The origin of pseudospin symmetry (PSS) and its breaking mechanism are explored by combining supersymmetry (SUSY) quantum mechanics, perturbation theory, and the similarity renormalization group (SRG) method. The Schrödinger equation is taken as an example, corresponding to the lowest-order approximation in transforming a Dirac equation into a diagonal form by using the SRG. It is shown that while the spin-symmetry-conserving term appears in the single-particle Hamiltonian H, the PSS-conserving term appears naturally in its SUSY partner HamiltonianH.The eigenstates of Hamiltonians H andH are exactly one-to-one identical except for the so-called intruder states. In such a way, the origin of PSS deeply hidden in H can be traced in its SUSY partner HamiltonianH. The perturbative nature of PSS in the present potential without spin-orbit term is demonstrated by the perturbation calculations, and the PSS-breaking term can be regarded as a very small perturbation on the exact PSS limits. A general tendency that the pseudospinorbit splittings become smaller with increasing single-particle energies can also be interpreted in an explicit way.

Starting from the relativistic form of the Bonn potential as a bare nucleon-nucleon interaction, the full Relativistic Brueckner-Hartree-Fock (RBHF) equations are solved for finite nuclei in a fully selfconsistent basis. This provides a relativistic ab initio calculation of the ground state properties of finite nuclei without any free parameters and without three-body forces. The convergence properties for the solutions of these coupled equations are discussed in detail at the example of the nucleus 16 O. The binding energies, radii, and spin-orbit splittings of the doubly magic nuclei 4 He, 16 O, and 40 Ca are calculated and compared with the earlier RBHF calculated results in a fixed Dirac Woods-Saxon basis and other non-relativistic ab initio calculated results based on pure two-body forces. PACS numbers: 21.10.-k, 21.30.Fe, 21.60.De,

A systematic and specific pattern due to the effects of the tensor forces is found in the evolution of spin-orbit splittings in neutron drops. This result is obtained from relativistic Brueckner-Hartree-Fock theory using the bare nucleon-nucleon interaction. It forms an important guide for future microscopic derivations of relativistic and nonrelativistic nuclear energy density functionals.

Starting with a bare nucleon-nucleon interaction, for the first time the full relativistic Brueckner-Hartree-Fock equations are solved for finite nuclei in a Dirac-Woods-Saxon basis. No free parameters are introduced to calculate the ground-state properties of finite nuclei. The nucleus 16 O is investigated as an example. The resulting ground-state properties, such as binding energy and charge radius, are considerably improved as compared with the non-relativistic Brueckner-Hartree-Fock results and much closer to the experimental data. This opens the door for ab initio covariant investigations of heavy nuclei.

Nuclear structure models built from phenomenological mean fields, the effective nucleon-nucleon interactions (or Lagrangians), and the realistic bare nucleon-nucleon interactions are reviewed. The success of covariant density functional theory (CDFT) to describe nuclear properties and its influence on Brueckner theory within the relativistic framework are focused upon. The challenges and ambiguities of predictions for unstable nuclei without data or for high-density nuclear matter, arising from relativistic density functionals, are discussed. The basic ideas in building an ab initio relativistic density functional for nuclear structure from ab initio calculations with realistic nucleon-nucleon interactions for both nuclear matter and finite nuclei are presented. The current status of fully self-consistent relativistic Brueckner-Hartree-Fock (RBHF) calculations for finite nuclei or neutron drops (ideal systems composed of a finite number of neutrons and confined within an external field) is reviewed. The guidance and perspectives towards an ab initio covariant density functional theory for nuclear structure derived from the RBHF results are provided. Summary and Perspectives 521. Introduction Brief introduction on nuclear theoryThe discoveries of radioactivity by Becquerel [1] and the Curies [2, 3] and the existence of a compact nucleus at the center of an atom by Rutherford et al. [4] opened the door of nuclear physics. During the hundred years of development in nuclear physics, there emerged several significant milestones, including the discovery of the neutron by Chadwick [5] which verified the composition of the nucleus as protons and neutrons, the meson-exchange theory for the strong interaction between nucleons by Yukawa [6], the independent-particle shell model of the nucleus by Goeppert-Mayer [7], Haxel, Jensen, and Suess [8], and the collective Hamiltonian for nuclear rotation and vibration by Rainwater [9], Bohr and Mottelson [10,11], etc.With the understanding of the composition of a nucleus as protons and neutrons [5] and the meson-exchange theory for the strong interaction between the nucleons [6], nuclear physicists hoped to describe the nucleus, a quantum many-body system, from the underlying nucleon-nucleon interaction. Euler, a student of Heisenberg, assumed the nuclear force as a two-body (2N) interaction with a Gaussian shape and calculated the infinite nuclear system, i.e., homogeneous nuclear matter, using second-order perturbation theory [12]. However, the strong repulsive core of the realistic nuclear force [13] prevents the application of perturbation theory.On the other hand, the nuclear structure model with a phenomenological mean field achieved great success. Goeppert-Mayer [7], Haxel, Jensen, and Suess [8] introduced a strong spin-orbit potential and proposed the nuclear independentparticle shell model that successfully explained the conventional magic numbers in nuclei. Rainwater [9], Bohr and Mottelson [10, 11] explored the nuclear deformation and proposed the nuclear collective mode...

