Pygmy dipole response of proton-rich argon nuclei in random-phase approximation and no-core shell model

Abstract

The occurrence of a pygmy dipole resonance in proton rich $^{32,34}\mathrm{Ar}$ is studied using the unitary correlator operator method interaction ${V}_{\mathrm{UCOM}}$, based on Argonne V18. Predictions from the random-phase approximation (RPA) and the shell model in a no-core basis are compared. It is found that the inclusion of configuration mixing up to two-particles--two-holes broadens the pygmy strength slightly and reduces sensibly its strength, as compared to the RPA predictions. For $^{32}\mathrm{Ar}$, a clear peak associated with a pygmy resonance is found. For $^{34}\mathrm{Ar}$, the pygmy states are obtained close to the giant dipole resonance and mix with it.