Nuclear symmetry energy and neutron skins derived from pygmy dipole resonances

TLDR

The study briefly discusses astrophysical implications for neutron stars. The authors aim to establish a correlation between pygmy dipole strength, nuclear symmetry energy parameters, and neutron‑skin thicknesses. They measured dipole‑strength distributions of neutron‑rich Sn and Sb isotopes using Coulomb dissociation of high‑energy radioactive beams and analyzed the data with the relativistic quasiparticle random‑phase approximation. The measurements reveal significant pygmy dipole strength that increases with proton‑to‑neutron asymmetry, and the analysis yields a one‑to‑one correlation allowing extraction of symmetry‑energy parameters ($a_4=32.0\pm1.8$ MeV, $p_0=2.3\pm0.8$ MeV fm⁻³) and neutron‑skin thicknesses of 0.24 ± 0.04 fm for $^{132}$Sn and 0.18 ± 0.035 fm for $^{208}$Pb.

Abstract

By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei $^{129\ensuremath{-}132}\mathrm{Sn}$ and $^{133,134}\mathrm{Sb}$, their dipole-strength distributions have been measured. A sizable fraction of ``pygmy'' dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei ($^{208}\mathrm{Pb}$ and $N=82$ isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (${a}_{4}=32.0\ifmmode\pm\else\textpm\fi{}1.8$ MeV and ${p}_{o}=2.3\ifmmode\pm\else\textpm\fi{}0.8$ MeV/fm${}^{3}$) are deduced as well as neutron-skin thicknesses ${R}_{n}\ensuremath{-}{R}_{p}$ of $0.24\ifmmode\pm\else\textpm\fi{}0.04$ fm for $^{132}\mathrm{Sn}$ and of $0.18\ifmmode\pm\else\textpm\fi{}0.035$ fm for $^{208}\mathrm{Pb}$, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.