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Theoretical Nuclear Physics

  • Masanobu Yahiro, Professor
  • Yoshifumi R. Shimizu, Associate Professor
  • Takuma Matsumoto, Assistant Professor
Our group is making advanced research work on nuclear physics from theoretical point of view. Here the nucleus is a finite quantum-mechanical many-body system, which is composed of two kinds of elementary particles, protons and neutrons. We are also working on hadron physics, where the quarks and gluons are basic ingredients and they are govern by the fundamental theory of strong interaction, quantum chromodynamics (QCD).

Our "Theoretical Nuclear Physics" group is working on nucleus and hadron physics, and covering a rather wide range of this field. We are now mainly making the following research works.

1. Hadron Physics

The hadron is a generic name of meson and baryon. Representative examples of the baryon are proton and neutron, and it is composed of three quarks interacting by exchanging many gluons. On the other hand the meson is composed of quark and anti-quark, which are strongly interacting through gluons. These hadrons are elementary ingredients of matter in our world.

The hadron physics is dealing with these quarks and gluons, and is an interdisciplinary field between elementary particle physics and nuclear physics. In order to understand the hadron physics, one has to solve the equation of the quantum chromodynamics (QCD), which is known to be the fundamental theory of strong interaction. However, this QCD is known to be one of the most difficult quantum field theories to solve. For example, there is the so-called problem of confinement; the quarks and gluons are confined in hadron and cannot be taken out. Thus it is very difficult to study the dynamics of quarks and gluons. In this way, the essential part of the hadron physics is not well understood yet and is a very challenging field.

One of the interesting topics is the "phase transitions" expected in QCD; study of the QCD phase diagram. When the temperature and/or the density of baryons are increased, it is predicted that the quarks and gluons are deconfined and can take freely moving state; this state is called the quarks-gluons plasma. This is one of the most interesting QCD phase transitions but is not well known yet. It is expected that such QCD phase transition might occur in the central part of very heavy stars like the neutron stars or the quark stars, and is under intense study by our group.

figure 2. Schematic picture of QCD phase diagram on T-m plane.

2. Physics of Unstable Nuclei

One of the most interesting problems of modern nuclear physics is to understand the properties of unstable nuclei. Nowadays a few thousand of nuclei, which have different combinations of proton number (Z) and neutron number (N), are known to exist. It is predicted about ten thousand or more may exist. Among them only a few hundred are the stable nuclei, and most of existing nuclei is thus unstable. Recent advent of experimental facilities made it possible to study extremely unstable nuclei, i.e. the number of neutrons is much larger than the number of protons or vice versa. It is speculated that such unstable nuclei with, for example, N > 2Z, play important role when very heavy elements like gold or uranium are synthesized in the course of cosmic evolution. Thus, understanding the properties of unstable nuclei is closely related to the fundamental issue of nucleosynthesis and cosmic evolution.

One of the most useful method to study the unstable nuclei is the breakup reaction. When a projectile unstable nucleus collides with the target nucleus, a projectile breakup into smaller pieces. By analyzing this pieces, one can study the properties of the unstable nucleus in details. Our group are trying the systematic study of this breakup reaction on unstable nuclei in collaboration with the experimental group in RIKEN. This is because we have a powerful theoretical tool to analyze the breakup reaction, the continuum discretized channel coupling method (CDCC), which has been developed by our group. We have been successfully applying CDCC to reveal new interesting properties of unstable nuclei.

figure 3. Nuclear chart: Black squares represent stable nuclei, and others are unstable nuclei.

3. Nuclear Structure at Extreme Situations

The nucleus shows various strange and yet interesting properties; understanding them is the main object of the study of nuclear structure. Recent advent of experimental facilities makes it possible to study nuclei at extreme situations, which is very different from their ground states. The unstable nuclei is one of such extreme situations. Another extreme situation related to the limit of increased number of nucleons is the study of superheavy nuclei; it is not well known that how many nucleons the nucleus can accommodate. It is also related to search the new chemical element.

The high-spin limit is one of the main topics of our nuclear structure group is studying; i.e. the question of how fast one can rotate the nucleus. When the nucleus is rotated rapidly, the strong Coriolis and centrifugal forces strongly affect the motions of nucleons inside the nucleus, which leads to various new phenomena that cannot appear in the ground state. For example, many of nucleus is known to be axially deformed like a lemon, and then rotates about the axis perpendicular to the symmetry axis. When the deformation is larger the nucleus gains the rotational energy. Therefore highly elongated shape may be expected at high-spin states; it is the limit of large deformation. To study the nucleus at extreme situations is very interesting, which tells us how the nuclear many-body system can change their form of existence.

figure 4. Schematic picture of nuclear rotational motion