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 quasiparticle 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.