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.