Fragmentation of spin-dipole strength in $^{90}$Zr and $^{208}$Pb

TL;DR

This paper extends covariant density functional theory to include particle-vibration coupling, revealing significant fragmentation and energy shifts in spin-dipole strength spectra of $^{90}$Zr and $^{208}$Pb, and estimating neutron skin thickness.

Contribution

It introduces an extended theoretical framework incorporating particle-vibration coupling into charge-exchange calculations for spin-dipole excitations.

Findings

Significant fragmentation of spin-dipole strength due to particle-vibration coupling.

Shift of strength distribution to higher energies.

Estimation of neutron skin thickness consistent with mean-field results.

Abstract

An extension of time-dependent covariant density functional theory that includes particle-vibration coupling is applied to the charge-exchange channel. Spin-dipole excitation spectra are calculated an compared to available data for $^{90}$Zr and $^{208}$Pb. A significant fragmentation is found for all three angular-momentum components of the spin-dipole strength as a result of particle-vibration coupling, as well as a shift of a portion of the strength to higher energy. A high-energy tail is formed in the strength distribution that linearly decreases with energy. Using a model-independent sum rule,the corresponding neutron skin thickness is estimated and shown to be consistent with values obtained at the mean-field level.