arXiv subjects

J. L. F. Barbon

Publications and source records attributed to J. L. F. Barbon.

At least 55 records · Page 3Linked to original sources

arXiv1998

Large N limit of Extremal Non-supersymmetric Black Holes

The large N limit of extremal non-supersymmetric Type-I five-dimensional string black holes is studied from the point of view of D-branes. We find that the agreement between the D-brane and the black-hole picture is due to an asymptotic restoration of supersymmetry in the large $N$ limit in which both pictures are compared. In that limit Type-I string perturbation theory is effectively embedded into a Type-IIB perturbation theory with unbroken supersymmetric charges whose presence guarantees the non-renormalization of mass and entropy as the effective couplings are increased. In this vein, we also study the near-horizon geometry of the Type-I black hole using D5-brane probes to find that the low energy effective action for the probe is identical to the corresponding one in the auxiliary Type-IIB theory in the large N limit.

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arXiv1998

Extensivity Versus Holography in Anti-de Sitter Spaces

We study the dimensionality manifested in the AdS/CFT correspondence. We show that the dimensionality as expressed by the high temperature behavior of a system has a holographic nature also at the quantum level. The emergence of the AdS black hole as a master field at high temperature leads to the screening of the extra dimensions in its excluded volume.

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arXiv1998

A Note on Softly Broken MQCD

We consider generic MQCD configurations with matter described by semi-infinte D4-branes and softly broken supersymmetry. We show that the matter sector does not introduce supersymmetry breaking parameters so that the most relevant supersymmetry breaking operator at low energies is the gaugino mass term. By studying the run-away properties of these models in the decoupling limit of the adjoint matter, we argue that these softly broken MQCD configurations fail to capture the infrared physics of QCD at scales below the gaugino mass scale.

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arXiv1997

D0-Branes as Instantons in D=4 Super Yang-Mills Theories

We discuss the role of D0-branes as instantons in the construction of SU(N) Super Yang-Mills and Super QCD theories in four space-time dimensions with D4-, D6- and NS-branes. [To appear in the proceedings of the conference "Quantum Aspects of Gauge Theories, Supersymmetry and Unification", held at Neuchatel University, Neuchatel, Switzerland, 18--23 September 1997.]

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arXiv1997

D0-Branes, Constrained Instantons and D=4 Super Yang-Mills Theories

We consider in more detail the role of D0-branes as instantons in the construction of SU(N) Super Yang-Mills and Super QCD theories in four space-time dimensions with D4, D6 and NS-branes. In particular, we show how the D0-branes describe both the exact and constrained instantons and reproduce the correct pattern of lifting of zero modes on the various branches of these models.

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arXiv1997

Fermion Exchange between D-instantons

We define fermionic collective coordinates for type-IIB Dirichlet instantons and discuss some effects of the associated fermionic zero modes within the dilute gas framework. We show that the standard rules for clustering of zero modes in the dilute limit, and the fermion-exchange interactions follow from world-sheet Ward identities. Fermion exchange is strongly attractive at string-scale distances, which makes the short-distance Hagedorn singularity between instantons and anti-instantons even stronger.

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arXiv1997

Dilute D-Instantons at Finite Temperature

We discuss Dirichlet instanton effects on type-IIB string Thermodynamics. We review some general properties of dilute D-instanton gases and use the low-energy supergravity solutions to define the normalization of the instanton measure, as well as the effects of long-range interactions. Thermal singularities in the single-instanton sector are due to tachyonic winding modes of Dirichlet open strings. Purely bosonic D-instantons induce in this way hard infrared singularities that ruin the weak-coupling expansion in the microcanonical ensemble. However, type-IIB D-instantons, give smooth contributions at the Hagedorn temperature, and the induced mass and coupling of the axion field are insufficient to change the first-order character of the phase transition in the mean field approximation.

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arXiv1997

Rotated Branes and N=1 Duality

We consider configurations of rotated NS-branes leading to a family of four-dimensional N=1 super-QCD theories, interpolating between four-dimensional analogues of the Hanany-Witten vacua, and the Elitzur-Giveon-Kutasov configuration for N=1 duality. The rotation angle is the N=2 breaking parameter, the mass of the adjoint scalar in the N=2 vector multiplet. We add some comments on the relevance of these configurations as possible stringy proofs of N=1 duality.

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arXiv1997

Remarks on the Classical Size of D-Branes

We discuss different criteria for `classical size' of extremal Dirichlet p-branes in type-II supergravity. Using strong-weak coupling duality, we find that the size of the strong-coupling region at the core of the (p<3)-branes, is always given by the asymptotic string scale, if measured in the weakly coupled dual string metric. We also point out how the eleven-dimensional Planck scale arises in the classical 0-brane solution, as well as the ten-dimensional Planck scale in the D-instanton solution.

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arXiv1996

Complex World-Sheets from N=2 Strings

We study some properties of target space strings constructed from (2,1) heterotic strings. We argue that world-sheet complexification is a general property of the bosonic sector of such target world-sheets. We give a target space interpretation of this fact and relate it to the non-gaussian nature of free String Field Theory. We provide several one loop calculations supporting the stringy construction of critical world-sheets in terms of (2,1) models. Using finite temperature boundary conditions in the underlying (2,1) string we obtain non-chiral target space spin structures, and point out some of the problems arising for chiral spin structures, such as the heterotic world-sheet. To this end, we study the torus partition function of the corresponding asymmetric orbifold of the (2,1) string.

