20516371
Characteristics of thermalization of boost-invariant plasma from holography.
We report on the approach toward the hydrodynamic regime of boost-invariant N=4 super Yang-Mills plasma at strong coupling starting from various far-from-equilibrium states at τ=0. The results are obtained through a numerical solution of Einstein's equations for the dual geometries, as described in detail in the companion article [M. P. Heller, R. A. Janik, and P. Witaszczyk, arXiv:1203.0755]. Despite the very rich far-from-equilibrium evolution, we find surprising regularities in the form of clear correlations between initial entropy and total produced entropy, as well as between initial entropy and the temperature at thermalization, understood as the transition to a hydrodynamic description. For 29 different initial conditions that we consider, hydrodynamics turns out to be definitely applicable for proper times larger than 0.7 in units of inverse temperature at thermalization. We observe a sizable anisotropy in the energy-momentum tensor at thermalization, which is nevertheless entirely due to hydrodynamic effects. This suggests that effective thermalization in heavy-ion collisions may occur significantly earlier than true thermalization.
7823932
Hydrodynamic Flow of the Quark-Gluon Plasma and Gauge/Gravity Correspondence
The contribution presents a summary of the Gauge/Gravity approach to the study of hydrodynamic flow of the quark-gluon plasma formed in heavy-ion collisions. Considering the ideal case of a supersymmetric Yang-Mills theory for which the AdS/CFT correspondence gives a precise form of the Gauge/Gravity duality, the properties of the strongly coupled expanding plasma are put in one-to-one correspondence with the metric of a 5-dimensional black hole moving away in the 5th dimension and its deformations consistent with the relevant Einstein equations. Several recently studied aspects of this framework are recalled and put in perspective. This paper is a written version of the four lectures given by the authors on that subject.
7028788
Supergravity description of boost invariant conformal plasma at strong coupling
We study string theory duals of the expanding boost invariant conformal gauge theory plasmas at strong coupling. The dual supergravity background is constructed as an asymptotic late-time expansion, corresponding to equilibration of the gauge theory plasma. The absence of curvature singularities in the first few orders of the late-time expansion of the dual gravitational background unambiguously determines the equilibrium equation of state, and the shear viscosity of the gauge theory plasma. While the absence of the leading pole singularities in the gravitational curvature invariants at third order in late-time expansion determines the relaxation time of the plasma, the subleading logarithmic singularity cannot be canceled within a supergravity approximation. Thus, a supergravity approximation to a dual description of the strongly coupled boost invariant expanding plasma is inconsistent. Nevertheless we find that the relaxation time determined from the cancellation of pole singularities is quite robust.
216050417
Towards a holographic realization of the quarkyonic phase
Large-Nc QCD matter at intermediate baryon density and low temperatures has been conjectured to be in the so-called quarkyonic phase, i.e., to have a quark Fermi surface and on top of it a confined spectrum of excitations. It has been suggested that the presence of the quark Fermi surface leads to a homogeneous phase with restored chiral symmetry, which is unstable towards creating condensates that break both the chiral and translational symmetry. Motivated by these exotic features, we investigate properties of cold baryonic matter in the single-flavor Sakai-Sugimoto model, searching for a holographic realization of the quarkyonic phase. We use a simplified mean-field description and focus on the regime of parametrically large baryon densities, of the order of the square of the ’t Hooft coupling, as they turn out to lead to new physical effects similar to the ones occurring in the quarkyonic phase. One effect—the appearance of a particular marginally stable mode breaking translational invariance and linked with the presence of the Chern-Simons term in the flavor-brane Lagrangian—is known to occur in the deconfined phase of the Sakai-Sugimoto model, but turns out to be absent here. The other, completely new phenomenon that we, preliminarily, study using strong simplifying assumptions are density-enhanced interactions of the flavor-brane gauge field with holographically represented baryons. These seem to significantly affect the spectrum of vector and axial mesons and might lead to approximate chiral symmetry restoration in the lowest part of the spectrum, where the mesons start to qualitatively behave like collective excitations of the dense baryonic medium. We discuss the relevance of these effects for holographic searches of the quarkyonic phase and conclude with a discussion of various subtleties involved in constructing a mean-field holographic description of a dense baryonic medium.
16202574
Hydrodynamic gradient expansion in gauge theory plasmas.
We utilize the fluid-gravity duality to investigate the large order behavior of hydrodynamic gradient expansion of the dynamics of a gauge theory plasma system. This corresponds to the inclusion of dissipative terms and transport coefficients of very high order. Using the dual gravity description, we calculate numerically the form of the stress tensor for a boost-invariant flow in a hydrodynamic expansion up to terms with 240 derivatives. We observe a factorial growth of gradient contributions at large orders, which indicates a zero radius of convergence of the hydrodynamic series. Furthermore, we identify the leading singularity in the Borel transform of the hydrodynamic energy density with the lowest nonhydrodynamic excitation corresponding to a 'nonhydrodynamic' quasinormal mode on the gravity side.
