Origin of fine structure of the giant dipole resonance in sd -shell nuclei

A set of high-resolution zero-degree inelastic proton scattering data on Mg24, Si28, S32, and Ca40 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 α clustering.