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arXiv1996

T-duality for open strings

The T-duality transformations between open and closed superstrings in different D-manifolds are generalized to curved backgrounds with commuting isometries. We address some global aspects like the occurrence of orientifold boundaries in general sigma models, higher genus world sheets, and the case of non-compact isometries. The various world volume effective actions are shown to transform properly under T-duality. We also include a brief discussion of the canonical transformations of boundary states in the operator formalism.

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arXiv1996

D-brane form factors at high energy

We study the high energy, fixed angle, asymptotics of D-brane form factors to all orders in string perturbation theory, using the Gross-Mende saddle point techniques. The effective interaction size of all D-branes grows linearly with the energy as (alpha') E/n, where n is the order of perturbation theory, except for the D-instanton, whose form factor is dominated by end-point contributions, and remains point-like at high orders. The qualitative features are independent of the R-R or NS-NS character of the states used to probe the D-brane.

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arXiv1995

Some properties of unstable monopoles in Super-QCD

We study embeddings of the Prasad-Sommerfield monopole solution in SU(N) Super-QCD (N>2), where the role of the Higgs field is played by the squarks in the fundamental representation. Classically, the resulting configurations live in a phase with unbroken SU(k) subgroups of SU(N) (as a result they are not topologically stable). The structure of zero modes is such that they can be naturally interpreted as massive chiral superfields with R charge one and baryon number zero, transforming in the adjoint representation of a dual gauge group defined using the Goddard-Nuyts-Olive (GNO) framework. We discuss the possible applications of these monopoles to N=1 duality, and more generally the possibility of relating GNO-type dual gauge groups to those appearing in N=1 duality.

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arXiv1995

Quantum black hole entropy and Newton constant renormalization

We discuss the status of the black hole entropy formula $S_{\rm BH} = A_H /4G$ in low energy effective field theory. The low energy expansion of the black hole entropy is studied in a non-equilibrium situation: the semiclassical decay of hot flat space by black hole nucleation. In this context the entropy can be defined as an enhancement factor in the semiclassical decay rate, which is dominated by a sphaleron-like saddle point. We find that all perturbative divergences appearing in Euclidean calculations of the entropy can be renormalized in low energy couplings. We also discuss some formal aspects of the relation between the Euclidean and Hamiltonian approaches to the one loop corrections to black hole entropy and geometric entropy, and we emphasize the virtues of the use of covariant regularization prescriptions. In fact, the definition of black hole entropy in terms of decay rates {\it requires} the use of covariant measures and accordingly, covariant regularizations in path integrals. Finally, we speculate on the possibility that low energy effective field theory could be sufficient to understand the microscopic degrees of freedom underlying black hole entropy. We propose a qualitative physical picture in which black hole entropy refers to a space of quasi-coherent states of infalling matter, together with its gravitational field. We stress that this scenario might provide a low energy explanation of both the black hole entropy and the information puzzle.

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arXiv1995

Generalized Abelian S-duality and coset constructions

Electric-magnetic duality and higher dimensional analogues are obtained as symmetries in generalized coset constructions, similar to the axial-vector duality of two dimensional coset models described by Rocek and Verlinde. We also study global aspects of duality between p-forms and (d-p-2)-forms in d-manifolds. In particular, the modular duality anomaly is governed by the Euler character as in four and two dimensions. Duality transformations of Wilson line operator insertions are also considered.

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arXiv1994

Remarks on Thermal Strings outside Black Holes

We discuss the semiclassical approximation to the level density of (super) strings propagating in non-compact coset manifolds $G/H$. We show that the WKB ansatz agrees with heuristic red-shift arguments with respect to the ``exact" sigma model metric, up to some deviations from minimal coupling, parametrized by the dilaton. This approximation is used to study thermal ensembles of free strings in black holes, with the ``brick wall" regularization of `t Hooft. In two dimensions the entropy diverges logarithmically with the horizon thickness, and a local Hagedorn transition occurs in higher dimensional models. We also observe that supersymmetry improves the regularity of strings at the horizon.

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arXiv1994

Horizon divergences of Fields and Strings in Black Hole backgrounds

General arguments based on curved space-time thermodynamics show that any extensive quantity, like the free energy or the entropy of thermal matter, always has a divergent boundary contribution in the presence of event horizons, and this boundary term comes with the Hawking-Bekenstein form. Although the coefficients depend on the particular geometry we show that intensive quantities, like the free energy density are universal in the vicinity of the horizon. {} From the point of view of the matter degrees of freedom this divergence is of infrared type rather than ultraviolet, and we use this remark to speculate about the fate of these pathologies in String Theory. Finally we interpret them as instabilities of the Canonical Ensemble with respect to gravitational collapse via the Jeans mechanism.

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arXiv1993

Some Global Aspects of Duality is String Theory

We explore some of the global aspects of duality transformations in String Theory and Field Theory. We analyze in some detail the equivalence of dual models corresponding to different topologies at the level of the partition function and in terms of the operator correspondence for abelian duality. We analyze the behavior of the cosmological constant under these transformations. We also explore several examples of non-abelian duality where the classical background interpretation can be maintained for the original and the dual theories. In particular we construct a non-abelian dual of $SL(2,R)$ which turns out to be a three-dimensional black hole

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