Subspace-projected multireference covariant density functional theory

Multireference density functional theory (MR-DFT) has been a pivotal method for studying nuclear low-lying states and neutrinoless double-beta ($0νββ$) decay. However, quantifying their theoretical uncertainties has been a significant challenge due to the computational demands. This study introduces a subspace-projected covariant density functional theory (SP-CDFT), which efficiently emulates MR-CDFT calculations for nuclear low-lying states. This approach leverages the eigenvector continuation method combined with the quantum-number projected generator coordinate method, based on a relativistic energy density functional (EDF). We apply SP-CDFT to investigate the correlations among the physical quantities of nuclear matter, nuclear low-lying spectroscopy, and the nuclear matrix elements (NMEs) of $0νββ$ decay in the two heaviest candidate nuclei. Our findings reveal generally strong correlations between the NMEs of $0νββ$ decay and the excitation energy of the $2_1^+$ state, as well as the $E2$ transition strength, although these correlations vary significantly among nuclei. This work also paves the way for refining nuclear EDF parameters using spectroscopic data.