Low-energy dipole strength and the critical case of 48Ca ⋆

Recent theoretical work has not led to a consensus regarding the nature of the low-energy E1 strength in the 40,44,48Ca isotopes, for which high-resolution (γ, γ′) data exist. Here we revisit this problem using the first-order quasi-particle random-phase approximation (QRPA) and different interactions. First we examine all even Ca isotopes with N = 14 − 40. All isotopes are predicted to undergo dipole transitions at low energy, of large and comparable isoscalar strength but of varying E1 strength. Provided a moderate and uniform energetic shift is introduced to the results, QRPA with the Gogny D1S interaction is able to account for the (γ, γ′) data, because, up to N = 28, it yields a rather pure isoscalar oscillation. A neutron-skin oscillation is anticipated for N ≥ 30. This contradicts existing predictions that 44,48Ca develop a neutron-skin mode. Which theoretical result is correct cannot be resolved conclusively using the available data. We propose that alpha-scattering, possibly followed by an electroexcitation experiment, could resolve the situation and thereby help to improve the different models aspiring to describe reliably the low-energy dipole strength of nuclei. Key words: low-energy dipole stength; Ca isotopes; quasiparticle RPA and second RPA; electron scattering; alpha scattering; phenomenological and realistic Hamiltonians; 1