Multiple excitation of giant dipole resonances in relativistic heavy ion collisions

The relativistic excitation of the double giant dipole resonance in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$ has been calculated, describing the resonance as a harmonic vibration of the mean field, damped through its coupling to doorway states. While the predictions associated with the one-phonon states provide an overall account of the experimental findings, the calculated cross section for the two-phonon states is much smaller than that extracted from the involved analysis of the data. The experimental elucidation of this discrepancy seems essential for assessing the validity of the standard picture of giant resonances.