Electroproduction of nucleon resonances

The electroproduction of nucleon resonances is discussed from both phenomenological and experimental points of view. Emphasis is given to recent data obtained by the 'coincidence' technique when the inelastically scattered electron and one of the decay products of the excited hadronic system are detected simultaneously. Most of the experimental results are confined to the hadronic mass range below 1.8 GeV/c2 and to values of the four-momentum transfer, Q2, less than 3 GeV2. Electroproduction is described as photoproduction by a single polarised virtual photon with negative mass squared given by Q2 so that real photoproduction is seen as the limiting case when Q2 goes to zero. The single-quark transition model of virtual photoproduction (SQTM) is taken to be the basis of the theoretical treatment and an explicit realisation in terms of the harmonic oscillator quark model is used extensively in comparison with the experimental results. It is seen that this quark model gives a good but by no means perfect description of the data even at moderate values of the excitation energy and four-momentum transfer.