Shigeki Sugimoto

Low Energy Hadron Physics in Holographic QCD

We present a holographic dual of four-dimensional, large N_c QCD with massless flavors. This model is constructed by placing N_f probe D8-branes into a D4 background, where supersymmetry is completely broken. The chiral symmetry breaking in QCD is manifested as a smooth interpolation of D8 - anti-D8 pairs in the supergravity background. The meson spectrum is examined by analyzing a five-dimensional Yang-Mills theory that originates from the non-Abelian DBI action of the probe D8-brane. It is found that our model yields massless pions, which are identified with Nambu-Goldstone bosons associated with the chiral symmetry breaking. We obtain the low-energy effective action of the pion field and show that it contains the usual kinetic term of the chiral Lagrangian and the Skyrme term. A brane configuration that defines a dynamical baryon is identified with the Skyrmion. We also derive the effective action including the lightest vector meson. Our model is closely related to that in the hidden local symmetry approach, and we obtain a Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin-type relation among the couplings. Furthermore, we investigate the Chern-Simons term on the probe brane and show that it leads to the Wess-Zumino-Witten term. The mass of the eta meson is also considered, and we formulate a simple derivation of the eta mass term satisfying the Witten-Veneziano formula from supergravity.

More on a Holographic Dual of QCD

We investigate the interactions among the pion, vector mesons and external gauge fields in the holographic dual of massless QCD proposed in a previous paper [T. Sakai and S. Sugi-moto, Prog. Theor. Phys. 113 (2005), 843; hep-th/0412141] on the basis of probe D8-branes embedded in a D4-brane background in type IIA string theory. We obtain the coupling constants by performing both analytic and numerical calculations, and compare them with experimental data. It is found that the vector meson dominance in the pion form factor as well as in the Wess-Zumino-Witten term holds in an intriguing manner. We also study the ω → πγ and ω → 3π decay amplitudes. It is shown that the interactions relevant to these decay amplitudes have the same structure as that proposed by Fujiwara et al. [T. Fujiwara, T. Kugo, H. Terao, S. Uehara and K. Yamawaki, Prog. Theor. Phys. 73 (1985), 926]. Various relations among the masses and the coupling constants of an infinite tower of mesons are derived. These relations play crucial roles in the analysis. We find that most of the results are consistent with experiments.

Baryons from Instantons in Holographic QCD

We consider aspects of dynamical baryons in a holographic dual of QCD that is formulated on the basis of a D4/D8-brane configuration. We construct a soliton solution carrying a unit baryon number and show that it is obtained as an instanton solution of four-dimensional Yang-Mills theory with fixed size. The Chern-Simons term on the flavor D8-branes plays a crucial role of protecting the instanton from collapsing to zero size. By quantizing the collective coordinates of the soliton, we derive the baryon spectra. Negative-parity baryons as well as baryons with higher spins and isospins can be obtained in a simple manner.

Holographic Baryons: Static Properties and Form Factors from Gauge/String Duality

In this paper, we study properties of baryons by using a holographic dual of QCD on the basis of the D4/D8-brane configuration, where baryons are described by a soliton. We first determine the asymptotic behavior of the soliton solution, which allows us to evaluate well-defined currents associated with the U(N_f)_L times U(N_f)_R chiral symmetry. Using the currents, we compute static quantities of baryons such as charge radii and magnetic moments, and make a quantitative test with experiments. It is emphasized that not only the nucleon but also excited baryons, such as Delta, N(1440), N(1535) etc., can be analyzed systematically in this model. We also investigate the form factors and find that our form factors agree well with the results that are well-established empirically. With the form factors, the effective baryon-baryon-meson cubic coupling constants among their infinite towers in the model can be determined. Some physical implications following from these results are discussed.

