The neutron dripline in calcium isotopes from a chiral interaction

TL;DR

The paper addresses the calcium neutron dripline and the broader reliability of ab initio predictions by introducing N$^3$LO$_{\rm Texas}$, a regulator-consistent chiral interaction that combines NN at N$^3$LO and 3N at NNLO with emulator-accelerated multi-start optimization of 28 LECs. It demonstrates that this interaction reproduces binding energies, radii, and spectra across nuclei up to $^{208}$Pb and places the calcium two-neutron dripline at $^{71}$Ca, significantly extending previous ab initio estimates. The work reconciles ab initio results with density-functional predictions, and highlights the crucial role of fitting both light-nucleus data and bulk properties (notably the $^{16}$O observables) to achieve accurate bulk and dripline behavior. It also establishes high-fidelity emulators that enable efficient exploration of the LEC parameter space, paving the way for Bayesian analyses that quantify EFT and many-body uncertainties in predictive nuclear theory.

Abstract

Interactions derived from effective field theories of quantum chromodynamics have thus far failed to bind calcium nuclei beyond neutron number $N=40$, while nuclear density functionals typically place the neutron dripline near $^{70}$Ca, at $N=50$. We present the chiral interaction N$^3$LO$_{\rm Texas}$, a combination of two- and three-nucleon potentials at fourth and third chiral order, respectively, with low-energy constants optimized using emulator-accelerated fits to few- and many-body data. This interaction accurately reproduces binding energies and charge radii of key nuclei with mass number $A=3$ to $208$, important excited states, and nuclear matter near saturation. Using ab-initio methods, we find that the calcium two-neutron dripline extends to $^{71}$Ca.

Figures (7)

• Figure 1: Ground-state energies ($E_{\rm gs}$), one-neutron ($S_{1n}$) and two-neutron ($S_{2n}$) separation energies for calcium isotopes computed with the interaction N$^3$LO$_{\rm Texas}$ using coupled-cluster (CC) and valence-space in-medium similarity renormalization group (VS-IMSRG) methods, compared with 1.8/2.0(EM) results stroberg2021 and data ensdfwang2021. Stars (open) are masses (extrapolations) from AME2020 wang2021. The red and gray bands indicate many-body uncertainty and different decoupled valence spaces, respectively.
• Figure 2: Similar to Fig. \ref{['Ca_BE_Sn']} but for oxygen isotopes. Experimental data taken from Refs. ensdfkondo2023.
• Figure 3: Binding energies per nucleon and charge radii for doubly closed-shell nuclei from $^{4}$He to $^{208}$Pb, computed with the J-NCSM, IMSRG(2), IMSRG(3f2) and CCSDT-3 methods using N$^3$LO$_{\rm Texas}$, and compared to 1.8/2.0(EM) and experiment ensdfangeli2013garciaruiz2016. Error bars on the IMSRG(3f2) results represent the uncertainty from extrapolation to the infinite model space. In the legend, "Triple" indicates the inclusion of perturbative triples he2024.
• Figure 4: Energy per nucleon for symmetric nuclear matter (top) and pure neutron matter (bottom). The gray rectangle marks the empirical saturation region from Ref. drischler2021. The pink ellipse shows a 95% confidence region for the saturation point inferred from Bayesian posterior distributions based on a set of density functional theory predictions drischler2024.
• Figure 5: Neutron-proton scattering phase shifts and mixing angles as functions of relative momenta, compared with the Granada phase shift analysis perez2013 and results from the 1.8/2.0(EM) hebeler2011simonis2017, $\Delta$NNLO$_{\rm GO}$(394) jiang2020 and N$^3$LO$_{\rm LEFT}$elhatisari2024 interactions.