On the importance of using exact pairing in the study of pygmy dipole resonance

TL;DR

This paper demonstrates that using exact pairing in modeling giant dipole resonance significantly affects the low-energy strength and line shape, especially in neutron-rich nuclei, highlighting the importance of precise pairing treatment.

Contribution

It introduces the use of exact pairing gaps in the phonon damping model to study GDR, revealing notable differences from BCS pairing results.

Findings

Exact pairing decreases two-neutron separation energy in light nuclei.

Exact pairing increases low-energy GDR strength in neutron-rich nuclei.

GDR line shape varies significantly with neutron number when using fixed model parameters.

Abstract

The strength functions of giant dipole resonance (GDR) in oxygen $^{18 - 24}$O, calcium $^{50 - 60}$Ca, and tin $^{120 - 130}$Sn isotopes are calculated within the phonon damping model under three approximations: without superfluid pairing, including BCS pairing, and exact pairing gaps. The analysis of the numerical results shows that exact pairing decreases the two-neutron separation energy in light nuclei, but increases it in heavy nuclei as compared to that obtained within the BCS theory. In neutron-rich medium and heavy nuclei, exact pairing significantly enhances the strength located at the low-energy tail of the GDR, which is usually associated with the pygmy dipole resonance. The line shape of the GDR changes significantly with increasing the neutron number within an isotopic chain if the model parameter is kept fixed at the value determined for the stable isotope.