12201332
Boost-invariant early time dynamics from AdS/CFT
Boost-invariant dynamics of a strongly-coupled conformal plasma is studied in the regime of early proper-time using the AdS/CFT correspondence. It is shown, in contrast with the late-time expansion, that a scaling solution does not exist. The boundary dynamics in this regime depends on initial conditions encoded in the bulk behavior of a Fefferman-Graham metric coefficient at initial proper-time. The relation between the early-time expansion of the energy density and initial conditions in the bulk of AdS is provided. As a general result it is proven that a singularity of some metric coefficient in Fefferman-Graham frame exists at all times. Requiring that this singularity at ? = 0 is a mere coordinate singularity without the curvature blow-up gives constraints on the possible boundary dynamics. Using a simple Pade resummation for solutions satisfying the regularity constraint, the features of a transition to local equilibrium, and thus to the hydrodynamical late-time regime, have been observed. The impact of this study on the problem of thermalization is discussed.
171006505
Mesure de la luminosité absolue et de la section efficace totale proton-proton dans l'expérience ATLAS au LHC
Le Large Hadron Collider (LHC) au CERN a Geneve delivrera bientot des collisions avec une energie jamais atteinte jusqu'alors dans un accelerateur de particules. Une energie dans le centre de masse entre 10 et 14 TeV permettra de depasser les frontieres de la physique actuelle. Le detecteur ATLAS fera la chasse au boson de Higgs et recherchera une nouvelle physique au dela du modele standard. Tout processus physique est decrit par sa section section efficace. Les detecteurs positionnes aux differents points de collision du LHC determinerons les taux de comptage associes aux divers processus. Cependant, pour en deduire la section efficace associee, il faut connaitre la luminosite. Pour l'experience ATLAS, une mesure relative de la luminosite peut etre fournie par quelques uns de ses sous-detecteurs. Cependant, pour calibrer ces detecteurs, une mesure absolue doit etre effectuee. Le detecteur ALFA a ete concu pour mesurer le spectre de diffusion elastique qui permettra de determiner la luminosite absolue et par la meme occasion, la section efficace totale proton-proton fournissant ainsi un outils de calibration tres precis, de l'ordre du %. Ces detecteurs, installes a 240 m de part et d'autre du point d'interaction sont appeles pots romains. Il s'agit d'un systeme mecanique permettant d'approcher un trajectographe a fibres scintillantes a une distance de l'ordre du millimetre du cœur du faisceau. La simulation de la mesure necessite l'utilisation d'un logiciel de transport de particules chargees. Ce logiciel doit etre soigneusement choisi car il sert a la determination des protons perdus dans la sequence de l'accelerateur, entre le point d'interaction jusqu'aux detecteurs. L'impact des incertitudes systematiques qui affectent la mesure de la luminosite et de la section efficace totale est egalement determine en utilisant la simulation. Les detecteur ALFA opere dans un environnement complexe et en consequence sa conception requiert une grande attention. Une large campagne de tests sur l'electronique front-end a ete effectuee. L'analyses des donnees resultant de ces tests a permis de demontrer que toutes les exigences etaient remplies. A chaque avancement majeur dans la conception du detecteur, celui-ci doit etre soumis a des tests en faisceau. Durant ces periodes, tous les aspects du detecteur sont etudies. L'algorithme de reconstruction des traces, les methodes pour extraire des donnees l'efficacite de detection ou encore le niveau de diaphonie sont autant de parametres qu'il a fallu determiner. Les conclusions de ces tests ont permis de valider les differents choix techniques effectues permettant ainsi le lancement de la fabrication en serie des huit detecteurs utiles a la mesure. L'installation prevue courant 2011 permettra de mesurer la luminosite et la section efficace totale proton-proton courant 2012.
122377679
Development of a detector (ALFA) to measure the absolute LHC luminosity at ATLAS
The ATLAS collaboration plans to determine the absolute luminosity of the CERN LHC at Interaction Point 1 by measuring the trajectory of protons elastically scattered at very small angles (μrad). A scintillating fibre tracker system called ALFA (Absolute Luminosity For ATLAS) is proposed for this measurement. Detector modules will be placed above and below the LHC beam axis in roman pot units at a distance of 240 m on cach side of the ATLAS interaction point. They allow the detectors to approach the beam axis to millimeter distance. Overlap detectors also based on the scintillating fibre technology, will measure the precise relative position of the two detector modules, Results obtained during beam tests at DESY and at CERN validate the detectors design and demonstrate the achievable resolution. We also report about radiation hardness studies of the scintillating fibres to estimate the lifetime of the ALFA system at different operating conditions of the LHC.