arXiv subjects

Konstadinos Sfetsos

Publications and source records attributed to Konstadinos Sfetsos.

At least 73 records · Page 4Linked to original sources

arXiv1999

Rotating NS5-brane solution and its exact string theoretical description

We construct the most general solution in type-II string theory that represents N coincident non-extremal rotating NS5-branes and determine the relevant thermodynamic quantities. We show that in the field theory limit, it has an exact description. In particular, it can be obtained by an O(3,3) duality transformation on the exact string background for the coset model SL(2,R)_{-N}/U(1) \times SU(2)_N. In the extreme supersymmetric limit we recover the multicenter solution, with a ring singularity structure, that has been discussed recently.

hep-th↗

arXiv1999

Branes for Higgs phases and exact conformal field theories

We consider multicenter supergravity solutions corresponding to Higgs phases of supersymmetric Yang-Mills theories with Z_N symmetric vacua. In certain energy regimes, we find a description in terms of a generalized wormhole solution that corresponds to the SL(2,R)/U(1) \times SU(2)/U(1) exact conformal field theory. We show that U-dualities map these backgrounds to purely gravitational ones and comment on the relation to the black holes arising from intersecting D1 and D5 branes. We also discuss supersymmetric properties of the various solutions and the relation to 2-dim solitons, on flat space, of the reduced axion-dilaton-gravity equations. Finally, we address the problem of understanding other supergravity solutions from the multicenter ones. As prototype examples we use rotating D3 branes and NS5 and D5 branes associated to non-Abelian duals of 4-dim hyper-Kahler metrics with SO(3) isometry.

hep-th↗

arXiv1999

Supergravity Models for 3+1 Dimensional QCD

The most general black M5-brane solution of eleven-dimensional supergravity (with a flat R^4 spacetime in the brane and a regular horizon) is characterized by charge, mass and two angular momenta. We use this metric to construct general dual models of large-N QCD (at strong coupling) that depend on two free parameters. The mass spectrum of scalar particles is determined analytically (in the WKB approximation) and numerically in the whole two-dimensional parameter space. We compare the mass spectrum with analogous results from lattice calculations, and find that the supergravity predictions are close to the lattice results everywhere on the two dimensional parameter space except along a special line. We also examine the mass spectrum of the supergravity Kaluza-Klein (KK) modes and find that the KK modes along the compact D-brane coordinate decouple from the spectrum for large angular momenta. There are however KK modes charged under a U(1)xU(1) global symmetry which do not decouple anywhere on the parameter space. General formulas for the string tension and action are also given.

hep-th↗

arXiv1998

On (multi-)center branes and exact string vacua

Multicenter supergravity solutions corresponding to Higgs phases of supersymmetric Yang-Mills theories are considered. For NS5 branes we identify three cases where there is a description in terms of exact conformal field theories. Other supergravity solutions, such as D3-branes with angular momentum, are understood as special limits of multicenter ones. Within our context we also consider 4-dim gravitational multi-instantons.

hep-th↗

arXiv1998

Poisson-Lie T-Duality beyond the classical level and the renormalization group

In order to study quantum aspects of $\s$-models related by Poisson--Lie T-duality, we construct three- and two-dimensional models that correspond, in one of the dual faces, to deformations of $S^3$ and $S^2$. Their classical canonical equivalence is demonstrated by means of a generating functional, which we explicitly compute. We examine how they behave under the renormalization group and show that dually related models have the same 1-loop beta functions for the coupling and deformation parameters. We find non-trivial fixed points in the ultraviolet, where the theories do not become asymptotically free. This suggests that the limit of Poisson--Lie T-duality to the usual Abelian and non-Abelian T-dualities does not exist quantum mechanically, although it does so classically.

hep-th↗

arXiv1998

Spinors and the AdS/CFT correspondence

We consider a free massive spinor field in Euclidean Anti-de Sitter space. The usual Dirac action in bulk is supplemented by a certain boundary term. The boundary conditions of the field are parametrized by a spinor on the boundary, subject to a projection. We calculate the dependence of the partition function on this boundary spinor. The result agrees with the generating functional of the correlation functions of a quasi-primary spinor operator, of a certain scaling dimension, in a free conformal field theory on the boundary.

