Giant Quadrupole Resonances in 208Pb, the nuclear symmetry energy and the neutron skin thickness

TL;DR

This paper investigates the properties of giant quadrupole resonances in 208Pb to constrain the nuclear symmetry energy and neutron skin thickness using both macroscopic and microscopic models, aligning experimental data with theoretical predictions.

Contribution

It provides a combined macroscopic and microscopic analysis of IVGQR in 208Pb, linking resonance energies to symmetry energy and neutron skin thickness, and offers new constraints on these nuclear properties.

Findings

Estimated symmetry energy at 0.1 fm^{-3}: 23.3 +/- 0.6 MeV

Neutron skin thickness of 208Pb: 0.14 +/- 0.03 fm

Slope parameter of symmetry energy: 37 +/- 18 MeV

Abstract

Recent improvements in the experimental determination of properties of the Isovector Giant Quadrupole Resonance (IVGQR), as demonstrated in the A=208 mass region, may be instrumental for characterizing the isovector channel of the effective nuclear interaction. We analyze properties of the IVGQR in 208Pb, using both macroscopic and microscopic approaches. The microscopic method is based on families of non-relativistic and covariant Energy Density Functionals (EDF), characterized by a systematic variation of isoscalar and isovector properties of the corresponding nuclear matter equations of state. The macroscopic approach yields an explicit dependence of the nuclear symmetry energy at some subsaturation density, for instance S(\rho=0.1 fm^{-3}), or the neutron skin thickness \Delta r_{np} of a heavy nucleus, on the excitation energies of isoscalar and isovector GQRs. Using available data it is found that S(\rho=0.1 fm{}^{-3})=23.3 +/- 0.6 MeV. Results obtained with the microscopic framework confirm the correlation of the \Delta r_{np} to the isoscalar and isovector GQR energies, as predicted by the macroscopic model. By exploiting this correlation together with the experimental values for the isoscalar and isovector GQR energies, we estimate \Delta r_{np} = 0.14 +/- 0.03 fm for 208Pb, and the slope parameter of the symmetry energy: L = 37 +/- 18 MeV.