Pygmy resonances and neutron skins

In a study motivated by a recent experiment, the distribution of electric dipole strength in the neutron-rich Ni-68 isotope was computed using a relativistic random-phase approximation with a set of effective interactions that-although well calibrated-predict significantly different values for the neutron-skin thickness in Pb-208. The emergence of low-energy pygmy strength that exhausts about 5%-8% of the energy-weighted sum rule (EWSR) is clearly identified. In addition to the EWSR, special emphasis is placed on the dipole polarizability. In particular, our results suggest a strong correlation between the dipole polarizability of Ni-68 and the neutron-skin thickness of Pb-208. Yet we find a correlation just as strong, and an even larger sensitivity, between the neutron-skin thickness of Pb-208 and the fraction of the dipole polarizability exhausted by the pygmy resonance. These findings suggest that dipole polarizability may be used as a proxy for the neutron skin.