hep-th↗

arXiv1997

Canonical equivalence of non-isometric sigma models and Poisson-Lie T-duality

We prove that a transformation, conjectured in our previous work, between phase-space variables in $\s$-models related by Poisson-Lie T-duality is indeed a canonical one. We do so by explicitly demonstrating the invariance of the classical Poisson brackets. This is the first example of a class of $\s$-models with no isometries related by canonical transformations. In addition we discuss generating functionals of canonical transformations in generally non-isometric, bosonic and supersymmetric $\s$-models and derive the complete set of conditions that determine them. We apply this general formalism to find the generating functional for Poisson-Lie T-duality. We also comment on the relevance of this work to D-brane physics and to quantum aspects of T-duality.

hep-th↗

arXiv1996

Poisson-Lie T-Duality and Supersymmetry

We review aspects of Poisson-Lie T-duality which we explicitly formulate as a canonical transformation on the world-sheet. Extensions of previous work on T-duality in relation to supersymmetry are also discussed. (Contribution to the proceedings of the 30th International Symposium Ahrenshoop on the Theory of Elementary Particles, Buckow, Germany, 26-31 August 1996)

hep-th↗

arXiv1996

Coset Models and Differential Geometry

String propagation on a curved background defines an embedding problem of surfaces in differential geometry. Using this, we show that in a wide class of backgrounds the classical dynamics of the physical degrees of freedom of the string involves 2-dim sigma-models corresponding to coset conformal field theories.

hep-th↗

arXiv1996

Non--Abelian Duality, Parafermions and Supersymmetry

Non--Abelian duality in relation to supersymmetry is examined. When the action of the isometry group on the complex structures is non--trivial, extended supersymmetry is realized non--locally after duality, using path ordered Wilson lines. Prototype examples considered in detail are, hyper--Kahler metrics with SO(3) isometry and supersymmetric WZW models. For the latter, the natural objects in the non--local realizations of supersymmetry arising after duality are the classical non--Abelian parafermions. The canonical equivalence of WZW models and their non--Abelian duals with respect to a vector subgroup is also established.

hep-th↗

arXiv1996

Duality and Restoration of Manifest Supersymmetry

World-sheet and spacetime supersymmetries that are manifest in some string backgrounds may not be so in their T-duals. Nevertheless, they always remain symmetries of the underlying conformal field theory. In previous work the mechanism by which T-duality destroys manifest supersymmetry and gives rise to non-local realizations was found. We give the general conditions for a 2-dim N=1 supersymmetric sigma-model to have non-local and hence non-manifest extended supersymmetry. We then examine T-duality as a mechanism of restoring manifest supersymmetry. This happens whenever appropriate combinations of non-local parafermions of the underlying conformal field theory become local due to non-trivial world-sheet effects. We present, in detail, an example arising from the model SU(2)/U(1) X SL(2,R)/U(1) and obtain a new exact 4-dim axionic instanton, that generalizes the SU(2) X U(1) semi-wormhole, and has manifest spacetime as well as N=4 world-sheet supersymmetry. In addition, general necessary conditions for abelian T-duality to preserve manifest N=4 world-sheet supersymmetry are derived and applied to WZW models based on quaternionic groups. We also prove some theorems for sigma-models with non-local N=4 world-sheet supersymmetry.

hep-th↗

arXiv1995

On T-Duality and Supersymmetry

The interplay between T-duality and supersymmetry in string theory is explored. It is shown that T-duality is always compatible with supersymmetry and simply changes a local realization to a non-local one and vice versa. Non-local realizations become natural using classical parafermions of the underlying conformal field theory. Examples presented include hyper-kahler metrics and the backgrounds for the SU(2) X U(1) and SU(2)/U(1) X U(1) X U(1) exact conformal field theories.

hep-th↗

arXiv1995

Stochastic Tachyon Fluctuations, Marginal Deformations and Shock Waves in String Theory

