F. Osterfeld

Nucleon resonances in the medium

Abstract The behaviour of nucleon resonances in nuclei excited by electromagnetic and hadronic probes is discussed. Special emphasis is placed on the Δ(1232) isobar propagation which is explained in terms of an interplay of mesonic, nuclear, and Δ dynamics. Inclusive cross section data of the (p,n) and (3He,t) charge exchange reactions show that there is a substantial downward energy shift of the Δ resonance in nuclei compared to the Δ excitation of the proton target. A large part of this shift is caused by a nuclear medium effect on the spin-longitudinal response function. This response function is shifted down in energy by 60 MeV relative to the spin-transverse response function measured in photon scattering and in inelastic electron scattering experiments. The shift is caused by the attractive π-exchange interaction between Δ-particle nucleon-hole tates leading to a collective pionic mode in the nucleus. The various damping mechanisms of this mode are discussed. The methods used for the description of Δ excitations in nuclei can also be applied to the study of other N ∗ resonances in the nucleus.

Delta excitations in nuclei

Abstract A new approach is proposed for the investigation of delta (Δ) excitations in nuclei induced by intermediate energy charge exchange reactions. The approach is based on the isobar-hole model which treats the dynamics of the Δ in the nucleus explicitly. The distortion effects of the projectile wave functions are included properly. The inclusive Δ-cross section is calculated. It is found that in a distorted wave calculation the peak position of the Δ-cross section is shifted towards lower excitation energies compared to a plane wave calculation. The shift amounts to 25 MeV in the 12 C(p,n)-reaction at 800 MeV incident energy and it amounts to 40 MeV in the 12 C( 3 He,t)-reactions at E = 2 GeV. The origin of this shift is explained in detail.

Dynamical Theory of Spin Excitations

We investigate in a systematic way magnetic resonances in light, medium and heavy mass nuclei. The numerical calculations have been performed within the framework of the extended theory of interacting Fermi systems. Here we incorporate explicitly the π-and ρ-exchange potential in a generalized spin-dependent particle-hole interaction. In addition we take into account the effect of the “dynamical theory of collective states” on the single particle energies (energy dependence of the effective mass). Special emphasis is given to the question whether the Δ(33)-resonance is responsible for the missing magnetic sum rule strength which experimentally was searched for in electron scattering and hadronic charge-exchange reactions with highly energetic protons. We point out that the inclusion of the (Pauli)-exchange terms in the particle-hole interaction strongly reduces the Δ(33)-hole quenching effect.