Electromagnetic Structure of the Giant Dipole Resonance

Abstract

The inelastic transverse form factor for electromagnetic excitation of the giant resonance is discussed and calculated on the basis of several different models. Particular attention is paid to the Brown theory of the giant resonance, which is reformulated here in such a way that no free parameters are left over, i.e., all parameters are determined from other experiments. Numerical calculations are carried out for ${\mathrm{C}}^{12}$ and compared with the 180\ifmmode^\circ\else\textdegree\fi{} electron scattering experiments and with photoabsorption data. It is found that the Brown theory successfully predicts the dip in the squared form factor and the shift in the resonance energy as the momentum transfer is changed, both of which are observed experimentally, and neither of which is predicted by the collective models for the charge motion of the giant resonance. More detailed comparison with the experimental data allows us to distinguish between different versions of the Brown theory, and it is found that the no-free-parameter results are consistent with all the experimental data considered. Results for all the models considered are presented and discussed.