Nuclear response theory for spin-isospin excitations in a relativistic quasiparticle-phonon coupling framework

TL;DR

This paper introduces a novel relativistic theoretical framework, pn-RQTBA, for modeling spin-isospin excitations in open-shell nuclei, improving the accuracy of beta-decay half-life predictions by incorporating quasiparticle-phonon coupling effects.

Contribution

The paper develops the pn-RQTBA method, extending the relativistic response theory to include quasiparticle-phonon coupling in the proton-neutron channel, enhancing the description of nuclear excitations.

Findings

Strong fragmentation and quenching of Gamow-Teller resonance observed.

Significant improvement in beta-decay half-life predictions.

Method successfully applied to neutron-rich Nickel isotopes.

Abstract

A new theoretical approach to spin-isospin excitations in open-shell nuclei is presented. The developed method is based on the relativistic meson-exchange nuclear Lagrangian of Quantum Hadrodynamics and extends the response theory for superfluid nuclear systems beyond relativistic quasiparticle random phase approximation in the proton-neutron channel (pn-RQRPA). The coupling between quasiparticle degrees of freedom and collective vibrations (phonons) introduces a time-dependent effective interaction, in addition to the exchange of pion and $\rho$-meson taken into account without retardation. The time-dependent contributions are treated in the resonant time-blocking approximation, in analogy to previously developed relativistic quasiparticle time blocking approximation (RQTBA) in the neutral (non-isospin-flip) channel. The new method is called proton-neutron RQTBA (pn-RQTBA) and applied to Gamow-Teller resonance in a chain of neutron-rich Nickel isotopes $^{68-78}$Ni. A strong fragmentation of the resonance along with quenching of the strength, as compared to pn-RQRPA, is obtained. Based on the calculated strength distribution, beta-decay half-lives of the considered isotopes are computed and compared to pn-RQRPA half-lives and to experimental data. It is shown that a considerable improvement of the half-life description is obtained in pn-RQTBA because of the spreading effects, which bring the lifetimes to a very good quantitative agreement with data.