Pion-nucleon correlations in finite nuclei in a relativistic framework: effects on the shell structure

TL;DR

This paper extends the relativistic particle-vibration coupling model by including spin- and isospin-flip modes, revealing their significant impact on the shell structure of tin isotopes through collective isovector meson exchange.

Contribution

The novel inclusion of isospin-flip excitation modes in the relativistic framework provides new insights into nucleon interactions and shell structure modifications.

Findings

Isospin-flip states have sizable low-energy transition probabilities.

Coupling of these states significantly affects the shell structure of $^{100,132}$Sn.

Softening of collective modes influences single-particle energy levels.

Abstract

The relativistic particle-vibration coupling (RPVC) model is extended by the inclusion of spin- and isospin-flip excitation modes into the phonon space, introducing a new mechanism of dynamical interaction between nucleons with different isospin in the nuclear medium. Protons and neutrons exchange by collective modes which are formed by isovector $\pi$ and $\rho$-mesons, in turn, softened considerably because of coupling to nucleons of the medium. These modes are investigated within the proton-neutron relativistic random phase approximation (pn-RRPA) and relativistic proton-neutron time blocking approximation (pn-RTBA). The appearance of isospin-flip states with sizable transition probabilities at low energies points out that they are likely to couple to the single-particle degrees of freedom and, in addition to isoscalar low-lying phonons, to modify their spectroscopic characteristics. Such a coupling is quantified for the shell structure of $^{100,132}$Sn and found significant for the location of the dominant single-particle states.