The origin of elements heavier than iron in the universe is one of the central problems in contemporary nuclear astrophysics. These heavy elements are primarily produced through the rapid neutron-capture process (r-process), which is widely believed to occur in astrophysical environments such as neutron-star mergers. A quantitative understanding of the r-process requires reliable nuclear-structure and reaction data for neutron-rich nuclei, in particular nuclear masses and single beta-decay half-lives. Since most of these nuclei are experimentally inaccessible, the required data rely heavily on theoretical calculations.

The quasiparticle random-phase approximation (QRPA), developed on the basis of phenomenological nuclear interactions or nuclear density functional theory, has achieved considerable success in the study of single beta decay in nuclei. However, calculated beta-decay half-lives are highly sensitive to the choice of effective interaction, model parameters, and the axial-vector coupling constant gA. Determining these phenomenological parameters in a reliable and systematic manner therefore remains one of the key open scientific challenges.

Over the past decade, the application of effective field theory (EFT) and similarity renormalization group (SRG) methods in nuclear physics has significantly advanced the development of ab initio nuclear-structure calculations. In the early 1990s, Weinberg proposed a framework for deriving nuclear forces from effective field theory. The central idea is to construct an effective Lagrangian consistent with the symmetries of quantum chromodynamics (QCD), organize all possible contributions in an expansion in powers of a small parameter, and compute observables order by order. Here, Q denotes the typical momentum of nucleons inside nuclei, and Lambda represents the chiral symmetry breaking scale (700-1000 MeV). For most nuclei, Q is approximately equal to the pion mass (140 MeV), which implies that chiral EFT explicitly includes pion degrees of freedom. Since the N3LO chiral nuclear interaction developed by Entem and Machleidt achieved an accuracy comparable to that of phenomenological realistic nuclear forces in fitting nucleon-nucleon scattering data, chiral interactions have become increasingly popular in nuclear-structure theory. One of their most important advantages is that both nuclear forces and electroweak current operators can be derived consistently within the same chiral EFT framework and truncated at the same order. The associated truncation uncertainties can be systematically constrained by power counting. Despite remaining open issues, these unique features have made chiral nuclear forces the interaction of choice in modern ab initio nuclear-structure calculations.

This project aims to start from chiral nuclear forces and derive effective interactions suitable for the quasiparticle random-phase approximation (QRPA) using similarity renormalization group methods. By incorporating the finite-amplitude method (FAM), we plan to develop an in-medium QRPA (IM-QRPA) framework and apply it to the study of single beta-decay half-lives in medium-mass and heavy deformed nuclei. This work is expected to provide valuable guidance for constraining free parameters in traditional QRPA models, such as the proton-neutron isoscalar pairing strength and the axial-vector coupling constant gA, to support ongoing and future experiments, and to deliver ab initio predictions of beta-decay half-lives for key nuclei involved in r-process nucleosynthesis.

More details: see the ChiBeta-MM program