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Asad Naqvi

Publications and source records attributed to Asad Naqvi.

25 records · Page 2Linked to original sources
arXiv2002

A Space-Time Orbifold: A Toy Model for a Cosmological Singularity

We explore bosonic strings and Type II superstrings in the simplest time dependent backgrounds, namely orbifolds of Minkowski space by time reversal and some spatial reflections. We show that there are no negative norm physical excitations. However, the contributions of negative norm virtual states to quantum loops do not cancel, showing that a ghost-free gauge cannot be chosen. The spectrum includes a twisted sector, with strings confined to a ``conical'' singularity which is localized in time. Since these localized strings are not visible to asymptotic observers, interesting issues arise regarding unitarity of the S-matrix for scattering of propagating states. The partition function of our model is modular invariant, and for the superstring, the zero momentum dilaton tadpole vanishes. Many of the issues we study will be generic to time-dependent cosmological backgrounds with singularities localized in time, and we derive some general lessons about quantizing strings on such spaces.

hep-th↗
arXiv2002

Giant Gravitons and a Correspondence Principle

We propose a correspondence between the physics of certain small charge black holes in AdS_k x S^l and large charge black holes in AdS_l x S^k. The curvature singularities of these solutions arise, following Myers and Tafjord, from a condensate of giant gravitons. When the number of condensed giants N_g is much greater than the number of background branes N, we propose that the system has an equivalent description in terms of N giant gravitons condensed in a background created by N_g branes. Our primary evidence is an exact correspondence between gravitational entropy formulae of small and large charge solutions in different dimensions.

hep-th↗
arXiv2001

Giant Gravitons in Conformal Field Theory

Giant gravitons in AdS_5 x S^5, and its orbifolds, have a dual field theory representation as states created by chiral primary operators. We argue that these operators are not single-trace operators in the conformal field theory, but rather are determinants and subdeterminants of scalar fields; the stringy exclusion principle applies to these operators. Evidence for this identification comes from three sources: (a) topological considerations in orbifolds, (b) computation of protected correlators using free field theory and (c) a Matrix model argument. The last argument applies to AdS_7 x S^4 and the dual (2,0) theory, where we use algebraic aspects of the fuzzy 4-sphere to compute the expectation value of a giant graviton operator along the Coulomb branch of the theory.

hep-th↗
arXiv2000

On Marginal Deformations in Superstring Field Theory

We use level truncated superstring field theory to obtain the effective potential for the Wilson line marginal deformation parameter which corresponds to the constant vacuum expectation value of the U(1) gauge field on the D-brane in a particular direction. We present results for both the BPS and the non-BPS D-brane. In the case of non-BPS D-brane the effective potential has branches corresponding to the extrema of the tachyon potential. In the branch with vanishing tachyon vev (M-branch), the effective potential becomes flatter as the level of the approximation is increased. The branch which corresponds to the stable vacuum after the tachyon has condensed (V-branch) exists only for a finite range of values of marginal deformation parameter. We use our results to find the mass of the gauge field in the stable tachyonic vacuum. We find this mass to be of a non-zero value which seems to stabilize as the level approximation is improved.

hep-th↗
arXiv2000

Tachyon Condensation on a non-BPS D-brane

We extend the recent computation of the tachyon potential by Berkovits, Sen and Zwiebach by including level two fields and keeping up to level four terms in the action. We find 90.5% of the expected result.

hep-th↗
arXiv1999

Marginal Deformations from Branes

We study brane configurations for four dimensional N=1 supersymmetric gauge theories with quartic superpotentials which flow in the infrared to manifolds of interacting superconformal fixed points. We enumerate finite N=2 theories, from which a large class of marginal N=1 theories descend. We give the brane descriptions of these theories in Type IIA and Type IIB string theory. The Type IIB descriptions are in terms of D3 branes in orientifold and generalized conifold backgrounds. We calculate the Weyl and Euler anomalies in these theories, and find that they are equal in elliptic models and unequal in a large class of finite N=2 and marginal N=1 non-elliptic theories.

hep-th↗