Ground and excited states of doubly open-shell nuclei fromab initiovalence-space Hamiltonians

Abstract

We present ab initio predictions for ground and excited states of doubly open-shell fluorine and neon isotopes based on chiral two-and three-nucleon interactions. We use the in-medium similarity renormalization group, to derive mass-dependent sd valence-space Hamiltonians. The experimental ground-state energies are reproduced through neutron number N = 14, beyond which a new targeted normal-ordering procedure improves agreement with data and large-space multi-reference calculations. For spectroscopy, we focus on neutron-rich 23−26 F and 24−26 Ne isotopes near N = 14, 16 magic numbers. In all cases we find agreement with experiment and established phenomenology. Moreover, yrast states are well described in 20 Ne and 24 Mg, providing a path towards an ab initio description of deformation in the medium-mass region. PACS numbers: 21.30.Fe, 21.60.Cs, 21.60.De, 21.10.-k With hundreds of undiscovered nuclei to be created and studied at rare-isotope beam facilities, the development of an ab initio picture of exotic nuclei is a central goal of modern nuclear theory. Three-nucleon (3N) forces are a key input to understand and predict the structure of medium-mass nuclei, from the neutron dripline in oxygen to the evolution of magic numbers in oxygen and calcium [1-11]. In addition, advances in large-space many-body methods have extended the scope of ab ini-tio theory to open-shell calcium and nickel isotopes, and beyond [12-14]. While ground-state properties of even-even isotopes are captured with these methods, excited states and/or odd-mass systems away from closed shells are more challenging. Furthermore, doubly open-shell nuclei may exhibit deformation, which is challenging to capture in large-space ab initio methods built on spherical reference states [15, 16]. These difficulties can be addressed straightforwardly within the framework of the nuclear shell model [17-19], where an effective valence-space Hamiltonian is constructed for particles occupying a small singe-particle space above some closed-shell configuration. Exact di-agonalization then accesses all nuclei and their structure properties in a given region and naturally captures deformation [20]. While the shell model approach is traditionally phenomenological, valence-space Hamiltonians obtained with many-body perturbation theory (MBPT) [21] including 3N forces describe separation energies and first-excited 2 + energies in the sd shell above 16 O [22, 23]. However, order-by-order convergence of is difficult to verify , especially for T = 0 components, and a successful description of exotic nuclei requires the use of extended valence spaces [24-27]. All-order diagrammatic extensions provide further insights [28] but exhibit dependence on the harmonic-oscillator spacing ω and have not been benchmarked with 3N forces. Recently, nonperturbative methods have been developed [29-33], which provide a promising path toward an ab initio description of nuclei between semi-magic isotopic chains, but have not been applied systematically beyond oxygen. In this article we p...

In-medium similarity renormalization group for closed and open-shell nuclei

“…This is illustrated beautifully in the examples of 20 Ne and 22 Ne, which both have intrinsically deformed ground states [186]. In section 7, we will show that Shell model calculations with IMSRG effective interactions based on the N N +3N (400) Hamiltonian [164,165] yield deformed ground states for 20,22 Ne. In these calculations, we also find intrinsically spherical 0 + 2 excited states whose absolute energies are in excellent agreement with the MR-IMSRG(2) energies.…”

“…We compare results from the original IMSRG+SM approach discussed in [164] and an improved version using the so-called targeted normal ordering (TNO, [165]), using 22 O and 24 O as reference states. Black bars indicate experimental data [110].…”

“…As we have shown in a series of publications [122,164,165], we can use IMSRG flows to construct nonempirical interactions for use in valence-space CI approaches like the interacting Shell model. In this way, we can systematically link Shell model phenomenology to the underlying nuclear interactions in the vacuum, and through them to QCD if we start from chiral N N and 3N interactions.…”

“…The oxygen isotopic chain has become a testing ground for ab initio nuclear structure methods in recent years [55,91,128,[159][160][161][162][163][164][165], mainly for two reasons: First, Otsuka et al [160] showed the impact of 3N forces on the location of the neutron drip line, and second, exact results from the importance-truncated NCSM (IT-NCSM) [56,91,166,167] are available for the ground states and low-lying excitations, allowing us to assess the many-body uncertainties of our calculations.…”

“…In the following, we will discuss the implementation of valence-space decoupling via a straightforward modification of the ground-state decoupling, and review results from recent applications [164,165,189].…”