What kind of material (elementary particles) is the Universe made of? What kind of law governs those elementary particles?
Physicists have introduced three generations of quarks and leptons (Kobayashi-Maskawa theory) as the fundamental building blocks, and exploited the gauge principle to explain interactions among them. The model is now recognized as the Standard Model (SM) of the elementary particles, which has succeeded in explaining a tremendous number of experimental results at quantitative level. Still, there are lots of unanswered questions in the world of the elementary particles, indicating the emergence of a breakthrough we expect to see when we solve those mysteries.
The origin of the masses of the elementary particles is one of unsolved mysteries. In the SM, the elementary particles obtain their masses through the interaction with the Higgs particle. However, most physicists consider that this is unsatisfactory not only because the Higgs particle has not been discovered yet, but also because it causes notorious hierarchy problem. Such a situation motivated physicists to suggest alternative scenarios, including supersymmetric models, extra-dimensional models, dynamical-symmetry-breaking models, etc. There are also phenomena which cannot be explained in the framework of the SM. Examples of those include: the existence of the dark matter confirmed by cosmological observations, small non-zero mass of the neutrinos indicated by the neutrino oscillations (Maki-Nakagawa-Sakata theory), muon anomalous magnetic moment obtained from the precision measurement. It is natural to imagine that there is physics beyond the SM which reasonably explains mysteries including those mentioned above.
There are also challenges within the SM itself. An example is the quantum chromodynamics (QCD), especially the study of low-energy phenomena of QCD such as color confinement and dynamical chiral symmetry breaking. It is true that studying low-energy phenomena of QCD is quite hard since those are essentially non-perturbative aspects of QCD, however, it is worth challenging. It is not only important for a deeper understanding of the SM, but also gives beneficial information for the search of physics beyond the SM. Non-perturbative nature of QCD is also important for the study of quark-gluon plasma (QGP) and color superconductivity, which are expected to be realized at high temperature and density, respectively. These are important for the study of the early Universe, the neutron stars, etc.
Members of the Division of Theoretical Particle Physics are working on variety of projects to solve mysteries of elementary particles including those explained above. The list of models beyond the SM we study includes: technicolor model, extra-dimensional model, supersymmetric model, grand unified theory (GUT), little Higgs model, etc. Technicolor model, for example, is a type of model which require new strongly coupled gauge dynamics. With output of numerical simulations carried out on the high-performance computers such as "Phi" in the Computational Theoretical Physics Laboratory, we pursue finding a realistic technicolor model. Extra-dimensional model is a conceptual breakthrough which altered our view of space-time. It naturally incorporates the mass of the elementary particles, as well as dark matter. It is also interesting since a new source of flavor violation exists in extra-dimensional models. We study various new physics candidates in view of the origin of the mass, as well as flavor structure, by confronting models with results from B-factory at KEK, neutrino oscillation experiments, direct/indirect detection experiments of the dark matter, etc., and we try to predict new physics signatures which we will see at the Large Hadron Collider (LHC), located at CERN near Geneva, Switzerland.
We also conduct research of the hadron physics in particular for the purpose of clarifying the nature of the quark-hadron many body system. Relativistic Heavy Ion Collider (RHIC) experiment at Brookhaven National Laboratory in USA has already succeeded in generating the QGP state. Furthermore, heavy ion collision experiment at CERN/LHC, which achieves higher energy collision compared to RHIC, is expected to produce more data, which enable us to explore the new world of the hadron physics. There are still lots of experimental results which we currently do not really understand, even qualitatively. We are tackling those mysteries of hadron physics by using variety of methods such as phenomenological approach and lattice simulations of QCD in corporation with the Computational Theoretical Physics Laboratory.
