Impact of pion dynamics on nuclear shell structure

TL;DR

This paper investigates how pion dynamics influence nuclear shell structure, revealing low-energy excitations linked to pion condensation and introducing a novel coupling approach in relativistic nuclear models.

Contribution

It introduces a new method for including pion-exchange effects and their coupling to elementary excitations in relativistic nuclear theory, enhancing the understanding of nuclear shell structure.

Findings

Low-energy pion-related excitations observed in exotic nuclei.

Coupling of pion modes to quasiparticle excitations is essential.

Impact of pion exchange on single-particle energies demonstrated.

Abstract

Spin-isospin response in exotic nuclear systems is investigated. It is found that in some nuclei excitations with pionic quantum numbers (0-, 1+, 2-, ...) appear at very low energies with large transition probabilities, which is an indication of the vicinity of the onset of pion condensation. As an example, 2- components of the spin-dipole resonance in 78-Ni and 132-Sn are considered. The existence of such modes points out to the necessity of taking into account their coupling to other elementary modes of excitation, e.g. single-quasiparticle ones. This coupling is introduced in the theory for the first time. Thereby, pion-exchange contribution to the nucleon-nucleon interaction is included in the relativistic framework beyond the Hartree-Fock approximation. Namely, classes of Feynman diagrams are selected according to their significance for nuclear spectroscopic characteristics, such as single-particle energies and strength functions, and included into the nucleonic self-energy in all orders of meson-exchange. As an illustration, the impact of these new contributions on the single-particle energies of 100-Sn is discussed.