Δ isobar in nuclei (review of experimental data)

Experimental and elementary theoretical work relevant to the electromagnetic and hadron excitation of the Δ isobar in nuclei is reviewed. The historical development of the notion of non-nucleon degrees of freedom, from the quasinucleon and the pion to quarks and gluons, is described, and the role of Δ excitations is discussed. Gamma, electron, proton, pion, and ion beam methodologies and detector and target designs are discussed. Preliminary suggestions about γ, e, p, π excitation mechanisms are made. Problem areas that need more research are highlighted, and trends for the future and prospective experiments are discussed.