Learning the structure of giant resonances from their $\gamma$-decay

The direct $\gamma$-decays of the giant dipole resonance (GDR) and the giant quadrupole resonance (GQR) of ${}^{208}$Pb to low-lying states are investigated by means of a microscopic self-consistent model. The model considers effects beyond the linear response approximation. The strong sensitivity of $\gamma$-decay to the isospin of the involved states is proven. By comparing their decay widths, a much larger weight of the $3_1^{-}$ component in the GQR wave function of ${}^{208}$Pb is deduced, with respect to the weight of the $2_1^{+}$ component in the GDR wave function. Thus, we have shown that $\gamma$-decay is a unique probe of the resonance wave functions, and a testground for nuclear structure models.