arXiv subjects

Ashoke Sen

Publications and source records attributed to Ashoke Sen.

At least 217 records · Page 12Linked to original sources

arXiv2000

Vortex Pair Creation on Brane-Antibrane Pair via Marginal Deformation

It has been conjectured that the vortex solution on a D-brane - anti-D-brane system represents a D-brane of two lower dimension. We establish this result by first identifying a series of marginal deformations which create the vortex - antivortex pair on the brane - antibrane system, and then showing that under this series of marginal deformations the original D-brane - anti-D-brane system becomes a D-brane - anti-D-brane system with two lower dimensions. Generalization of this construction to the case of solitons of higher codimension is also discussed.

hep-th↗

arXiv1999

Tachyon condensation in string field theory

It has been conjectured that at a stationary point of the tachyon potential for the D-brane of bosonic string theory, the negative energy density exactly cancels the D-brane tension. We evaluate this tachyon potential by off-shell calculations in open string field theory. Surprisingly, the condensation of the tachyon mode alone into the stationary point of its cubic potential is found to cancel about 70% of the D-brane tension. Keeping relevant scalars up to four mass levels above the tachyon, the energy density at the shifted stationary point cancels 99% of the D-brane tension.

hep-th↗

arXiv1999

Universality of the Tachyon Potential

Using string field theory, we argue that the tachyon potential on a D-brane anti-D-brane system in type II string theory in arbitrary background has a universal form, independent of the boundary conformal field theory describing the brane. This implies that if at the minimum of the tachyon potential the total energy of the brane antibrane system vanishes in a particular background, then it vanishes in any other background. Similar result holds for the tachyon potential of the non-BPS D-branes of type II string theory, and the D-branes of bosonic string theory.

hep-th↗

arXiv1999

Supersymmetric World-volume Action for Non-BPS D-branes

We construct the world-volume action for non-BPS D-branes in type II string theories. This action is invariant under all the unbroken supersymmetries in the bulk, but these symmetries are realised as spontaneously broken symmetries in the world-volume theory. Coupling of this action to background supergravity fields is straightforward. We also discuss the fate of the U(1) gauge field on the D-brane world-volume after tachyon condensation.

hep-th↗

arXiv1999

Non-supersymmetric D-Brane Configurations with Bose-Fermi Degenerate Open String Spectrum

The spectrum of open strings on various non-BPS D-brane configurations in type II string theory on a K3 orbifold is analysed. At a generic point in the corresponding moduli space the spectrum of open strings does not have any degeneracy between bosonic and fermionic states. However, there exist special values for these moduli for which many non-BPS D-brane configurations have an exactly bose-fermi degenerate open string spectrum at all mass levels. In this case the closed string exchange interaction between a pair of such D-brane configurations vanishes at all distances.

hep-th↗

arXiv1999

Stable Non-BPS States in F- theory

F-theory on K3 admits non-BPS states that are represented as string junctions extending between 7-branes. We classify the non-BPS states which are guaranteed to be stable on account of charge conservation and the existence of a region of moduli space where the 7-branes supporting the junction can be isolated from the rest of the branes. We find three possibilities; the 7-brane configurations carrying: (i) the D_1 algebra representing a D7-brane near an orientifold O7-plane, whose stable non-BPS state was identified before, (ii) the exotic affine E_1 algebra, whose stable non-BPS state seems to be genuinely non-perturbative, and, (iii) the affine E_2 algebra representing a D7-brane near a pair of O7-planes. As a byproduct of our work we construct explicitly all 7-brane configurations that can be isolated in a K3. These include non-collapsible configurations of affine type.

hep-th↗

arXiv1999

`Blowing up' D-branes on Non-supersymmetric Cycles

In the orbifold limit of K3, one can give exact conformal field theory description of D-branes wrapped on certain non-supersymmetric cycles of K3. We study the effect of switching on the `non-geometric blow up modes' corresponding to anti-symmetric tensor gauge field flux through the 2-cycles on these D-branes. Working to first order in the blow up parameter, we determine the region of the moduli space in which these D-branes are stable. Across the boundary of this region, the D-brane wrapped on the non-supersymmetric cycle decays to a pair of D-branes, each wrapped on a supersymmetric cycle, via a second order phase transition.

hep-th↗

arXiv1999

Non-BPS States and Branes in String Theory

We review the recent developments in our understanding of non-BPS states and branes in string theory. The topics include 1) construction of unstable non-BPS D-branes in type IIA and type IIB string theories, 2) construction of stable non-BPS D-branes on various orbifolds and orientifolds of type II string theories, 3) description of BPS and non-BPS D-branes as tachyonic soliton solutions on brane-antibrane pair of higher dimension, and 4) study of the spectrum of non-BPS states and branes on a system of coincident D-brane - orientifold plane system. Some other related results are also discussed briefly.

