Particle Theory and Cosmology Group

Professor : Ken-ichi Hikasa and Masahiro Yamaguchi
Associate Professor : Hiroshi Ishikawa, Yukinari Sumino, Fuminobu Takahashi and Satoshi Watamura
Assistant Professor : Masahiro Hotta, Yasuhiro Shimizu and Youichi Yamada


The subjects of particle physics are the elementary particles---the most basic constituents of matter, and their interactions---the most fundamental law of Nature. Particle physics is also called high energy physics, since the short-distance physics corresponds to the high-energy physics because of the uncertainty principle in quantum mechanics.

Theory of elementary particles is based on relativistic quantum field theories, of which gauge theories are most important. Four fundamental interactions are currently known to govern the forces among elementary particles: They are electromagnetic, weak, strong, and gravitational interactions. The first three have been found to have unified description in the form of a gauge theory, the Standard Model. The understanding of the three interactions is surely one of the most important achievements of physics in the last century.

Although the Standard Model is very successful, it leaves many unanswered questions, some of which are theoretical and others based on experiments. Various candidates for physics beyond the Standard Model have been introduced to solve them. Supersymmetry and Grand Unification have received much attention in this context. There may exist extra spacial dimensions other than the three we know. We study various aspects of these new ideas and scenarios, from the viewpoints of theoretical, phenomenological, model-building etc.

Particle physics is intimately related to cosmology. Many kinds of particles, inaccessible in laboratory, must have played essential roles in the beginning of the universe. The present structure of the universe might be a result of interactions among many unknown particles activated by a high temperature at that time. In fact, cosmological observations have lead to clues and evidences for physics beyond the Standard Model. We study cosmic dark matter, cosmology of gravitinos, inflation model of the universe, and the origin of the baryon number in the universe, and so on.

Gravity is described by general relativity as a classical theory. However, its quantization is an important problem yet to be solved. Quantum gravity will be essential in understanding the very early universe, the evaporation of black holes and so on. Superstring theory is an attractive candidate to unify all interactions including gravity. Recently there are important developments regarding this theory, including string duality, Dirichlet-branes, Dirichlet-instantons, M theory, and F theory.

Since quantum field theory is the basic tool in particle theory, we are very much interested in every aspect of quantum field theory. It is worthwhile to investigate it and develop it by its own sake. We are attempting a new quantum field theory based on noncommutative geometry.

Our scope thus extends from the ultimate microscope (particle physics) to the ultimate macroscope (cosmology), with the help of quantum field theory.