Research of the Particle Theory Group

The purpose of our group is to study the microscopic structure of matter beyond the standard model, and we are conducting research within international collaborations on neutrino physics and mechanisms of mass generation of elementary particles.


1. Neutrino Physics, Astroparticle Physics

Neutrino mass was discovered by the Superkamiokande experiment, which opened the door to a new frontier. This was followed by the solar neutrino and the KamLAND reactor experiments, which elucidated the existence of the three-flavor mixing of leptons. Stimulated by these developments, efforts have been made to clarify the overall structure of the leptonic mixing matrix, i.e., to discover the leptonic CP phase and determine the neutrino mass pattern. These goals are expected to be achieved by long baseline experiments with intense beams from accelerators in the near future. Our group has been conducting basic theoretical research to contribute to the experiments. Long baseline experiments with intense beams will enable us to look for deviations from the standard framework of three-flavor massive neutrinos, and they may offer us a hint about physics beyond the Standard Model. Such scenarios include non-standard interactions of neutrinos, light sterile neutrinos, violation of unitarity due to heavy particles. Our group has been studying possible physics at long baseline experiments such as T2K (JPARC at Tokai to Superkamiokande) or the proposed neutrino factory (with neutrino source obtained from muon decays). The existence of neutrino masses and mixings have quite important implications for understanding the structure of the fundamental hierarchy of matter, and it is expected that they reflect physics at a much higher energy scale, such as Grand Unified Theories. The information obtained through neutrinos is complementary to that obtained by the LHC (Large Hadron Collider) which has been running since 2010, and we are trying to probe the deep structure of nature by combining the information on quarks and leptons.

2. Origin of elementary particle mass (Physics of spontaneous broken electroweak symmetry)

The origin of elementary particle mass is a mystery. While it was experimentally established that the gauge symmetry called the electroweak symmetry exists, this symmetry requires masses of elementary particles to vanish. Since we know that the electron mass does not vanish, this symmetry must be broken spontaneously. The mechanism of this spontaneous symmetry breaking is unknown, and various theories have been proposed. We are studying the possibility that this mechanism can be understood using string theory. The LHC experiment, started at CERN in 2010, is intended to elucidate physics of electroweak symmetry breaking. String theory is a framework which enables the unified treatment of matter and its interactions, including gravity. In the present framework of quantum field theory, we first assume matter particles and turn on the interactions; therefore, we cannot predict the interaction which mediates electroweak symmetry breaking, and this is the origin of the mystery. String theory has the potential to solve this problem. However, (super) string theory is still incomplete and exhibits many theoretical problems. We are conducting research on string theory related to the accelerator experiment mentioned above, research toward resolution of the theoretical problems of string theory, and, research about possible string theory implications for cosmology.