## Topics of research: Physics on extra dimensions and superstring theories

1. Construction of realistic grand unified theory

Grand unified theories (GUTs) are attractive theories that describe three forces (the strong, weak and electromagnetic forces) except for the gravitational force as a single gauge interaction, but they have not yet been verified experimentally. The strong force plays a role of the binding of nucleon and the weak force plays a role of the decay of particles (a typical one is the beta-decay).
We expect that a structure of nature is as simple as possible at a fundamental level and **it is possible to understand different phenomena and features in a unified fashion using GUTs.** Hence, GUTs are promising candidates as a theory beyond the standard model (SM) of particle physics. There are, however, several open questions. For instance, GUTs predict the decay of proton, but such a process has not yet been observed. An unnatural fine-tuning is required among parameters to construct the Higgs sector in the SM.
A symmetry called "supersymmetry (SUSY)" has been paid much attention to as a new physical concept to build a realistic theory. The SUSY is a symmetry between two types of elementary particles called "boson" and "fermion". The SUSY has not also yet been found, but it is still appealing because it can partially solve the fine-tuning problem.

2. Exploration of physics on extra dimensions

Q.Why extra dimensions?

A simple answer is "because there are mountains, i.e., big problems". In fact, we have various problems that cannot be solved using theories in four-dimensional space-time, and suppose that an extension of the framework could be effective. **The unification of physical laws can be implemented in the introduction of extra dimensions.** Furthermore, superstring theories (SSTs) that are powerful candidates of "theory of everything" containing quantum gravity are defined in ten-dimensional space-time, that is, SSTs build in extra dimensions.

Q. Why have extra dimensions not been observed?

Tentative answers are "because the size of extra dimensions is small enough" and "because the SM particles cannot move the directions of extra dimensions", but a real one remains unclear. Models based on "a brane world scenario" have been intensively studied. The brane world scenario is the idea that our world seems like a four-dimensional wall in a higher-dimensional space-time. For instance, one of \(M^4\) stands for our four-dimensional space-time in Fig.1. To clarify whether the SUSY and/or extra dimensions exist or nor, I have tried to construct a realistic SUSY GUT in a higher-dimensional space-time and to pursue a method of verification.

## My dream: I long to uncover an ultimate law of physics.

Quest of unknown laws

I have searched for an ultimate law of physics, believing in its existence. The belief is based on things like precedents. Concretely speaking, there are many laws in electromagnetism such as Coulomb's law, Lentz's law, Faraday's law and so on, and they are derived from a set of equations called "Maxwell's equations". Hence, I guess **that some physical laws must be unified under a more fundamental law and to witness such a realization would be one of the best part of physics.** Why don't you join us?

*Maxwell's equations: A set of differential equations that describe electromagnetic phenomena in a unified form.*

Grasp of fundamental concepts

Most physical concepts are not completely understood at a deeper level. Let us ask the questions such that "what is time?", "what is space?", "what is a fundamental object or constituent?", "what is the electric charge?", "why does electron exist?" and so on. We have no final answers of the above-mentioned questions.

We expect that equations in underlying theories must be beautiful. By putting the question "what does beautiful mean?" aside for now, **I think that to understand the beauty of equations would be a real joy of physics.** Why don't you join us?

*Dirac equation: A relativistic wave equation that describes the motion of particle with spin 1/2 and that stems from the marriage of special relativity and quantum mechanics.*

Full of mystery

96 percent of substances in our universe have not yet been identified from the energy budgetary perspective. An unknown energy called "dark energy" and an energy of extraordinary matter(s) called "dark matter" comprise 74 and 22 percent, respectively. An origin of universe also remains undetermined. Our universe is full of mystery.

*Einstein equation: A tensor equation that describes the interaction of gravitation and the dynamics of space and time based on general relativity.*

Beauty of equations

For "what is the beauty of equations?", "symmetry" can be a key word. A symmetry means an invariance of a physical system under some transformation. The more the system has a symmetry, the more the system looks beautiful. Symmetries assist us finding its physical properties (the existence of conservation quantities, the form of interactions, the classification of particles and so on). Symmetries can also play a vital role of a guiding principle to construct theories. We can touch natural providences through magnificent concepts such as symmetries, and it would drive our fascination with physics. Why don't you join us?

Transport myself back in time

I wanted to know an exact meaning of equations and understand their beauty as soon as possible, shortly after I entered university. At present, I long to discover a fundamental law and a basic equation beyond laws and equations in hand, hoping that it must be extremely excellent and beautiful. On the way to the goal, I often murmur "It is totally beyond my knowledge, but it is quite intriguing!".