Density of States and Tachyons in Open and Closed String Theory

In this note we reexamine the possibility of constructing stable non-supersymmetric theories that exhibit an exponential density of states. For weakly coupled closed strings there is a general theorem, according to which stable theories with an exponential density of states must exhibit an almost exact cancellation of spacetime bosons and fermions (not necessarily level by level). We extend this result to open strings by showing that if the above cancellation between bosons and fermions does not occur, the open strings do not decouple from a closed string tachyon even in the NCOS scaling limit. We conclude with a brief comment on the proposed generalization of the AdS/CFT correspondence to non-supersymmetric theories.Comment: 11 pages, harvmac, v2: improved discussion, references added, v3: version published in JHE

(Multi)Matrix Models and Interacting Clones of Liouville Gravity

Large-N matrix models coupled via multitrace operators are used to define, via appropriate double-scaling limits, solvable models of interacting multi-string theories. It is shown that although such theories are non-local at the world-sheet level they have a simple description of the spacetime physics. Such theories share the main characteristics of similarly coupled higher-dimensional CFTs. An interpretation has been given in the past of similar continuum limits in terms of Liouville interactions that violate the Seiberg bound. We provide a novel interpretation of this relation which agrees with the current understanding of Liouville theory and analogous observations in the AdS/CFT correspondence.Comment: 45 pages, 4 figures; v2 references added, version to appear in JHE

Large-N limits of 2d CFTs, Quivers and AdS_3 duals

We explore the large-N limits of 2d CFTs, focusing mostly on WZW models and their cosets. The $SU(N)_k$ theory is parametrized in this limit by a 't Hooft-like coupling. We show a duality between strong coupling, where the theory is described by almost free fermions, and weak coupling where the theory is described by bosonic fields by an analysis of spectra and correlators. The AdS$_3$ dual is described, and several quantitative checks are performed. Besides the more standard states that should correspond to bulk black holes we find ground states with large degeneracy that can dominate the standard Cardy entropy at weak coupling and are expected to correspond to regular horizonless semiclassical bulk solutions.Comment: latex, 54pages; v2 minor changes, addition of a new appendix on YM-CS theories on AdS3, version published in JHEP; v3 several misprints have been corrected in the formulae of section

A Lagrangian for self-dual strings

We propose a Lagrangian for the low-energy theory that resides at the (1 + 1)-dimensional intersection of N semi-infinite M2-branes ending orthogonally on M M5-branes in R1,2×C4/ZkR1,2×C4/Zk (for arbitrary positive integers N, M, k). We formulate this theory as a 2d boundary theory with explicit N=(1,1)N=(1,1) supersymmetry that contains two superfields in the bi-fundamental representation of U(N )×U(M ) interacting with the (2+1)-dimensional U(N )k × U(N )−k ABJM Chern-Simons-matter theory in the bulk. We postulate that the boundary theory exhibits in the deep infrared supersymmetry enhancement to N=(4,2)N=(4,2) , or N=(4,4)N=(4,4) depending on the value of k. Arguments in favor of the proposal follow from the study of the open string theory of a U-dual type IIB Hanany-Witten setup. To formulate the bulk-boundary interactions special care is taken to incorporate all the expected boundary effects on gauge symmetry, supersymmetry, and other global symmetries

Supersymmetric Perturbations of the M5 brane

We study long-wavelength supersymmetric deformations of brane solutions in supergravity using an extension of previous ideas within the general scheme of the blackfold approach. As a concrete example, we consider long-wavelength perturbations of the planar M2-M5 bound state solution in eleven-dimensional supergravity. We propose a specific ansatz for the first order deformation of the supergravity fields and explore how this deformation perturbs the Killing spinor equations. We find that a special part of these equations gives a projection equation on the Killing spinors that has the same structure as the κ-symmetry condition of the abelian M5 brane theory. Requiring a match between supergravity and gauge theory implies a specific non-linear gauge-gravity map between the bosonic fields of the abelian M5 brane theory and the gravity-induced fluid-like degrees of freedom of the blackfold equations that control the perturbative gravity solution. This observation sheds new light on the SUGRA/DBI correspondence

Exact correlation functions in SU(2) N=2 superconformal QCD

We report an exact solution of 2- and 3-point functions of chiral primary fields in SU(2) N=2 super-Yang-Mills theory coupled to four hypermultiplets. It is shown that these correlation functions are non-trivial functions of the gauge coupling, obeying differential equations which take the form of the semi-infinite Toda chain. We solve these equations recursively in terms of the Zamolodchikov metric that can be determined exactly from supersymmetric localization on the four-sphere. Our results are verified independently in perturbation theory with a Feynman diagram computation up to 2-loops. This is a short version of a companion paper that contains detailed technical remarks, additional material and aspects of an extension to SU(N) gauge group.Comment: 5 page

Stable Non-Supersymmetric Vacua in the Moduli Space of Non-Critical Superstrings

We study a set of asymmetric deformations of non-critical superstring theories in various dimensions. The deformations arise as Kaehler and complex structure deformations of an orthogonal two-torus comprising of a parallel and a transverse direction in the near-horizon geometry of NS5-branes. The resulting theories have the following intriguing features: Spacetime supersymmetry is broken in a continuous fashion and the masses of the lightest modes are lifted. In particular, no bulk or localized tachyons are generated in the non-supersymmetric vacua. We discuss the effects of these deformations in the context of the holographic duality between non-critical superstrings and Little String Theories and find solutions of rotating fivebranes in supergravity. We also comment on the generation of a one-loop cosmological constant and determine the effects of the one-loop backreaction to leading order.Comment: lanlmac, 41 pages, 1 figure; v2 an erroneous statement corrected in section 6, version published in NP