We find non-critical string backgrounds in five and eight dimensions, holographically related to four-dimensional conformal field theories with N = 0 and N = 1 supersymmetries. In the five-dimensional case we find an AdS 5 background metric for a string model related to non-supersymmetric, conformal QCD with large number of colors and flavors and discuss the conjectured existence of a conformal window from the point of view of our solution. In the eight-dimensional string theory, we build a family of solutions of the form AdS 5 ×S 3 withS 3 a squashed three-sphere. For a special value of the ratio N f /N c , the background can be interpreted as the supersymmetric near-horizon limit of a system of color and flavor branes on R 1,3 times a known four-dimensional generalization of the cigar. The N = 1 dual theory with fundamental matter should have an IR fixed point only for a fixed ratio N f /N c . General features of the string/gauge theory correspondence for theories with fundamental flavors are also addressed.

We study the holographic dual model of quenched flavors immersed in a quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This is modeled by embedding a probe D7 brane in a background where the backreaction of massless D7 branes has been taken into account. The background, and hence the effects, are perturbative in the Veneziano parameter N f /N c , therefore giving small shifts of all magnitudes like the constituent mass, the quark condensate, and several transport coefficients. We provide qualitative results for the effect of flavor degrees of freedom on the probes. For example, the meson melting temperature is enhanced, while the screening length is diminished. The drag force is also enhanced.

We provide a cross-check of AdS/CFT and a-charge maximization for a four dimensional N = 1 SCFT with irrational R-charges. The gauge theory is the low energy effective theory of N D3branes at the tip of the complex cone over the first del Pezzo surface. By carefully taking into account the subtle issue of flavor symmetry breaking at the fixed point, we show, using amaximization, that this theory has in fact irrational central charge and R-charges. Our results perfectly match with those inherited from the recently discovered supergravity dual background.Along analogous lines, we make novel predictions for the still unknown AdS dual of the quiver theory for the second del Pezzo surface. This should flow to a SCFT with irrational charges, too. All of our results differ from previous findings in the literature and outline interesting subtleties in a-maximization and AdS/CFT techniques overlooked in the past.

We study the IR dynamics of the cascading non-conformal quiver theory on N regular and M fractional D3 branes at the tip of the complex cone over the first del Pezzo surface. The horizon of this cone is the irregular Sasaki-Einstein manifold Y 2,1 . Our analysis shows that at the end of the cascade supersymmetry is dynamically broken.

We present string duals of four dimensional N = 2 pure SU(N) SYM theory. The theory is obtained as the low energy limit of D5-branes wrapped on non-trivial two-cycles. Using seven dimensional gauged supergravity and uplifting the result to ten dimensions, we obtain solutions corresponding to various points of the N = 2 moduli space. The more symmetric solution may correspond to a point with rotationally invariant classical vevs. By turning on seven dimensional scalar fields, we find a solution corresponding to a linear distribution of vevs. Both solutions are conveniently studied with a D5-probe, which also confirms many of the standard expectations for N = 2 solutions.

We compute the one-loop string corrections to the Wilson loop, glueball Regge trajectory and stringy hadron masses in the Witten model of nonsupersymmetric, large-N Yang-Mills theory. The classical string configurations corresponding to the above field theory objects are, respectively: open straight strings, folded closed spinning strings, and strings orbiting in the internal part of the supergravity background. For the rectangular Wilson loop we show that besides the standard Lüscher term, string corrections provide a rescaling of the field theory string tension. The one-loop corrections to the linear glueball Regge trajectories render them nonlinear with a positive intercept, as in the experimental soft Pomeron trajectory. Strings orbiting in the internal space predict a spectrum of hadroniclike states charged under global flavor symmetries which falls in the same universality class of other confining models.

We present the string dual to SU(N c ) N = 4 SYM, coupled to N f massless fundamental flavors, at finite temperature and baryon density. The solution is determined by two dimensionless parameters, both depending on the 't Hooft coupling λ h at the scale set by the temperature T : ǫ h ∼ λ h N f /N c , weighting the backreaction of the flavor fields, where n b is the baryon density. For small values of these two parameters the solution is given analytically up to second order. We study the thermodynamics of the system in the canonical and grand-canonical ensembles. We then analyze the energy loss of partons moving through the plasma, computing the jet quenching parameter and studying its dependence on the baryon density. Finally, we analyze certain "optical" properties of the plasma. The whole setup is generalized to non abelian strongly coupled plasmas engineered on D3-D7 systems with D3-branes placed at the tip of a generic singular Calabi-Yau cone. In all the cases, fundamental matter fields are introduced by means of homogeneously smeared D7-branes and the flavor symmetry group is thus a product of abelian factors.

We consider the addition of a large number of massive dynamical flavors to the Klebanov-Witten theory, the quiver gauge theory describing the low energy dynamics of N c D3-branes at the conifold singularity. Massive flavors are introduced by means of N f D7-branes which are holomorphically embedded and smeared along the transverse directions. After some general comments on the validity of the smearing procedure, we find the full backreacted supergravity solution corresponding to a particular class of massive embeddings. The solution depends on a running effective number of flavors, whose functional form follows from the smeared embedding. The running reflects the integrating in/out of massive degrees of freedom in the dual field theory as the energy scale is changed. We study how the dynamics of the theory depends on the flavor parameters, mainly focusing on the static quark-antiquark potential. As expected, we find that the dynamical flavors tend to screen the static color charges.