Research Interests

Last Update: March 2, 2015

I am interested in the physics beyond the Standard Model. The Standard Model is the model of elementary particles and their interactions which explains almost all the present experimental results. But there are some problems in the model. One of the largest problems is that the masses of elementary particles are not explained. The values, or origin, of the masses of six quarks and six leptons are not explained, but simply described by arbitrary parameters. We do not yet really understand the hierarchical masses of elementary particles and also their mixing. This is the flavor problem in the standard model of elementary particles.

I have learned much efforts to solve this problem in the framework of the quantum field theory, and I myself made efforts by sometimes proposing ideas and sometimes constructing explicit models. I think it is a common fact that the models to derive realistic masses of quarks and leptons become very complicated with many parameters or some unjustified assumptions. Some ideas and models go even beyond the framework of the quantum field theory by introducing non-renormalizable interactions, extra dimensions with non-trivial background, and so on. I agree that we may have to go beyond the framework of the quantum field theory, but I think we should work within the well-defined framework.

String Theory is an framework which naturally includes quantum gravity, and the perturbative superstring theory is an well-defined framework. The standard strategy of the model building based on the perturbative superstring theory, which is typically the theory with ten-dimensional space-time, is to make extra six-dimensional space "compact" and small enough, and obtain our four-dimensional space-time. The features in our four-dimensional space-time, namely field contents, gauge symmetry and so on, originate from the geometry of the six-dimensional compact space and background fields inside. There is a long history of model building based on the heterotic string theory, since the heterotic string theory (and type I theory) contains gauge symmetry from the beginning. Although the type IIA and IIB theory originally contains only the gravity (supergravity) in ten-dimensions, gauge symmetries can be included by introducing D-branes. The model building in type IIA and IIB theories with D-branes is an important direction towards realistic model, because we can expect models with stable compact space of extra six-dimensional space in this direction.

I would like to challenge the flavor problem in the standard model, including the dynamics of the electroweak symmetry breaking, not in the framework of quantum field theory, but in the framework of the perturbative type II string theory with D-branes. I would like to contribute by proposing practical and concrete ideas towards constructing realistic models. For example, I have already proposed an idea towards obtaining non-trivial flavor structure in the model with D-branes which intersect with each other in compact six-dimensional space (so called intersecting D-brane model) by introducing the compositeness of quarks and leptons. I have recognized that non-trivial dynamics of D-branes in the situation without supersymmetry is an key issue to understand spontaneous gauge symmetry breaking (or electroweak gauge symmetry breaking) in the framework of String Theory. I have also recognized that exploring String Theory without supersymmetry is necessary and inevitable to obtain stable six-dimensional compact space. Understanding the physics of String Theory without supersymmetry is the subject of my recent research towards the goal.