Nuclear Force from String Theory

We compute nuclear force in a holographic model of QCD on the basis of a D4-D8 brane configuration in type IIA string theory. Repulsive core of nucleons is quite important in nuclear physics, but its origin has not been well-understood in strongly-coupled QCD. We find that string theory via gauge/string duality deduces this repulsive core at short distance between nucleons. Since baryons in the model are realized as solitons given by Yang-Mills instanton configuration on flavor D8-branes, ADHM construction of two instantons probes well the nucleon interaction at short scale, which provides the nuclear force quantitatively. We obtain, as well as a tensor force, a central force which is strongly repulsive as suggested in experiments and lattice results. In particular, the nucleon-nucleon potential V(r) (as a function of the distance) scales as 1/r^2, which is peculiar to the holographic model. We compare our results with one-boson exchange model using the nucleon-nucleon-meson coupling obtained in our previous paper (arXiv:0806.3122).

Orientifold planes, type I Wilson lines and non-BPS D-branes

There is a longstanding puzzle concerned with the existence of ?p?-planes with p ? 6, which are 0orientifold p-planes of negative charge with stuck Dp-branes. We study the consistency of configurations with various orientifold planes and propose a resolution to this puzzle. It is argued that ?6?-planes are possible in massive IIA theory with odd cosmological constant, while ?7?-planes and ?8?-planes are not allowed. Various relations between orientifold planes and non-BPS D-branes are also addressed.

Mesons as Open Strings in a Holographic Dual of QCD

We study meson spectrum obtained from massive open string modes in a holographic dual of QCD constructed on the basis of a D4/D8-brane configuration in type IIA string theory. The spectrum includes mesons with spin higher than one. Taking into account the effect of curved background perturbatively, we obtain a mass formula for these mesons. The mass formula exhibits the linear Regge behavior at the leading order with subleading nonlinear corrections. We argue that the string spectrum captures some features of the observed meson spectrum. For example, a2(1320), b1(1235), π(1300), a0(1450), etc., are identified as the first excited massive open string states and ρ3(1690), π2(1670), etc., are identified as the second excited states. Subject Index: 129, 168

QED and string theory

We analyze the D9- system in type IIB string theory using Dp-brane probes. It is shown that the world-volume theory of the probe Dp-brane contains two-dimensional and four-dimensional QED in the cases with p = 1 and p = 3, respectively, and some applications of the realization of these well-studied quantum field theories are discussed. In particular, the two-dimensional QED (the Schwinger model) is known to be a solvable theory and we can apply the powerful field theoretical techniques, such as bosonization, to study the D-brane dynamics. The tachyon field created by the D9- strings appears as the fermion mass term in the Schwinger model and the tachyon condensation is analyzed by using the bosonized description. In the T-dualized picture, we obtain the potential between a D0-brane and a D8- pair using the Schwinger model and we observe that it consists of the energy carried by fundamental strings created by the Hanany-Witten effect and the vacuum energy due to a cylinder diagram. The D0-brane is treated quantum mechanically as a particle trapped in the potential, which turns out to be a system of a harmonic oscillator. As another application, we obtain a matrix theory description of QED using Taylors T-duality prescription, which is actually applicable to a wide variety of field theories including the realistic QCD. We show that the lattice gauge theory is naturally obtained by regularizing the matrix size to be finite.

Confinement and Dynamical Symmetry Breaking in Non-SUSY Gauge Theory from S-Duality in String Theory

We discuss an attempt to understand confinement and dynamical symmetry breaking in a 4-dimensional non-supersymmetric gauge theory using S-duality in type IIB string theory. The electric theory is a USp (2n) gauge theory and its magnetic dual is argued to be an SO(2n )o rSO(2n� 1) gauge theory. These theories are obtained as the low-energy effective theory of O3-D3 systems in type IIB string theory, which are related by S-duality. Confinement and dynamical symmetry breaking in the USp (2n) gauge theory are caused by the condensation of scalar fields in the magnetic dual description, which is consistent with the scenario of the dual Meissner mechanism for the confinement. Subject Index: 101, 102, 120, 123

Supersymmetric gauge theory with space-time-dependent couplings

We study deformations of