Research of the Particle Theory Group

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

1. Neutrino Physics, Astroparticle Physics

Neutrino mass was discovered by the Superkamiokande experiment, and a door to a new frontier was opened. 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 purposes are expected to be attained by the 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. On the other hand, long baseline experiments with intense beams will enable us to look for deviation from the standard framework of three flavor massive neutrinos, and they may offer us a hint on the physics beyond the Standard Model. Such scenarios include non-standard interactions of neutrinos, light sterile neutrinos, violation of unitarily due to heavy particles. Our group has been studying possible physics at long baseline experiments such as T2K (JPARC at Tokai to Superkamiokande) or 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 much higher energy scale such as the Grand Unified Theories. The information obtained through neutrinos is complementary to that from 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 masses of elementary particles (Physics of spontaneous broken electroweak symmetry)

The origin of masses of elementary particles 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 should be broken spontaneously. The mechanism of the spontaneous breaking of this symmetry is unknown, and various theories have been proposed. We are studying the possibility in which this mechanism is understood by string theory. The LHC experiment, which was started at CERN in 2010, is intended to elucidate physics of the electroweak symmetry breaking. String theory is a framework which enables unified treatment of matter and interactions including gravity. In the present framework of quantum field theory, we first assume matter particles and then we turn on the interactions. Therefore we cannot predict on the interaction which mediates the 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 has a lot of theoretical problems. We are conducting research on string theory which is related to the accelerator experiment mentioned above, research toward resolution of the theoretical problems of string theory, and furthermore its implication for cosmology.