We study gravity duals of large N non-supersymmetric gauge theories with matter in the fundamental representation by introducing a D7-brane probe into deformed AdS backgrounds. In particular, we consider a D7-brane probe in both the AdS Schwarzschild black hole solution and in the background found by Constable and Myers, which involves a non-constant dilaton and S 5 radius. Both these backgrounds exhibit confinement of fundamental matter and a discrete glueball and meson spectrum.We numerically compute theΨΨ condensate and meson spectrum associated with these backgrounds.In the AdS-black hole background, a quark-bilinear condensate develops only at non-zero quark mass.We speculate on the existence of a third order phase transition at a critical quark mass where the D7 embedding undergoes a geometric transition. In the Constable-Myers background, we find a chiral symmetry breaking condensate as well as the associated Goldstone boson in the limit of small quark mass. The existence of the condensate ensures that the D7-brane never reaches the naked singularity at the origin of the deformed AdS

Abstract. We review recent progress in studying mesons within gauge/gravity duality, in the context of adding flavour degrees of freedom to generalizations of the AdS/CFT correspondence. Our main focus is on the "top-down approach" of considering models constructed within string theory. We explain the stringtheoretical constructions in detail, aiming at non-specialists. These give rise to a new way of describing strongly coupled confining large-N gauge theories similar to large-N QCD. In particular, we consider gravity dual descriptions of spontaneous chiral symmetry breaking, and compare with lattice results. A further topic covered is the behaviour of flavour bound states in finite-temperature field theories dual to a gravity background involving a black hole. We also describe the "bottom up" phenomenological approach to mesons within AdS/QCD. Some previously unpublished results are also included.PACS. 11.15.-q Gauge field theories -11.25.Tq Gauge/string duality -12.40.Yx Hadron mass models and calculations -14.40.-n Mesons

We study four-dimensional superconformal field theories coupled to three-dimensional superconformal boundary or defect degrees of freedom. Starting with bulk N = 2, d = 4 theories, we construct abelian models preserving N = 2, d = 3 supersymmetry and the conformal symmetries under which the boundary/defect is invariant. We write the action, including the bulk terms, in N = 2, d = 3 superspace. Moreover we derive Callan-Symanzik equations for these models using their superconformal transformation properties and show that the beta functions vanish to all orders in perturbation theory, such that the models remain superconformal upon quantization. Furthermore we study a model with N = 4 SU (N ) Yang-Mills theory in the bulk coupled to a N = 4, d = 3 hypermultiplet on a defect. This model was constructed by DeWolfe, Freedman and Ooguri, and conjectured to be conformal based on its relation to an AdS configuration studied by Karch and Randall. We write this model in N = 2, d = 3 superspace, which has the distinct advantage that non-renormalization theorems become transparent. Using N = 4, d = 3 supersymmetry, we argue that the model is conformal.

We study a defect conformal field theory describing D3-branes intersecting over two space-time dimensions. This theory admits an exact Lagrangian description which includes both two-and four-dimensional degrees of freedom, has (4,4) supersymmetry and is invariant under global conformal transformations. Both two-and four-dimensional contributions to the action are conveniently obtained in a two-dimensional (2,2) superspace. In a suitable limit, the theory has a dual description in terms of a probe D3-brane wrapping an AdS 3 ϫS 1 slice of AdS 5 ϫS 5 . We consider the AdS/CFT dictionary for this setup. In particular we find classical probe fluctuations corresponding to the holomorphic curve wyϭc␣Ј. These fluctuations are dual to defect fields containing massless two-dimensional scalars which parametrize the classical Higgs branch, but do not correspond to states in the Hilbert space of the CFT. We also identify probe fluctuations which are dual to BPS superconformal primary operators and to their descendants. A nonrenormalization theorem is conjectured for the correlators of these operators, and verified to order g 2 .

The AdS 3 /CFT 2 correspondence is checked beyond the supergravity approximation by comparing correlation functions. To this end we calculate 2-and 3-point functions on the sphere of certain chiral primary operators for strings on AdS 3 × S 3 × T 4 . These results are then compared with the corresponding amplitudes in the dual 2-dimensional conformal field theory. In the limit of small string coupling, where the sphere diagrams dominate the string perturbation series, beautiful agreement is found.