Angel Paredes

Chiral symmetry breaking as open string tachyon condensation

Abstract We consider a general framework to study holographically the dynamics of fundamental quarks in a confining gauge theory. Flavors are introduced by placing a set of (coincident) branes and antibranes on a background dual to a confining color theory. The spectrum contains an open string tachyon and its condensation describes the U ( N f ) L × U ( N f ) R → U ( N f ) V symmetry breaking. By studying worldvolume gauge transformations of the flavor brane action, we obtain the QCD global anomalies and an IR condition that allows to fix the quark condensate in terms of the quark mass. We find the expected N f 2 Goldstone bosons (for m q = 0 ), the Gell-Mann–Oakes–Renner relation (for m q small) and the η ′ mass. Remarkably, the linear confinement behavior for the masses of highly excited spin-1 mesons, m n 2 ∼ n is naturally reproduced.

Non-critical holography and four-dimensional CFT's with fundamentals

We find non-critical string backgrounds in five and eight dimensions, holographically related to four-dimensional conformal field theories with = 0 and = 1 supersymmetries. In the five-dimensional case we find an AdS5 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 AdS5 ? 3 with 3 a squashed three-sphere. For a special value of the ratio Nf/Nc, the background can be interpreted as the supersymmetric near-horizon limit of a system of color and flavor branes on 1,3 times a known four-dimensional generalization of the cigar. The = 1 dual theory with fundamental matter should have an IR fixed point only for a fixed ratio Nf/Nc. General features of the string/gauge theory correspondence for theories with fundamental flavors are also addressed.

Flavoring the gravity dual of Script N = 1 Yang-Mills with probes

We study two related problems in the context of a supergravity dual to = 1 SYM. One of the problems is finding kappa symmetric D5-brane probes in this particular background. The other is the use of these probes to add flavors to the gauge theory. We find a rich and mathematically appealing structure of the supersymmetric embeddings of a D5-brane probe in this background. Besides, we compute the mass spectrum of the low energy excitations of = 1 SQCD (mesons) and match our results with some field theory aspects known from the study of supersymmetric gauge theories with a small number of flavors.

Unquenched Flavor in the Gauge/Gravity Correspondence

Within the AdS/CFT correspondence, we review the studies of field theories with a large number of adjoint and fundamental fields, in the Veneziano limit. We concentrate in set-ups where the fundamentals are introduced by a smeared set of D-branes. We make emphasis on the general ideas and then in subsequent chapters that can be read independently and describe particular considerations in various different models. Some new material is presented along the various sections.

D3-D7 quark-gluon plasmas

We present the string dual to finite temperature SU(Nc) = 4 SYM coupled to massless fundamental matter introduced by Nf D7 branes, with Abelian flavor symmetry. The analytic solution includes the backreaction of the flavors up to second order in the parameter that weighs the internal flavor loops, h = (λhNf)/(8π2Nc), λh being the t Hooft coupling at the temperature of the dual Quark-Gluon Plasma. We study the thermodynamics of the system and its departure from conformality, which is a second order effect. We then analyze the energy loss of partons moving through the plasma, finding that the fundamental degrees of freedom enhance the jet quenching. The whole setup is generalized to D3-D7 systems with D3-branes placed at the tip of a generic singular Calabi-Yau cone over a five-dimensional Sasaki-Einstein manifold. We finally provide the equations for the inclusion of massive flavors in the = 4 SYM plasma.

Elaborations on the string dual to N = 1 SQCD

In this paper we make further refinements to the duality proposed between N=1 SQCD and certain string (supergravity plus branes) backgrounds, working in the regime of comparable large number of colors and flavors. Using the string theory solutions, we predict different field theory observables and phenomena like Seiberg duality, gauge coupling and its running, the behavior of Wilson and t Hooft loops, anomalous dimensions of the quark superfields, quartic superpotential coupling and its running, continuous and discrete anomaly matching. We also give evidence for the smooth interpolation between the Higgs and confining vacua. We provide several matchings between field theory and string theory computations.

Fundamental matter, meson spectroscopy and non-critical string/gauge duality

We discuss the incorporation of quarks in the fundamental representation of the color group into the non-critical string/gauge duality. We focus on confining theories and address this question using two different approaches: (i) by introducing flavor probe branes and (ii) by deriving backreacted flavored near extremal gravity backgrounds. In the former approach we analyze the near extremal AdS6 model with D4 and anti-D4 probe flavor branes included. We study the meson spectrum and discuss the role played by the constituent quark mass, related to the integration constant that defines the embedding. As for the second approach we derive a class of flavored AdSn+1 ? Sk black hole solutions. In particular we write down the flavored AdS6 and AdS5 black holes and the near extremal AdS5 ? S1 backgrounds. We analyze several gauge dynamical properties associated with these models.

An AdS/QCD model from tachyon condensation: II

A simple holographic model is presented and analyzed that describes chiral symmetry breaking and the physics of the meson sector in QCD. This is a bottom-up model that incorporates string theory ingredients like tachyon condensation which is expected to be the main manifestation of chiral symmetry breaking in the holographic context. As a model for glue the Kuperstein-Sonnenschein background is used. The structure of the flavor vacuum is analyzed in the quenched approximation. Chiral symmetry breaking is shown at zero temperature. Above the deconfinement transition chiral symmetry is restored. A complete holographic renormalization is performed and the chiral condensate is calculated for different quark masses both at zero and non-zero temperatures. The 0++, 0−+, 1++, 1−− meson trajectories are analyzed and their masses and decay constants are computed. The asymptotic trajectories are linear. The model has one phenomenological parameter beyond those of QCD that affects the 1++, 0−+ sectors. Fitting this parameter we obtain very good agreement with data. The model improves in several ways the popular hard-wall and soft wall bottom-up models.

Multiflavour excited mesons from the fifth dimension

Abstract We study the Regge trajectories and the quark–antiquark energy in excited hadrons composed by different dynamical mass constituents via the gauge/string correspondence. First we exemplify the procedure in a supersymmetric system, D3–D7, in the extremal case. Afterwards we discuss the model dual to large- N c QCD, D4–D6 system. In the latter case we find the field theory expected gross features of vector like theories: the spectrum resembles that of heavy quarkonia and the Chew–Frautschi plot of the singlet and first excited states is in qualitative agreement with those of lattice QCD. We stress the salient points of including different constituents masses.

AdS/QCD model from an effective action for open string tachyons

We construct a new, simple phenomenological model along the lines of AdS/QCD. The essential new ingredient is the brane-antibrane effective action including the open string tachyon proposed by Sen. Chiral symmetry breaking happens because of tachyon dynamics. We fit a large number of low-spin meson masses at the 10%-15% level. The only free parameters involved in the fits correspond to the overall QCD-scale and the quark masses. Several aspects of previous models are qualitatively improved.