Skyrme-Hartree-Fock-Bogoliubov mass models on a 3D mesh: II. Time-reversal symmetry breaking

TL;DR

This paper introduces the Skyrme-based BSkG2 nuclear model that lifts the time-reversal symmetry assumption, allowing for more accurate predictions of nuclear properties by including time-odd effects, and achieves high accuracy in mass and fission barrier predictions.

Contribution

The study develops a new nuclear energy density functional model that does not rely on the Equal Filling Approximation, improving the description of odd-mass and odd-odd nuclei.

Findings

Achieves 0.678 MeV rms deviation on nuclear masses

Provides accurate fission barrier predictions for actinides

Enables analysis of spin and current densities in ground states

Abstract

Models based on nuclear energy density functionals can provide access to a multitude of observables for thousands of nuclei in a single framework with microscopic foundations. Such models can rival the accuracy of more phenomenological approaches, but doing so requires adjusting parameters to thousands of nuclear masses. To keep such large-scale fits feasible, several symmetry restrictions are generally imposed on the nuclear configurations. One such example is time-reversal invariance, which is generally enforced via the Equal Filling Approximation (EFA). Here we lift this assumption, enabling us to access the spin and current densities in the ground states of odd-mass and odd-odd nuclei and which contribute to the total energy of such nuclei through so-called "time-odd" terms. We present here the Skyrme-based BSkG2 model whose parameters were adjusted to essentially all known nuclear masses without relying on the EFA, refining our earlier work [G. Scamps et al., EPJA 57, 333 (2021), arXiv:2011.07904]. Moving beyond ground state properties, we also incorporated information on the fission barriers of actinide nuclei in the parameter adjustment. The resulting model achieves a root-mean-square (rms) deviation of (i) 0.678 MeV on 2457 known masses, (ii) 0.027 fm on 884 measured charge radii, (iii) 0.44 MeV and 0.47 MeV, respectively, on 45 reference values for primary and secondary fission barriers of actinide nuclei, and (iv) 0.49 MeV on 28 fission isomer excitation energies. We limit ourselves here to a description of the model and the study the impact of lifting the EFA on ground state properties such as binding energies, deformation and pairing, deferring a detailed discussion of fission to a forthcoming paper.