Properties of Confinement in Holography

We review certain properties of confinement with added focus on the ones we study with holography. Then we discuss observables whose unique behavior can indicate the presence of confinement. Using mainly the Wilson loop in the gauge/gravity formalism, we study two main features of the QCD string: the string tension dependence on the temperature while in the confining phase, and the logarithmic broadening of the flux tube between the heavy static charges that turns out to be a generic property of all confining theories. Finally, we review the k-string bound state and we show that for a wide class of generic theories the k-string observables can be expressed in terms of the single meson bound state observables.Comment: 20 pages, 2 figures, Contribution to the Proceedings of the Workshop on Quantum Fields and Strings, (Corfu, September 2014

Observables in Strongly Coupled Anisotropic Theories

We review certain anisotropic gauge/gravity dualities, focusing more on a theory with space dependent axion term. Then we discuss and also present some new results for several observables: the static potential and force, the imaginary part of the static potential, the quark dipole in the plasma wind, the drag force and diffusion time, the jet quenching of heavy and light quarks, the energy loss of rotating quarks, the photon production and finally the violation of the holographic viscosity over entropy bound. The corresponding weakly coupled results are also discussed. Finally we investigate the bounds of the parameters of the current strongly coupled anisotropic theories attempting to match them with the observed quark-gluon plasma and report the problems appear.Comment: 36 pages, 9 figures, Contribution to the Proceedings of the XVIII European Workshop on String Theory, (Corfu, September 2012), v3: Minor change

k-Strings as Fundamental Strings

It has been noticed that the k-string observables can be expressed in terms of the fundamental string ones. We identify a sufficient condition for a generic gravity dual background which when satisfied the mapping can be done. The condition is naturally related to a preserved quantity under the T-dualities acting on the Dp-brane describing the high representation Wilson loops. We also find the explicit relation between the observables of the heavy k-quark and the single quark states. As an application to our generic study and motivated by the fact that the anisotropic theories satisfy our condition, we compute the width of the k-string in these theories to find that the logarithmic broadening is still present, but the total result is affected by the anisotropy of the space.Comment: 1+21 page

Tension of Confining Strings at Low Temperature

In the low temperature confining phase of QCD or QCD-like theories it is challenging to capture the temperature dependence of observables through AdS/CFT. Using the blackfold approach we compute the quark-antiquark linear static potential in the low temperature confining phase, taking into account the thermal excitations of the string. We find the explicit temperature dependence of the string tension and notice that, as naturally expected, tension decreases as temperature increases. We have also generalized the blackfold approach for the computation of the Wilson loops, making it directly applicable to a large class of backgrounds.Comment: 22 pages, 2 figure

On the holographic width of flux tubes

We investigate the width of the flux tube between heavy static quark charges. Using the gauge/gravity duality, we find the properties of the minimal connected surface related to the width of the bound state. We show that in the confining phase, the logarithmic broadening predicted by the effective string description and observed in lattice simulations is a generic property of all confining backgrounds. We also study the transverse fluctuations of the string connecting two static quarks in curved backgrounds. Our formalism is applied to AdS space where we compute the expectation value of the square of transverse deviations of the string, a quantity related to the width.Comment: 1+28 pages, 6 figure

Non-integrability in non-relativistic theories

Generic non-relativistic theories giving rise to non-integrable string solutions are classified. Our analysis boils down to a simple algebraic condition for the scaling parameters of the metric. Particular cases are the Lifshitz and the anisotropic Lifshitz spacetimes, for which we find that for trivial dilaton dependence the only integrable physical theory is that for z=1. For the hyperscaling violation theories we conclude that the vast majority of theories are non-integrable, while only for a small class of physical theories, where the Fermi surfaces belong to, integrability is not excluded. Schrodinger theories are also analyzed and a necessary condition for non-integrability is found. Our analysis is also applied to cases where the exponential of the dilaton is a monomial of the holographic coordinate.Comment: 1+20 pages, v2:minor corrections, references adde

Flavor Corrections in the Static Potential in Holographic QCD

We examine the static potential in the presence of flavors in the perturbative backreacted D4/D8 system from localized D8 branes, focusing in particular on the Sakai-Sugimoto model. For the case of long strings we find the flavor corrections to the static potential which are of exponential form. We then investigate shorter Wilson loops and express their energy analytically in terms of the lengths of two neighboring Wilson loops. Moreover, we find that the static force for all the cases in the backreacted background is reduced compared to one in the probe limit, as expected due to screening effects. We also compare the string world-sheets in the two backgrounds and find how they get modified by the backreaction. Our results are supported by numerical computations as well. Finally we discuss our results in comparison with the lattice data and comment on the issue of physical scales which seem to lie at the heart of the reason that obstructs our model at this level to fully describe QCD.Comment: 28 pages, 3 figure