical potential is extended in such a way that the Δ dynamics can be described straightforwardly. The in-medium pion-nucleon T-matrix is decomposed into the πN single scattering and the πN scattering brought about the Δ-nucleus interaction. The effects of nucleon binding and Δ spreading are represented by one-body Δ-nucleus potentials, and evaluated by summing up the Δ-nucleus multiple-scattering series. The Δ-nucleus transition matrix thus obtained is folded with the doorway state vertex function which links the space of the pion and the nucleus to that of the Δ and the residual nucleus. The Pauli-principle effect is calculated in the π-nucleus subspace which includes unphysical Pauli-violating states. The mixing of the (3, 3) amplitude and the background amplitude due to the exclusion principle, which is neglected in the Δ-hole model, is incorporated in this way. The validity of the standard static treatment of pion-nucleus scattering is also tested in various aspects in comparison with our non-static approach. It is found that phenomenological strengths of the spreading potential determined from the fit to the elastic scattering data have magnitudes comparable to those obtained for light closed-shell nuclei in the Δ-hole model. The pion-induced transition to the weakly excited 0 +1 (3.56 MeV) state of 6Li is worked out in great detail. An elaborate evaluation of the transition via DWIA fails to reproduce the observed excitation function even qualitatively. To remove this discrepancy the effective ΔN interaction is introduced. By inspection of the selection rules it is proved that the ΔN interaction in the 3 S 1 ( I = 1) channel contributes dominantly to the transition. With a strongly repulsive force for this non-absorptive ΔN interaction the data are reproduced satisfactorily in a wide energy region.