Exploring origin of matter in the femto-scale

What is "matter"? This is a deep question that has been asked since the Greek era. By definition, matter is a reality with a “structure”. Elementary particles born at the beginning of the universe are the smallest components that do not have a size (structure), but whether or not they gather to acquire an internal structure determines the world in which materialization is possible.

What makes this materialization possible is the so-called "strong interaction", a force that is not touched in everyday life. Of course, in our world, electromagnetic and gravitational interactions create a larger layer of matter, but it is the world dominated by this strong interaction (or nuclear force) that first acquires the "structure". In order to distinguish such substances from the substances in the everyday world, they are called QCD* matter, baryonic matter, nuclear matter**. This is the first substance created in the universe.

*QCD (quantum chromodynamics) : A theory based on quantum field theory that describes a strong interaction called quantum chromodynamics.
**QCD matter, nuclear matter : Hadrons consisting of quarks, and nuclei consisting of nucleons and other hadrons can be said to be matter, but in other words, they are also called finite quantum many-body systems. In particular, atomic nuclei and hadrons are often distinguished from infinite systems, so when we say QCD matter and nuclear matter, we often refer to the case of infinite many-body system in a narrow sense.

It is a world of nuclear material that we rarely touch on the earth we live in, but when we look at the activities of the universe, we can see that nuclear material plays a major role. On the ground, the nuclei are quiet and confined in electrons, but they are still active in the universe in large numbers, from fusion reactions of stars to supernova explosions and neutron stars.

Curiously, in the vast world of the universe, tiny nucleons in the femto-scale gather to self-organize and create a system that transforms order into various forms. At one end of that system, the elements that shape our existence were also created. Life in its present form could exist only if there was nucleosynthesis performed in the stars. In this way, the material creation system in the universe may be expressed by the similarity of life phenomena.

Material creation is happening right now in distant stars, but it is easy to imagine that it is difficult to understand what is actually happening inside by observing from a distance.

Therefore, we use accelerators on the ground to artificially create atomic nuclei (which do not exist on the ground, called unstable nuclei) that would be made in stars, and realize a pseudo-nuclear material environment. We are doing research to experimentally elucidate what is happening in the universe and what kind of "thing" is nuclear material. To list the main research contents below,

• Study of the equation of state from 0 to finite temperature of neutron matter. --> static aspects of the matter. Equilibrium and uniform.
• Study of density / isospin dependence of spontaneous cluster expression mechanism occurring in nuclear matter. --> dynamical aspects of the matter. Solvent and solute. Non-equilibrium and non-uniform. Active matter.
• Development of new technology and equipment for precision reaction measurement with rare RI beams.

Research aimed at understanding the mass existence limit and internal structure of infinite nuclear materials, such as neutron stars floating in the universe, through a system called unstable nuclei, which is a finite system with large isospin asymmetry (proton density / neutron density bias). If you would like to know more details, please feel free to contact our laboratory.

Our main site will be accelerator facilities on the earth. Among them, the main battlefield is a facility with a heavy ion accelerator that can produce various atomic nuclei. Since it is necessary to investigate nuclei that do not exist on the ground (unstable nuclei), we select and use unstable ones from the crushed fragments that are formed when accelerated heavy ions hit another nuclei and break them.

RIBF, located at RIKEN in Japan, is currently the facility that can produce the world's most diverse and high-intensity unstable nuclear beams. In the world, FAIR is under construction at GSI in Germany and FRIB is under construction at MSU in the United States to catch up with and overtake RIBF. The discovery of the famous Nihonium (113th element) was also made at this RIBF.

Our group has developed a new detector (recoil particle spectrometer; RPS) to realize high-precision reaction measurement in unstable nuclei, and has performed the world's first measurement (commonly known as ESPRI project). In 2019, we finally succeeded in measuring proton scattering in an unstable nucleus called 132Sn (protons: 50, neutrons: 82), which is the flagship nucleus in unstable nuclear research, and students are currently taking the initiative in analysis. Currently, a new project called ESPRI+ is underway to get closer to the neutron star system (which has a high proportion of neutrons).

The main experiment will be conducted at the heavy ion facility as mentioned above, but the development up to this point is also very important research. In many experiments, new technologies and detection devices are proposed and used, and tests to determine their performance are indispensable. In such cases, development experiments will be conducted at accelerator facilities throughout Japan. Not only in physics, but also in the development of equipment, you will be challenged with new things, so you can experience a wide range of knowledge. Please feel free to contact us if you would like to know more about the experiment.