Dynamics

These lectures discuss the properties and interactions with nuclei of Δ's. To do so, the authors must establish first a common idea about what a Δ is. This will be the topic of discussion in Chapter II, wherein an isobar model is developed which allows us to discuss precisely the propagation and dynamics of the Δ as an unstable particle. In Chapter III, the extraction of Δ-nucleus interaction parameters from an analysis of intermediate energy pion- and photon-initiated nuclear reactions is discussed. The central result is the strength of the Δ-nucleus optical potential, which is found to have a large imaginary part associated with intermediate coupling to purely nuclear states (i.e., pion annihilation channels). The isospin signature provided by the pion annihilation dynamics both lends support to this interpretation and leads to new predictions for inelastic pion reactions. Finally, in Chapter IV the role of Δ's in low energy nuclear properties is described. This subject has received considerable attention because of the special role played by the Δ in nuclear spin-isospin excitations (the Δ is a static spin-flip nucleon excitation in the quark model). Focus will be on the ambiguities inherent in such discussions both because of the dynamical nature of the resonance discussed in Chapter II and because the quarks are the true nucleon internal degrees of freedom