arXiv subjects

S. F. Hassan

Publications and source records attributed to S. F. Hassan.

44 records · Page 3Linked to original sources

arXiv1998

Gauge Theory Description of D-brane Black Holes: Emergence of the Effective SCFT and Hawking Radiation

We study the hypermultiplet moduli space of an N=4, U(Q_1)xU(Q_5) gauge theory in 1+1 dimensions to extract the effective SCFT description of near extremal 5-dimensional black holes modelled by a collection of D1- and D5-branes. On the moduli space, excitations with fractional momenta arise due to a residual discrete gauge invariance. It is argued that, in the infra-red, the lowest energy excitations are described by an effective c=6, N=4 SCFT on T^4, also valid in the large black hole regime. The ``effective string tension'' is obtained using T-duality covariance. While at the microscopic level, minimal scalars do not couple to (1,5) strings, in the effective theory a coupling is induced by (1,1) and (5,5) strings, leading to Hawking radiation. These considerations imply that, at least for such black holes, the calculation of the Hawking decay rate for minimal scalars has a sound foundation in string theory and statistical mechanics and, hence, there is no information loss.

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arXiv1997

D-brane Black Holes: Large-N Limit and the Effective String Description

We address the derivation of the effective conformal field theory description of the 5-dimensional black hole, modelled by a collection of D1- and D5- branes, from the corresponding low energy U(Q_1)xU(Q_5) gauge theory. Finite horizon size at weak coupling requires both Q_1 and Q_5 to be large. We derive the result in the moduli space approximation (say for Q_1>Q_5) and appeal to supersymmetry to argue its validity beyond weak coupling. As a result of a combination of quenched Z_{Q_1} Wilson lines and a residual Weyl symmetry, the low-lying excitations of the U(Q_1)xU(Q_5) gauge theory are described by an effective N=4 superconformal field theory with c=6 in 1+1 dimensions, where the space is a circle of radius RQ_1Q_5. We also discuss the appearance of a marginal perturbation of the effective conformal field theory for large but finite values of Q_5.

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arXiv1997

Introduction to S-Duality in N=2 Supersymmetric Gauge Theory. (A pedagogical review of the work of Seiberg and Witten)

In these notes we attempt to give a pedagogical introduction to the work of Seiberg and Witten on S-duality and the exact results of N=2 supersymmetric gauge theories with and without matter. The first half is devoted to a review of monopoles in gauge theories and the construction of supersymmetric gauge theories. In the second half, we describe the work of Seiberg and Witten.

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arXiv1995

T-Duality and Non-Local Realizations of Supersymmetry in String Theory

We study non-local realizations of extended worldsheet supersymmetries and the associated space-time supersymmetries which arise under a T-duality transformation. These non-local effects appear when the supersymmetries do not commute with the isometry with respect to which T-duality is performed.

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arXiv1995

T-Duality and Non-Local Supersymmetries

We study the non-localization of extended worldsheet supersymmetry under T-duality, when the associated complex structure depends on the coordinate with respect to which duality is performed. First, the canonical transformation which implements T-duality is generalized to the supersymmetric non-linear $σ$-models. Then, we obtain the non-local object which replaces the complex structure in the dual theory and write down the condition it should satisfy so that the dual action is invariant under the non-local supersymmetry. For the target space, this implies that the supersymmetry transformation parameter is a non-local spinor. The analogue of the Killing equation for this non-local spinor is obtained. It is argued that in the target space, the supersymmetry is no longer realized in the standard way. The string theoretic origin of this phenomenon is briefly discussed.

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arXiv1995

O(d,d;R) Deformations of Complex Structures and Extended Worldsheet Supersymmetry

It is shown that the O(d,d;R) deformations of the superstring vacua and the O(d,d+16;R) deformations of the heterotic string vacua preserve extended worldsheet supersymmetry and, hence, generate superconformal deformations. The transformations of the complex structures are given explicitly and the action of the discrete duality subgroup is discussed. The results are valid when the complex structures are independent of the d coordinates which appear in the transformations. It is shown that generic deformations do not preserve the known superfield formulations of (2,2) extended supersymmetry. The analysis is performed by decomposing the transformations in terms of the metric vielbein and by introducing space-time connections induced due to the non-linear action of the O(d,d;R) and O(d,d+16;R) deformations on the background fields.

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arXiv1992

Marginal Deformations of WZNW and Coset Models from O(d,d) Transformation

We show that O(2,2) transformation of SU(2) WZNW model gives rise to marginal deformation of this model by the operator $\int d^2 z J(z)\bar J(\bar z)$ where $J$, $\bar J$ are U(1) currents in the Cartan subalgebra. Generalization of this result to other WZNW theories is discussed. We also consider O(3,3) transformation of the product of an SU(2) WZNW model and a gauged SU(2) WZNW model. The three parameter set of models obtained after the transformation is shown to be the result of first deforming the product of two SU(2) WZNW theories by marginal operators of the form $\sum_{i,j=1}^2 C_{ij} J_i \bar J_j$, and then gauging an appropriate U(1) subgroup of the theory. Our analysis leads to a general conjecture that O(d,d) transformation of any WZNW model corresponds to marginal deformation of the WZNW theory by combination of appropriate left and right moving currents belonging to the Cartan subalgebra; and O(d,d) transformation of a gauged WZNW model can be identified to the gauged version of such marginally deformed WZNW models.

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arXiv1991

Twisting Classical Solutions in Heterotic String Theory

We show that, given a classical solution of the heterotic string theory which is independent of $d$ of the space time directions, and for which the gauge field configuration lies in a subgroup that commutes with $p$ of the $U(1)$ generators of the gauge group, there is an $O(d)\otimes O(d+p)$ transformation, which, acting on the solution, generates new classical solutions of the theory. With the help of these transformations we construct black 6-brane solutions in ten dimensional heterotic string theory carrying independent magnetic, electric and antisymmetric tensor gauge field charge, by starting from a black 6-brane solution that carries magnetic charge but no electric or antisymmetric tensor gauge field charge. The electric and the magnetic charges point in different directions in the gauge group.

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