[ Japanese ]

The atomic nucleus is a self-organized many-body quantum system of nucleons – protons and neutrons, and is governed by the interaction acting among them, i.e., the nuclear force. The nucleus exhibits many interesting and unique facets of structures and reaction dynamics, thus offering many possibilities for research, while at the same time there are numerous problems that remain to be solved. By now thousands of nuclides are predicted by theory. It has nowadays become possible to produce and accelerate radioactive nuclei at experiments, which has opened many new avenues for low-energy nuclear physics. For instance, since in nuclei, electromagnetic, strong and weak fundamental interactions play crucial roles, the understanding structure and reaction properties of nuclei is key to elucidate the origin of matters (i.e., synthesis of heavy chemical elements), and fundamental symmetries in Nature. Thus the knowledge of the nuclear structure and reaction finds applications in other fields of research. We investigate the quantum many-body problems concerning the nuclear structures and reactions, which are also of astrophysical interest.

Nuclear deformations and collective excitations

The nucleus takes on certain geometrical shapes that correspond to a spherical vibrator, and an ellipsoidal deformed rotor. This feature is known as collective motion, which exhibits remarkable regularities and symmetries of nuclear structure as observed in the vibrational or rotational energy spectra and characteristic patterns of electromagnetic transition rates. The questions as to how the atomic nucleus organizes itself into a variety of shapes and what are the microscopic origins have been, since the discovery of the unified theory of single-particle and collective motions by A. Bohr and B.R. Mottelson (Nobel Prize in physics in 1975), a central problem in low-energy nuclear physics, are also common to quantum many-body systems in general. We develop a theory allowing for a universal and microscopic description of the structure and dynamics of heavy nuclei, constructed by using the nuclear density functional theory (DFT) and the Interacting Boson Model (IBM). Recently we apply the method to describe those nuclear structure phenomena like shape coexistence and pear-shaped deformations in nuclei, which are declared objectives of the low-energy experiments around the world. We are involved in international collaborations with both theoretical and experimental nuclear physicists, aiming to interpret and analyze the low-energy structures of heavy nuclei which are of interest for experiments using RI beams.

Fundamental nuclear processes

Nuclear β decay is a fundamental process in which protons in the nucleus convert into neutrons, or vice versa. The β decay is important for determining the low-lying structure of individual nuclei, and is also crucial to model astrophysical nucleosynthesis of heavy elements. In rare cases, two successive β decays may occur (which are called double-β or ββ decay) between the neighboring even-even nuclei, emitting two electron or positrons and some light light particles such as antineutrinos or neutrinos. Especially the ββ decay that does not emit neutrinos (0νββ decay) is of fundamental importance, because this decay mode would imply violations of various symmetry properties required for the electroweak fundamental interaction. In nuclear theory, calculations of the nuclear matrix elements for these fundamental nuclear decays require the precise modeling of the nuclear low-lying states for each nucleus involved in the process (even-even, odd-even, and odd-odd). We have proposed a theoretical scheme that is constructed by using the microscopic framework of nuclear density functional theory, which allows for a simultaneous description of the nuclear low-lying states and β and ββ decay properties.

Nuclear clustering from realistic nuclear forces

Due to the strong nucleon-nucleon correlations, the nuclei exhibit a rich variety of structure phenomena. For those unstable nuclei close to the neutron dripline, the correlations among nucleons are even more enhanced, leading to many interesting features that are not observed in stable nuclei. We particularly investigate the clustering aspects of light nuclei, i.e., the nuclear structure characterized by the excitations of alpha-like particles (4He). As a promising tool to approach the clustering phenomena, we have developed the “Anti-symmetrized Molecular Dynamics (AMD)” model, which simultaneously incorporates both the shell-model-like structure resulting from the nuclear mean field and the cluster-like structure corresponding to the molecule-like picture. We also derive an effective nucleon-nucleon interaction from the realistic nuclear force. Recently we try to solve the so-called cosmological 7Li problem using the AMD model implemented in the eigenvalue problem within the generator coordinate method.

Nuclear reactions

Nuclear reactions provide crucial information about nuclear structure, and also give insight into astrophysical nucleosynthesis in the Early Universe. In addition, the study of nuclear reaction mechanism finds several practical applications, e.g., in nuclear engineering. We develop the Continuum-Discretized Coupled-Channels (CDCC) model to describe accurately the break-up reactions, which is among the most important reaction processes. More recently, we are trying to optimize parameters for nuclear reaction models using methods of machine learning, aiming to a reliable estimation of nuclear data.