Neutron drops confined in an external field are studied in the framework of relativistic Brueckner-Hartree-Fock theory using the bare nucleon-nucleon interaction. The ground state energies and radii of neutron drops with even numbers from N = 4 to N = 50 are calculated and compared with results obtained from other nonrelativistic ab initio calculations and from relativistic density functional theory. Special attention has been paid to the magic numbers and to the sub-shell closures. The single-particle energies are investigated and the monopole effect of the tensor force on the evolutions of the spin-orbit and the pseudospin-orbit splittings is discussed. The results provide interesting insight of neutron rich systems and can form an important guide for future density functionals.Recently, the self-consistent relativistic Brueckner-Hartree-Fock (RBHF) theory for finite nuclei has been established, and the results are in much better agreement with experimental data than the nonrelativistic calculations with the 2N interaction only [22,23]. Indeed, it is known since more than 30 years that relativistic Brueckner-Hartree-Fock theory gives a much better description of the nuclear matter saturation properties than nonrelativistic BHF theories [38][39][40]. In nonrelativistic many-body investigations on the influence of various types of 3N-interactions, it was found that a relativistic effect, the so-called Z-diagram, plays a major role [41].Having these progresses in mind, it is important to study the neutron drops in more detail in the framework of RBHF theory and compare the results with other nonrelativistic ab initio calculations using various 2N or 2N + 3N interactions, as well as calculations using various density functionals. This can also provide valuable insight to improve current relativistic density functionals. In Ref. [28], a systematic and specific pattern due to the tensor forces in the evolution of spin-orbit splittings based on RBHF theory is reported.In this work, we investigate neutron drops confined in an external harmonic oscillator potential using relativistic Brueckner-Hartree-Fock theory, and present the numerical details and calculated results in detail. In Sec. II, we give a brief outline of the RBHF framework for neutron drops. The numerical details are discussed in Sec. III. Results and discussion for neutron drops with an even number of neutrons from N = 4 to 50 will be presented in Sec. IV. Finally, a summary and perspectives for future investigations will be given in Sec.V.

Following a previous paper [Haozhao Liang et al., Phys. Rev. C 87, 014334 (2013)], we discuss the spin-orbit effects on the pseudospin symmetry (PSS) within the framework of supersymmetric quantum mechanics. By using the perturbation theory, we demonstrate that the perturbative nature of PSS maintains when a substantial spin-orbit potential is included. With the explicit PSS-breaking potential, the spin-orbit effects on the pseudospin-orbit splittings are investigated in a quantitative way.

A new Skyrme functional devised to account well for standard nuclear properties as well as for spin and spin-isospin properties is presented. The main novelty of this work relies on the introduction of tensor terms guided by ab initio relativistic Brueckner-Hartree-Fock calculations of neutron-proton drops. The inclusion of tensor term does not decrease the accuracy in describing bulk properties of nuclei, experimental data of some selected spherical nuclei such as binding energies, charge radii, and spin-orbit splittings can be well fitted. The new functional is applied to the investigation of various collective excitations such as the Giant Monopole Resonance (GMR), the Isovector Giant Dipole Resonance (IVGDR), the Gamow-Teller Resonance (GTR), and the Spin-Dipole Resonance (SDR). The overall description with the new functional is satisfactory and the tensor terms are shown to be important particularly for the improvement of the Spin-Dipole Resonance results. Predictions for the neutron skin thickness based on the non-energy weighted sum rule of the Spin-Dipole Resonance are also given. PACS numbers: