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 Some previously unpublished results are also included.PACS. 11.15.-q Gauge field theories -11.25.Tq Gauge/string duality -12.40.Yx Hadron mass models and calculations -14.40.-n Mesons

Recently, a D-brane construction in type IIA string theory was shown to yield the electric/magnetic duality of four dimensional N =1 supersymmetric U (N c ) gauge theories with N f flavours of quark. We present here an extension of that construction which yields the electric/magnetic duality for the SO(N c ) and U Sp(N c ) gauge theories with N f quarks, by adding an orientifold plane which is consistent with the supersymmetry. Due to the intersection of the orientifold plane with the NS-NS fivebranes already present, new features arise which are crucial in determining the correct final structure of the dualities.

We study the family of ten dimensional type IIB supergravity solutions corresponding to renormalisation group flows from N = 4 to N = 2 supersymmetric SU (N ) Yang-Mills theory. Part of the solution set corresponds to a submanifold of the Coulomb branch of the gauge theory, and we use a D3-brane probe to uncover details of this physics. At generic places where supergravity is singular, the smooth physics of the probe yields the correct one-loop form of the effective low energy gauge coupling. The probe becomes tensionless on a ring at finite radius. Supergravity flows which end on this "enhançon" ring correspond to the vacua where extra massless degrees of freedom appear in the gauge theory, and the gauge coupling diverges there. We identify an SL(2, Z Z) duality action on the enhançon ring which relates the special vacua, and comment on the massless dyons within them. We propose that the supergravity solution inside the enhançon ring should be excised, since the probe's tension is unphysical there.