My research specializes in gravity, low energy effective string theory, and nonperturbative aspects of superstring theory and Yang-Mills theory. Much of my work involve solitonic objects, such as magnetic monopoles and D-branes, and the low energy dynamics thereof. With help of supersymmetry, my collaborators and myself invented and made use of many new aspects of such solitonic system. All of these developments are crucial ingredients in recent progress in string theory and Yang-Mills theory.

Also of interest to me is the so-called tachyon condensation. Tachyon is a name given to a hypothetical particle that moves faster than light, but, in real system, existence of "tachyon" simply implies the system is unstable and is prone to decay to another, stable system. In fact, one of earliest problem with string theories was how to remove such tachyons and at the same time describe gravity. This restriction narrows down variety of consistent string theories drastically. In the absence of manageable string field theory, tachyon condensation allows to probe some of off-shell nature of the theory, although its usefulness remains somewhat restricted. One possible outcome here could be a reformulation of the whole string theory in terms of open string field theory, which in some sense would be a fruition of the induced gravity idea.

I also work on application of string theory to more realistics systems. Notably, two main areas where string theory has been actively used are cosmology and theory of strong interactions. For the former, string theory is now finally making contact with nature in a way that was initially hoped for by the pioneers. Vacuum structure of the theory is now much better understood in a way that we now have some hope for explaining our universe. I am pursuing this problme in two different aspects. The first is a direct application to inflationary cosmology which is often based on the tachyon condensation. The second is to understand vacuum structure of string theory which is not completely explored yet. In particular, IIA and Heterotic theories need be better understood. 

On another side,  string theory is being used as a tool for solving a long-standing problem of strong interactions. Most of the visible mass of the universe originate in the force called QCD, whose definition is well formed but whose actual dynamics have been beyond any analytical attempts. AdS/CFT, an off-shoot from string theory, addressed such strongly interaction systems in some limit, and the so-called holographic QCD emerged as a way of solving QCD. This approach relies on an approximations which is quite valid for real QCD, yet the solution exhibits interesting and semi-realistic behavior. Pushing this method further with a goal for solving QCD proper is one problem I am pursuing at the moment.

Most recently, I began to come back to the problem of BPS states in supersymmetric theories, which is a subject I pursued in the weakly coupled Yang-Mills theories in late 1990's. While this old studies provided many interesting insights and results, one of which is the realization that most BPS states have the shape of loose bound states. This explained why sometimes BPS states disappear across the so-called marginal stability domain wall. The subject was later picked up again in the context of supersymmetric black holes and studied by many researchers under the name of wall-crossing. Nevertheless, the question of wall-crossing remained much abstract and mathematical, riddled with conjectures and hypotheses. This most recent study takes a constructive approach, much like the weak coupling study of late 1990's, but without weak coupling assumption. This allows a most general and systematic construction of BPS states, directly out of field theory.  

MAGNETIC MONOPOLE DYNAMICS, SUPERSYMMETRY, AND DUALITY.
Phys. Rept. 43: 65-236, 2007.

HOLOGRAPHIC DUETERON AND NUCLEON-NUCLEON POTENTIAL
JHEP 0904 (2009) 086 

NUCLEON FORM FACTORS AND HIDDEN SYMMETRY  IN HOLOGRAPHIC QCD
Phys. Rev. D77 (2008) 014030

DYNAMICS OF BARYONS FROM STRING THEORY AND VECTOR DOMINANCE.
JHEP 0709 (2007) 063 

CHIRAL DYNAMICS OF BARYONS FROM STRING THEORY.
Phys. Rev. D76 (2007) 061901 

A HETEROTIC FLUX BACKGROUND AND CALIBRATED FIVEBRANES.
JHEP 0611:040, 2006

COMMENTS ON HETEROTIC FLUX COMPACTIFICATION
JHEP 0607:030, 2006

REHEATING THE UNIVERSE AFTER STRING THEORY INFLATION
Phys. Rev. D72:106001, 2005

WINDING STRINGS AND D-BRANE DECAY WITH FLUX.
JHEP 0501:015, 2005

OPEN / CLOSED DUALITY, UNSTABLE D-BRANES, AND COARSE GRAINED CLOSED STRINGS.
Nucl. Phys. B686:31-52,2004

TACHYON CONDENSATES, CARROLLIAN CONTRACTION OF LORENTZ GROUP, AND FUNDAMENTAL STRINGS.
JHEP 0209:061,2002

STRING FLUID FROM UNSTABLE D-BRANES.
Nucl.Phys.B596:136-150,2001

CONFINEMENT ON THE BRANE.
Nucl.Phys.B580:289-310,2000

MEMBRANES FROM FIVE-BRANES AND FUNDAMENTAL STRINGS FROM DP BRANES.
Nucl.Phys.B550:214-224,1999

GENERAL LOW-ENERGY DYNAMICS OF SUPERSYMMETRIC MONOPOLES.
Phys.Rev.D63:065020,2001

COUNTING YANG-MILLS DYONS WITH INDEX THEOREMS.
Phys.Rev.D62:125006,2000

MONOPOLE DYNAMICS AND BPS DYONS N=2 SUPERYANG-MILLS THEORIES.
Phys.Rev.D61:125012,2000

LOW-ENERGY DYNAMICS FOR 1/4 BPS DYONS.
Phys.Rev.D61:025001,2000