Searcharxiv⌕ Search

arXiv subjects

Yoonbai Kim

Publications and source records attributed to Yoonbai Kim.

At least 55 records · Page 3Linked to original sources
arXiv2008

Multi-BPS D-vortices

We investigate the BPS configuration of the multi D-vortices produced from the D2${\bar {\rm D}}$2 system. Based on the DBI-type action with a Gaussian-type runaway potential for a complex tachyon field, the BPS limit is achieved when the tachyon profile is thin. The solution states randomly-distributed $n$ static D-vortices with zero interaction. With the obtained BPS configuration, we derive the relativistic Lagrangian which describes the dynamics of free massive D-vortices. We also discuss the 90${}^{\circ}$ and 180${}^{\circ}$ scattering of two identical D-vortices, and present its implications on the reconnection in the dynamics of cosmic superstrings.

hep-th↗
arXiv2008

Tachyon Vacuum Solution in Open String Field Theory with Constant B Field

We show that Schnabl's tachyon vacuum solution is an exact solution of the equation of motion of Witten's open bosonic string field theory in the background of constant antisymmetric two-form field. The action computed at the vacuum solution is given by the Dirac-Born-Infeld factor multiplied to that without the antisymmetric tensor field.

hep-th↗
arXiv2008

Negative-Tension Branes and Tensionless 1/2 Brane in Boundary Conformal Field Theory

In the framework of boundary conformal field theory we consider a flat unstable D$p$-brane in the presence of a large constant electromagnetic field. Specifically, we study the case that the electromagnetic field satisfy the following three conditions: (i) a constant electric field is turned on along the $x^1$ direction ($E_{1}\ne 0$); (ii) the determinant of the matrix $(η+ F)$ is negative so that it lies in the physical region ($-\det (η+ F)>0$); (iii) the 11-component of its cofactor is positive to the large electromagnetic field. In this case, we identify exactly marginal deformations depending on the spatial coordinate $x^1$. They correspond to tachyon profiles of hyperbolic sine, exponential, and hyperbolic cosine types. Boundary states are constructed for these deformations by utilizing T-duality approach and also by directly solving the overlap conditions in BCFT. The exponential type deformation gives a tensionless half brane connecting the perturbative string vacuum and one of the true tachyon vacua, while the others have negative tensions. This is in agreement with the results obtained in other approaches.

hep-th↗
arXiv2008

Lower Dimensional Branes in Boundary Conformal Field Theory

In the presence of constant background electromagnetic fields, we discuss three types of exactly marginal boundary tachyon operators for static kinks in boundary conformal field theory. Functional forms of three operators are hyperbolic sine, hyperbolic cosine, and exponential types, and they describe codimension-one solitons when the transverse electric field has overcritical value. The energy-momentum tensor and the source for antisymmetric tensor field are computed in the path integral approach for the exponential-type tachyon vertex operator.

hep-th↗
arXiv2008

Relativistic Dynamics of Multi-BPS D-vortices and Straight BPS D-strings

Moduli space dynamics of multi-D-vortices from D2${\bar {\rm D}}$ (equivalently, parallel straight D-strings from D3${\bar {\rm D}}$3) is systematically studied. For the BPS D-vortices, we show through exact calculations that the classical motion of randomly-distributed $n$ D-vortices is governed by a relativistic Lagrangian of free massive point-particles. When the head-on collision of two identical BPS D-vortices of zero radius is considered, it predicts either 90${}^{\circ}$ scattering or 0${}^{\circ}$ scattering equivalent to 180${}^{\circ}$ scattering. Since the former leads to a reconnection of two identical D-strings and the latter does to a case of their passing through each other, two possibilities are consistent with the prediction of string theory. It is also shown that the force between two non-BPS vortices is repulsive. Although the obtained moduli space dynamics of multi-BPS-D-vortices is exact in classical regime, the quantum effect of an F-string pair production should be included in determining the probabilities of the reconnection and the passing through for fast-moving cosmic superstrings.

hep-th↗
arXiv2007

Cosmic D- and DF-strings from D3Dbar3: Black Strings and BPS Limit

We study D- and DF-strings in a D3${\bar {\rm D}}3$ system by using Dirac-Born-Infeld type action. In the presence of an electric flux from the transverse direction, we discuss gravitating thick D-string solutions of a spatial manifold, ${\rm S}^{2}\times {\rm R}^{1}$, in which straight D-strings stretched along the R${}^{1}$ direction are attached to the south and north poles of the two-sphere. There is a horizon along its equator, which means the structure of black strings is formed. We also discuss the BPS limit for thin parallel D- and DF-strings in both flat and curved spacetime. We obtain the BPS sum rule for an arbitrarily-separated multi-string configuration with a Gaussian type tachyon potential. At the site of each thin BPS D(F)-string, the pressure takes a finite value. We find that there exists a maximum deficit angle $π$ in the conical geometry induced by thin BPS D- and DF-strings.

hep-th↗
arXiv2007

BPS Limit of Multi- D- and DF-strings in Boundary String Field Theory

A BPS limit is systematically derived for straight multi- D- and DF-strings from the D3D3bar system in the context of boundary superstring field theory. The BPS limit is obtained in the limit of thin D(F)-strings, where the Bogomolnyi equation supports singular static multi-D(F)-string solutions. For the BPS multi-string configurations with arbitrary separations, BPS sum rule is fulfilled under a Gaussian type tachyon potential and reproduces exactly the descent relation. For the DF-strings ((p,q)-strings), the distribution of fundamental string charge density coincides with its energy density and the Hamiltonian density takes the BPS formula of square-root form.

hep-th↗
arXiv2006

String Cosmology of the D-brane Universe

We analyze homogeneous anisotropic cosmology driven by the dilaton and the self-interacting ``massive'' antisymmetric tensor field which are indispensable bosonic degrees with the graviton in the NS-NS sector of string theories with D-branes. We found the attractor solutions for this system, which show the overall features of general solutions, and confirmed it through numerical analysis. The dilaton possesses the potential due to the presence of the D-brane and the curvature of extra dimensions. In the presence of the non-vanishing antisymmetric tensor field, the homogeneous universe expands anisotropically while the D-brane term dominates. The isotropy is recovered as the dilaton rolls down and the curvature term dominates. With the stabilizing potential for the dilaton, the isotropy can also be recovered.

hep-th↗
arXiv2006

Cosmological Aspects of the D-brane World

The D-brane world is an idea that we are living on the D-brane imbedded in a 10- or 11-dimensional spacetime of string theories, aiming at the construction of realistic models from string theories. We investigate the cosmological aspects of the D-brane world, focusing on homogeneous anisotropic cosmology driven by the dilaton and the NS-NS 2-form field which becomes massive in the presence of the D-brane. The dilaton possesses the potential due to the presence of the D-brane, various form field fluxes, and the curvature of extra dimensions. In the absence of stabilizing potential, we found the attractor solutions for this system which show the overall features of general solutions. In the presence of the non-vanishing NS-NS 2-form field, the homogeneous universe expands anisotropically while the D-brane term dominates. The isotropy is recovered as the dilaton rolls down and the curvature term dominates. With the stabilizing potential for the dilaton, the anisotropy developed by the initial NS-NS 2-form field flux is erased as the NS-NS 2-form field begins to oscillate around the minimum, forming the B-matter, and the isotropic matter-dominated universe is obtained.

hep-th↗
arXiv2006

Cosmological implication of antisymmetric tensor field on D-brane

We discuss the cosmological implications of an antisymmetric tensor field when it experiences a kind of ``Higgs mechanism,'' including nonlocal interaction terms, on D-branes. Even when a huge magnetic-condensation-inducing anisotropy is assumed in the early universe, the expansion of the universe leads to an isotropic $B$-matter-dominated universe.

hep-ph↗
arXiv2006

DF-strings from D3${\bar {\bf D}}$3 as Cosmic Strings

We study Dirac-Born-Infeld type effective field theory of a complex tachyon and U(1)$\times$U(1) gauge fields describing a D3${\bar {\rm D}}$3 system. Classical solutions of straight global and local DF-strings with quantized vorticity are found and are classified into two types by the asymptotic behavior of the tachyon amplitude. For sufficiently large radial distances, one has linearly-growing tachyon amplitude and the other logarithmically-growing tachyon amplitude. A constant radial electric flux density denoting the fundamental-string background makes the obtained DF-strings thick. The other electric flux density parallel to the strings is localized, which represents localization of fundamental strings in the D1-F1 bound states. Since these DF-strings are formed in the coincidence limit of the D3${\bar {\rm D}}$3, these cosmic DF-strings are safe from inflation induced by the approach of the separated D3 and ${\bar {\rm D}}3$.

