Nuclear response theory with multiphonon coupling in a covariant framework

TL;DR

This paper develops a covariant multiphonon response theory within density functional theory to improve the description of nuclear excited states, especially fine spectral features at low energies in medium-mass and heavy nuclei.

Contribution

It introduces a novel covariant multiphonon response approach that extends existing models to better capture fine spectral details in nuclear excitations.

Findings

Successfully formulated covariant multiphonon response equations.

Applied the theory to medium-mass and heavy nuclei.

Achieved improved description of low-energy spectral features.

Abstract

Background: Nuclear excited states within a wide range of excitation energies are formally described by the linear response theory. Besides its conventional formulation within the quasiparticle random phase approximation (QRPA) representing excited states as two correlated quasiparticles (2q), there exist extensions for 4q configurations. Such extended approaches are quite successful in the description of gross properties of nuclear spectra, however, accounting for many of their fine features requires further extension of the configuration space. Purpose: This work aims at the development of an approach which is capable of such an extension as well as of reproducing and predicting fine spectral properties, which are of special interest at low energies. Method: The method is based on the covariant density functional theory and time blocking approximation, which is extended for couplings between quasiparticles and multiphonon excitations. Results: The covariant multiphonon response theory is developed and adopted for nuclear structure calculations in medium-mass and heavy nuclei. The equations are formulated in both general and coupled forms in the spherical basis. Conclusions: The developed covariant multiphonon response theory represents a new generation of the approaches to nuclear response, which aims at a unified description of both high-frequency collective states and low-energy spectroscopy in medium-mass and heavy nuclei.