Since the discovery of neutrino oscillations by Super-Kamiokande in 1998, the three mixing angles and the absolute values of two mass-squared differences of neutrinos have been determined. What remains to be determined is the mass ordering of the three mass eigenstates, the octant of the atmospheric neutrino mixing angle θ₂₃ , and the CP-violating phase. There are indications of a normal mass ordering similar to that of quarks, a value of θ₂₃ greater than π /4, and the CP-violating phase being around -π / 2, but these are not yet definitively determined. The T2HK experiment in Japan, starting in 2027, is expected to resolve these issues. This article provides a basic explanation of neutrino oscillations, summarizes the results to date within the three-generation neutrino framework, and looks ahead to future prospects, including scenarios beyond the three-generation framework.

Colloidal particles spontaneously assemble in dispersions to form various ordered structures, including colloidal crystals and regular clusters. The self-assembly of colloids has been actively studied as a model for phase transitions in atomic and molecular systems. Colloidal crystals are one of the ‘photonic crystals’ whose refractive index changes with the structural period of light wavelengths. In particular, the diamond lattice of colloidal particles is a ‘perfect photonic crystal’ with a full band gap for incident light from all directions. The formation of the diamond lattice and tetrahedral clusters, the structural units of the diamond lattice, have been actively studied. Particles with a high refractive index are useful for optical materials. However, the high refractive index particles often have a high specific gravity, which causes them to settle in the medium and this can affect their association behavior. Microgravity environments, where the sedimentation is safely negligible, are useful for experiments on colloidal systems. In 2020, in collaboration with JAXA and many other relevant organizations, we experimented on the self-assembly of charged colloidal particles on the Japanese Experiment Module Kibo of the International Space Station. Experimental results on the number of aggregates and structural symmetry suggested that slight settling and convection on the ground have a significant effect on the association of colloids. We expect that the methods and experimental systems used in this space experiment will be useful also for future space experiments on various kinds of soft-matter.

In the early stages of crystal formation, crystals grow not only by the monomer-by-monomer addition mechanisms described in the classical nucleation theory but also by the aggregation of particles, ranging from multi-ion complexes to fully formed nanocrystals. These particle-based growth processes have been considered as “non-classical” crystallization pathways and have been actively studied in recent years. The results have revealed that the initial stages of crystal formation are more complex than we had imagined, but the detailed atomic processes have not yet been elucidated.

As a method to analyze such processes, we focus on Global Reaction Route Mapping (GRRM) program that has recently been developed in the field of quantum chemistry to search for molecular reaction pathways. Using this method, an automatic search for the reaction pathway based on the quantum chemical calculations can be performed without first assuming its mechanism, even when it is not known what kind of product structure will appear. This would make it ideal for elucidating processes in the early stages of crystal formation.

In this study, the GRRM program was used to analyze the initial stages of calcium carbonate polymorph formation. Stable structures of anhydrous and Mg-containing calcium carbonate clusters and their transition processes were derived, showing that Mg²⁺ is readily incorporated into the clusters and changes their stable configuration. In the CaCO₃ tetramer structure containing Mg²⁺, the stacking of Ca²⁺ and CO₃^2- ions was similar to the aragonite structure, which suggests that Mg²⁺ is actively involved in metastable aragonite formation.

These results indicate that the GRRM program is also effective for the analysis of the initial process of crystal formation. It is expected that this can be applied to various systems to clarify the specific picture of these processes.

We have developed a spectroscopic method to directly determine the transition frequency of atoms from the time evolution of Rabi oscillation and named it Rabi-oscillation spectroscopy. Rabi-oscillation spectroscopy does not need to sweep the frequency or draw a resonance curve to determine the resonance frequency. This makes it possible to use data taken under different conditions with different power of the electromagnetic waves in an integrated manner and enables more efficient measurements. We have applied the Rabi-oscillation spectroscopy to the precise measurement of muonium hyperfine structure and verified the effectiveness of this new spectroscopic technique. Rabi-oscillation spectroscopy can be applied in principle to various spectroscopic experiments, and is expected to be particularly useful for spectroscopy of short-lived exotic atoms and nuclei produced at accelerators, where efficiency is important because the number of the atoms and nuclei is very limited.