  The fine structure of the IsoVector Giant Dipole Resonance (IVGDR) in the
doubly-magic nuclei $^{40,48}$Ca observed in inelastic proton scattering
experiments under $0^\circ$ is used to investigate the role of different
mechanisms contributing to the IVGDR decay width. Characteristic energy scales
are extracted from the fine structure by means of wavelet analysis. The
experimental scales are compared to different theoretical approaches allowing
for the inclusion of complex configurations beyond the mean-field level.
Calculations are performed in the framework of RPA and beyond-RPA in a
relativistic approach based on an effective meson-exchange interaction, with
the UCOM effective interaction and, for the first time, with realistic two-
plus three-nucleon interactions from chiral effective field theory employing
the in-medium similarity renormalization group. All models highlight the role
of Landau fragmentation for the damping of the IVGDR, while the differences in
the coupling strength between one particle-one hole (1p-1h) and two
particle-two hole (2p-2h) correlated (relativistic) and non-correlated
(non-relativistic) configurations lead to very different pictures of the
importance of the spreading width resulting in wavelet scales being a sensitive
measure of their interplay. The relativistic approach with particle-vibration
coupling, in particular, shows impressive agreement with the number and
absolute values of the scales extracted from the experimental data.
