A set of high-resolution zero-degree inelastic proton scattering data on ${}^{24}\mathrm{Mg}$, ${}^{28}\mathrm{Si}$, ${}^{32}\mathrm{S}$, and ${}^{40}\mathrm{Ca}$ provides new insight into the long-standing puzzle of the origin of fragmentation of the giant dipole resonance (GDR) in $sd$-shell nuclei. Understanding is achieved by comparison with random phase approximation calculations for deformed nuclei using for the first time a realistic nucleon-nucleon interaction derived from the Argonne V18 potential with the unitary correlation operator method and supplemented by a phenomenological three-nucleon contact interaction. A wavelet analysis allows one to extract significant scales both in the data and calculations characterizing the fine structure of the GDR. The fair agreement for scales in the range of a few hundred keV supports the surmise that the fine structure arises from ground-state deformation driven by $\alpha$ clustering.

• Received 3 September 2014
• Revised 8 March 2018

DOI:https://doi.org/10.1103/PhysRevC.97.044325