hep-th↗
arXiv2006

Tachyon Kinks in Boundary String Field Theory

We study tachyon kinks with and without electromagnetic fields in the context of boundary string field theory. For the case of pure tachyon only an array of kink-antikink is obtained. In the presence of electromagnetic coupling, all possible static codimension-one soliton solutions such as array of kink-antikink, single topological BPS kink, bounce, half kink, as well as nonBPS topological kink are found, and their properties including the interpretation as branes are analyzed in detail. Spectrum of the obtained kinks coincides with that of Dirac-Born-Infeld type effective theory.

hep-th↗
arXiv2005

BPS D-branes from an Unstable D-brane

We search for exact tachyon kink solutions of DBI type effective action describing an unstable D-brane with worldvolume gauge field turned in both the flat and a curved background. There are various kinds of solutions in the presence of electromagnetic fields in the flat space, such as periodic arrays, topological tachyon kinks, half kinks, and bounces. We identify a BPS object, D($p$-1)F1 bound state, which describes a thick brane with string flux density. The curved background of interest is the ten-dimensional lift of the Salam-Sezgin vacuum and, in the asymptotic limit, it approaches ${\rm R}^{1,4}\times {\rm T}^2\times {\rm S}^3$. The solutions in the curved background are identified as composites of lower-dimensional D-branes and fundamental strings, and, in the BPS limit, they become a D4D2F1 composite wrapped on ${\rm R}^{1,2}\times {\rm T}^2$ where ${\rm T}^2$ is inside ${\rm S}^3$.

hep-th↗
arXiv2005

Global and Local D-vortices

Codimension-two objects on a system of brane-antibrane are studied in the context of Born-Infeld type effective field theory with a complex tachyon and U(1)$\times$U(1) gauge fields. When the radial electric field is turned on in D2${\bar {\rm D}}$2, we find static regular global and local D-vortex solutions which could be candidates of straight cosmic D-strings in a superstring theory. A natural extension to DF-strings is briefly discussed.

hep-th↗
arXiv2005

BPS D-branes from an Unstable D-brane in a Curved Background

We find exact tachyon kink solutions of DBI type effective action describing an unstable D5-brane with worldvolume gauge field turned on in a curved background. The background of interest is the ten-dimensional lift of the Salam-Sezgin vacuum and, in the asymptotic limit, it approaches ${\rm R}^{1,4}\times {\rm T}^2\times {\rm S}^3$. The solutions are identified as composites of lower-dimensional D-branes and fundamental strings, and, in the BPS limit, they become a D4D2F1 composite wrapped on ${\rm R}^{1,2}\times {\rm T}^2$ where ${\rm T}^2$ is inside ${\rm S}^3$. In one class of solutions we find an infinite degeneracy with respect to a constant magnetic field along the direction of NS-NS field on ${\rm S}^3$.

hep-th↗
arXiv2005

Exact Rolling Tachyon in Noncommutative Field Theory

We study the exact rolling tachyon solutions in DBI type noncommutative field theory with a constant open string metric and noncommutative parameter on an unstable D$p$-brane. Functional shapes of the obtained solutions span all possible homogeneous rolling tachyon configurations; that is, they are hyperbolic-cosine, hyperbolic-sine, and exponential under $1/\cosh$ runaway NC tachyon potential. Even if general DBI type NC electric field is turned on, only a constant electric field satisfies the equations of motion, and again, exact rolling tachyon solutions are obtained.

hep-th↗
arXiv2005

Cosmology of Antisymmetric Tensor Field in D-brane Universe

We analyze homogeneous, anisotropic cosmology driven by a self-interacting ``massive'' antisymmetric tensor field $B_{μν}$ which is present in string theories with D-branes. Time-dependent magnetic $B$ field existing in the early universe can lead to the Bianchi type I universe. Evolutions of such a tensor field are solved exactly or numerically in the universe dominated by vacuum energy, radiation, and $B$ field itself. The matter-like behavior of the $B$ field (dubbed as ``$B$-matter'') ensures that the anisotropy disappears at late time and thus becomes unobservable in a reasonable cosmological scenario. Such a feature should be contrasted to the cosmology of the conventional massless antisymmetric tensor field.

hep-ph↗