Welcome

Our laboratory studies theory for elementary particles, which is the basic elements of the universe. What is the world made from? How do they interact? What are the laws of physics behind them? How does the universe evolve under these laws? In order to answer these “ultimate questions,” we try to discover more fundamental laws of nature following past theoretical and experimental studies. That is the research on elementary particle theory.

The equations of QFT are nonlinear involving an infinite number of variables, and it is usually impossible to find their exact solutions for a system with physical interest. The perturbation theory, which approximates a field by a superposition of “ripples,” is a typical systematic approximation of QFT. The perturbative formulation of QFT is well understood. In addition, the existence of elementary particles is derived naturally from the view of perturbative quantization.

On the other hand, it is also known that there are indeed phenomena in nature that cannot be understood by this “perturbation theory”. The spontaneous breaking of chiral symmetry in strong interactions and tunneling effects are the examples. To study such “non-perturbative” phenomena from the first principles, a non-perturbative formulation of QFT and analysis based on it are necessary.

In fact, surprisingly, non-perturbative formulation is not known even for the Standard Model of elementary particles. Then, it is an important challenge in particle theory to find a non-perturbative formulation of the Standard Model. This problem is closely related to the phenomena of anomalies, in which quantum effects break the symmetry of classical mechanics. Moreover, while conventional particle models are mainly based on perturbative picture, there is also a significant prospect that some models based on the non-perturbative dynamics of QFT will become important in the future. We are actively engaged in such theoretical and numerical researches that open up new possibilities and unexplored ground for QFT.

The most fundamental physics theory so far verified experimentally, is called the Standard Model (SM) for elementary particles. The SM  describes physics precisely to about 10 to the minus 18 meters (1/1000th of a nucleus). In 2012, the Higgs boson is observed, so the cutting-edge searches of LHC experiments have moved to precise verification of the Higgs boson. On the other hand, various phenomena beyond the Standard Model have been discovered as results of progress in experiments and observations. In those, there is no doubt that neutrinos have non-zero small masses and that dark matter exists. Then, it is becoming increasingly important to reveal the origin of these phenomena.

In particular, we are interested in how “the electroweak symmetry breaking,” which is the origin of all particle masses in SM, is realized. In SM, the Higgs boson mass is a parameter, but the observed mass is not a theoretical “natural” value. It is believed that the problem implicit in SM is strongly linked to the origin of the electroweak symmetry breaking. We imagine what such physics might be and how to verify it from experiments and observations. Moreover, we also explore new possibilities, such as the understanding by boundary conditions at very high energies, which is a relatively new idea.

Neutrino research is a field in which Japan leads the world, but the origin of neutrino mass is still unclear. Various possibilities are being discussed, including the relationship with Grand Unified Theory and dark matter. In addition, there have been many experiments aimed at detecting dark matter, and stronger restrictions continue to be updated. Thus, new types of dark matter not previously considered are highlighted.

“Superstring theory” is the most promising candidate for a unified theory that describes the four forces observed in nature (gravity, electromagnetic, strong, and weak force) in quantum ways. This is a theory that uses “strings” as the fundamental elements, not elementary particles, and predicts an extra dimensional space of 6 dimensions in addition to the 4 dimensions we recognize (3 dimensions of space + 1 dimension of time). This extra dimensional space is considered to be compact and small enough not to be observed, and we cannot directly perceive it. Over the past 36 years since the birth of string theory, a vast number of 6-dimensional compact spaces, including Calabi-Yau manifolds, have been investigated. However, the full picture of the compactification rule is still unknown, and the Standard Model of elementary particles has not been derived.

For matter particles, the geometric quantities and geometric symmetries of the compact spaces determine the number of generations, generation structures, coupling strengths, CP symmetry breaking, etc. We focus on the rich geometric structure of the 6-dimensional compact spaces and study particle phenomenology based on superstring theory. We also research the compact spaces appearing in string theory and their vacuum structure using machine learning and deep learning, which have been remarkably developed in recent years.