## Particle and Nuclear Theory

### Particle Theory and Cosmology

http://www.tuhep.phys.tohoku.ac.jp/index.html

#### Staff

Professor : | Fuminobu Takahashi | Ken-ichi Hikasa | Masahiro Yamaguchi | |

Associate Professor : | Hiroshi Ishikawa | Yukinari Sumino | Satoshi Watamura | |

Assistant Professor : | Atsushi Naruko | Masahiro Hotta | Youichi Yamada | Norimi Yokozaki |

#### Rsearch

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.

The theory of particle physics is based on relativistic quantum field theory. In particular, gauge theories are most important. Four interactions are known presently to act among particles: They are the electromagnetic interaction, the weak interaction, the strong interaction and the gravitational interaction. The first three interactions are merged into a single gauge theory, the Standard Model, which is surely one of the most important achievements in physics of the last century.

Although the standard model is very successful, it leaves many unanswered questions. 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 might exist new strong interactions such as technicolor. We study various aspects of these “physics beyond the standard model”, e.g., unification of the coupling constants, proton decay, production of supersymmetric particles at colliders, neutrino mass, solar neutrinos, W boson scattering, CP violation and flavor-changing neutral current, etc..

Particle physics is intimately related to cosmology. Many kinds of particles, inaccessible in a laboratory, must have played essential roles at the beginning of the universe. The present structure of the universe might be a result of interactions among many unknown particles activated by high temperature at that time. We study dark matter models and their phenomenological and cosmological implications, inflation models, and the origin of the baryon number in the universe.

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. We study the singularity structure and topology of the space-time under strong gravitation. This is related to the problem of information loss in the quantum theory of black holes.

It is tempting to unify gravity as well. Superstring theory is a good candidate to unify all interactions. Recently there are big developments in this theory. We study string duality, Dirichlet membranes, Dirichlet instantons, F theory, and M 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 for its own sake. We are attempting to construct a new quantum field theory based on noncommutative geometry. It is also important to apply it to various branches of physics. For instance, condensed matter physics is a good field to test it. There is a good chance to gain entirely new insights from a quantum-field-theoretical point of view. Indeed, we have already obtained very new results by applying it to the analysis of quantum Hall effects.

In this way, we are investigating various topics of particle physics, cosmology and quantum field theory widely from the ultimate microscope to the ultimate macroscope.