Pygmy Resonances and Neutron Skins

Abstract

Motivated by a recent experiment, the distribution of electric dipole strength in the neutron-rich 68Ni 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 208Pb. 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 68Ni and the neutron-skin thickness of 208Pb. Yet we find a correlation just as strong and an even larger sensitivity between the neutron-skin thickness of 208Pb and the fraction of the dipole polarizability exhausted by the Pygmy resonance. These findings suggest that the dipole polarizability may be used as a proxy for the neutron skin.

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