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The standard model has been established as the theory of particle physics
at the weak scale experimentally.
However, it has several mysteries and puzzles inside
such as the gauge and flavor structures
and it also has many free parameters.
Then, it is believed that there would be underlying theory to
Furthermore, the standard model has no caididate for the dark matter,
and also quantum gravity is outside of the target of the standard model.
Thus, there must exist the more underlying theory beyond the standard model
to explain them.
Superstring theory is a promising candidate for the unified theory of all the interactions including gravity and all the matter fields and scalar fields. The typical scale of superstring theory is the Planck scale, which is very high compared with the weak scale. Thus, it is quite important to study in two approaches, the top-down approach from superstring theory and the bottom-up approach with taking into account experiments, complimentarily.