hep-th↗

arXiv1999

Descent Relations Among Bosonic D-branes

We show that the tachyonic kink solution on a pair of D-p-branes in the bosonic string theory can be identified with the D-(p-1)-brane of the same theory. We also speculate on the possibility of obtaining the D-(p-1)-brane as a tachyonic lump on a single D-p-brane. We suggest a possible reinterpretation of the first result which unifies these two apparently different descriptions of the D-(p-1) brane.

hep-th↗

arXiv1999

BPS D-branes on Non-supersymmetric Cycles

In certain regions of the moduli space of K3 and Calabi-Yau manifolds, D-branes wrapped on non-supersymmetric cycles may give rise to stable configurations. We show that in the orbifold limit, some of these stable configurations can be described by solvable boundary conformal field theories. The world-volume theory of N coincident branes of this type is described by a non-supersymmetric U(N) gauge theory. At the boundary of the region of stability, there are marginal deformations connecting the non-supersymmetric brane to a pair of D-branes wrapped on supersymmetric cycles. We also discuss various relationships between BPS and non-BPS D-branes of type II string theories.

hep-th↗

arXiv1998

Developments in Superstring Theory

In this talk I review recent developments in superstring theory. In the first half of the talk I discuss some of the earlier developments in this subject. This includes a review of the status of string theory as a unified theory of gravity and other interactions, the role of duality in string theory, and application of string theory to the problem of information loss near a black hole. In the second half of the talk I review the developments of the last two years. This includes application of duality symmetries, Matrix theory, Maldacena conjecture, and derivation of gauge theory results from string theory. In choosing the list of topics for this talk, I have focussed on those which are likely to be of some interest to non-string theorists, thereby leaving out many of the marvelous technical results in this subject.

hep-ph↗

arXiv1998

Type I D-particle and its Interactions

In a previous paper (hep-th/9808141) we showed that the type I string theory contains a stable non-BPS D-particle carrying SO(32) spinor charge. In this paper we formulate the rules for computing the spectrum and interaction of open strings with one or both ends lying on this D-particle.

hep-th↗

arXiv1998

SO(32) Spinors of Type I and Other Solitons on Brane-Antibrane Pair

We construct the SO(32) spinor state in weakly coupled type I string theory as a kink solution of the tachyon field on the D-string - anti-D-string pair and calculate its mass. We also give a description of this system in terms of an exact boundary conformal field theory and show that in this description this state can be regarded as a non-supersymmetric D0-brane in type I string theory. This construction can be generalised to represent the D0-brane in type IIA string theory as a vortex solution of the tachyon field on the membrane anti-membrane pair, and the D-string of type I string theory as a topological soliton of the tachyon field on the D5-brane anti- D5-brane pair.

hep-th↗

arXiv1998

Stable Non-BPS Bound States of BPS D-branes

S-duality symmetry of type IIB string theory predicts the existence of a stable non-BPS state on an orbifold five plane of the type IIB theory if the orbifold group is generated by the simultaneous action of (-1)^{F_L} and the reversal of sign of the four coordinates transverse to the orbifold plane. We calculate the mass of this state by starting from a pair of D-strings carrying the same charge as this state, and then identifying the point in the moduli space where this pair develops a tachyonic mode, signalling the appearance of a bound state of this configuration into the non-BPS state.

hep-th↗

arXiv1998

Tachyon Condensation on the Brane Antibrane System

A coincident D-brane - anti-D-brane pair has a tachyonic mode. We present an argument showing that at the classical minimum of the tachyonic potential the negative energy density associated with the potential exactly cancels the sum of the tension of the brane and the anti-brane, thereby giving a configuration of zero energy density and restoring space-time supersymmetry.

hep-th↗

arXiv1998

Black holes and Elementary String States in N=2 Supersymmetric String Theories

We compare the logarithm of the degeneracy of BPS saturated elementary string states and the string modified Bekenstein-Hawking entropy of the corresponding black holes in N=2 supersymmetric heterotic string compactification to four dimensions. As in the case of N=4 supersymmetric theory, the two results match up to an overall undetermined numerical factor. We also show that this undetermined numerical constant is identical in the N=2 and N=4 supersymmetric theories, therby showing that the agreement between the Bekenstein-Hawking entropy and the microscopic entropy for N=2 theories does not require any new identity, other than the one already required for the N=4 theory. A similar result holds for type II string compactification as well.

hep-th↗

arXiv1998

Stable Non-BPS States in String Theory

Many string theories contain states which are not BPS, but are stable due to charge conservation. In many cases the description of these states in the strong coupling limit remains unknown despite the existence of a weakly coupled dual theory. However, we show that in some cases duality symmetries in string theory do enable us to identify these states in the strong coupling limit and calculate their masses. We also speculate that in some of the other cases the missing states might arise from non-supersymmetric analog of D0-branes.

hep-th↗

arXiv1998

An Introduction to Non-perturbative String Theory

In this review I discuss some basic aspects of non-perturbative string theory. The topics include test of duality symmetries based on the analysis of the low energy effective action and the spectrum of BPS states, relationship between different duality symmetries, an introduction to M- and F-theories, black hole entropy in string theory, and Matrix theory.

hep-th↗