Starting with exact solutions to string theory on curved spacetimes we obtain deformations that represent gravitational shock waves. These may exist in the presence or absence of sources. Sources are effectively induced by a tachyon field that randomly fluctuates around a zero condensate value. It is shown that at the level of the underlying conformal field theory (CFT) these deformations are marginal and moreover all \a'-corrections are taken into account. Explicit results are given when the original undeformed 4-dimensional backgrounds correspond to tensor products of combinations of 2-dimensional CFT's, for instance SL(2,R)/R \times SU(2)/U(1).

hep-th↗

arXiv1994

On Gravitational Shock Waves in Curved Spacetimes

Some years ago Dray and 't Hooft found the necessary and sufficient conditions to introduce a gravitational shock wave in a particular class of vacuum solutions to Einstein's equations. We extend this work to cover cases where non-vanishing matter fields and cosmological constant are present. The sources of gravitational waves are massless particles moving along a null surface such as a horizon in the case of black holes. After we discuss the general case we give many explicit examples. Among them are the $d$-dimensional charged black hole (that includes the 4-dimensional Reissner-Nordström and the $d$-dimensional Schwarzschild solution as subcases), the 4-dimensional De-Sitter and Anti-De-Sitter spaces (and the Schwarzschild-De-Sitter black hole), the 3-dimensional Anti-De-Sitter black hole, as well as backgrounds with a covariantly constant null Killing vector. We also address the analogous problem for string inspired gravitational solutions and give a few examples.

hep-th↗

arXiv1994

Gauged WZW models and Non-abelian duality

We consider WZW models based on the non-semi-simple algebras that they were recently constructed as contractions of corresponding algebras for semi-simple groups. We give the explicit expression for the action of these models, as well as for a generalization of them, and discuss their general properties. Furthermore we consider gauged WZW models based on these non-semi-simple algebras and we show that there are equivalent to non-abelian duality transformations on WZW actions. We also show that a general non-abelian duality transformation can be thought of as a limiting case of the non-abelian quotient theory of the direct product of the original action and the WZW action for the symmetry gauge group $H$. In this action there is no Lagrange multiplier term that constrains the gauge field strength to vanish. A particular result is that the gauged WZW action for the coset $(G_k \otimes H_l)/H_{k+l}$ is equivalent, in the limit $l\to \infty$, to the dualized WZW action for $G_k$ with respect to the subgroup $H$.

hep-th↗

arXiv1994

Gauging a Non-Semi-Simple WZW Model

We consider gauged WZW models based on a four dimensional non-semi-simple group. We obtain conformal $\s$-models in $D=3$ spacetime dimensions (with exact central charge $c=3$) by axially and vectorially gauging a one-dimensional subgroup. The model obtained in the axial gauging is related to the $3D$ black string after a correlated limit is taken in the latter model. By identifying the CFT corresponding to these $\s$-models we compute the exact expressions for the metric and dilaton fields. All of our models can be mapped to flat spacetimes with zero antisymmetric tensor and dilaton fields via duality transformations.

hep-th↗

arXiv1993

Exact string backgrounds from WZW models based on non-semi-simple groups

We formulate WZW models based on a centrally extended version of the Euclidean group in $d$-dimensions. We obtain string backgrounds corresponding to conformal $\s$-models in $D=d^2$ space-time dimensions with exact central charge $c=d^2$ and $d(d-1)/2$ null Killing vectors. By identifying the corresponding conformal field theory we show that the one loop results coincide with the exact ones up to a shifting of a parameter.

hep-th↗

arXiv1993

Effective Action and Exact Geometry in Chiral Gauged WZW Models

Following recent work on the effective quantum action of gauged WZW models, we suggest such an action for {\it chiral} gauged WZW models which in many respects differ from the usual gauged WZW models. Using the effective action we compute the conformally exact expressions for the metric, the antisymmetric tensor, and the dilaton fields in the $\s$-model arising from a general {\it chiral } gauged WZW model. We also obtain the general solution of the geodesic equations in the exact geometry. Finally we consider in some detail a three dimensional model which has certain similarities with the three dimensional black string model. Finally we consider in some detail a three dimensional model which has certain similarities with the three dimensional black string model.

hep-th↗