This paper discusses possible phenomenological implications for p+A and A+A collisions of the results of recent numerical AdS/CFT calculations examining asymmetric collisions of planar shocks. In view of the extreme Lorentz contraction, we model highly relativistic heavy ion collisions (HICs) as a superposition of collisions between many near-independent transverse "pixels" with differing incident longitudinal momenta. It was found that also for asymmetric collisions the hydrodynamization time is in good approximation a proper time, just like for symmetric collisions, depending on the geometric mean of the longitudinally integrated energy densities of the incident projectiles. For realistic collisions with fluctuations in the initial energy densities, these results imply a substantial increase in the hydrodynamization time for highly asymmetric pixels. However, even in this case the local hydrodynamization time still is significantly smaller than perturbative results for the thermalization time.
We perform a numerical simulation of the evolution of inhomogeneities with transverse profile in a collision of gravitational shockwaves in asymptotically anti-de Sitter spacetime. This constitutes a step closer towards an accurate holographic description of the thermalization of a strongly coupled plasma, which can model the dynamics of heavy ion collisions. The results indicate that the considered inhomogeneities typically become hydrodynamical earlier or at the same moment when hydrodynamics applies to the background, even though they decay slowly.
Collisions of asymmetric planar shocks in maximally supersymmetric Yang-Mills theory are studied via their dual gravitational formulation in asymptotically anti-de Sitter spacetime. The post-collision hydrodynamic flow is found to be very well described by appropriate means of the results of symmetric shock collisions. This study extends, to asymmetric collisions, previous work of Chesler, Kilbertus, and van der Schee examining the special case of symmetric collisions. Given the universal description of hydrodynamic flow produced by asymmetric planar collisions one can model, quantitatively, n…
Within the AdS/CFT correspondence, the shockwave metric has been widely used as a gravity dual for a fast moving nucleus. Here we propose a picture for particle production in deep inelastic scattering off a shockwave with the projectile represented by the $\mathcal{R}$-current. By using the method developed in \cite{Avsar:2009xf} to find an explicit expression for the scattered field, we are able to show that the scattered field can be written in terms of time-like and space-like vacuum states where the time-like states are identified with the outgoing particles. To support this picture, we ca…
We reanalyze high energy QCD scattering regimes from scattering in cut-off AdS via gravity-gauge dualities (a la Polchinski-Strassler). We look at 't Hooft scattering, Regge behaviour and black hole creation in AdS. Black hole creation in the gravity dual is analyzed via gravitational shockwave collisions. We prove the saturation of the QCD Froissart unitarity bound, corresponding to the creation of black holes of AdS size, as suggested by Giddings.
In a previous paper, we have analyzed high energy QCD from AdS-CFT and proved the saturation of the Froissart bound (a purely QCD proof of which is still lacking). In this paper we describe the calculation in more physical terms and map it to QCD language. We find a remarkable agreement with the 1952 Heisenberg description of the saturation (pre-QCD!) in terms of shockwave collisions of pion field distributions. It provides a direct map between gauge theory physics and the gravitational physics on the IR brane of the Randall-Sundrum model. Saturation occurs through black hole production on the…
We relate bulk fields in Randall-Sundrum AdS_5 phenomenological models to the world-volume fields of probe D7 branes in the Klebanov-Witten background of type IIB string theory. The string constructions are described by AdS_5 X T^{1,1} in their near-horizon geometry, with T^{1,1} a 5d compact internal manifold that yields N=1 supersymmetry in the dual 4d gauge theory. The effective 5d Lagrangian description derived from the explicit string construction leads to additional features that are not usually encountered in phenomenological model building.
We construct a fully back-reacted holographic dual of a four-dimensional field theory which exhibits chiral symmetry breaking. Two possible models are considered by studying the effects of a five-dimensional field, dual to the $q\bar{q}$ operator. One model has smooth geometry at all radii and the other dynamically generates a cutoff at finite radius. Both of these models satisfy Einstein's field equations. The second model has only three free parameters, as in QCD, and we show that this gives phenomenologically consistent results. We also discuss the possibility that in order to obtain linear…
The analytical treatment of the nonperturbative QCD dynamics is one of main open questions of the strong interactions. Currently, it is only possible to get some qualitative information about this regime considering other QCD-like theories, as for example the N=4 super Yang-Mills (SYM), where one can perform calculations in the nonperturbative limit of large 't Hooft coupling using the Anti-de Sitter space/Conformal field theory (AdS/CFT). Recently, the high energy scattering amplitude was calculated in the AdS/CFT approach, applied to deep inelastic scattering (DIS) and confronted with the $F…
We give an example of modeling phenomenological heavy-quark potentials in a five-dimensional framework nowadays known as AdS/QCD. In particular we emphasize the absence of infrared renormalons.
We propose that the space-time evolution of strongly coupled matter formed by ultra-relativistic heavy ion collisions can be modelled by phenomenological equations involving the energy-momentum tensor and conserved currents alone. These equations can describe the late stage of local chemical and thermal equilibration of the matter formed after collisions, and its subsequent transition to hydrodynamic expansion in an unified framework. The full set of equations include local energy, momentum and charge conservation; but also additional equations for evolution of non-equilibrium variables. These…
We present a phenomenological unpolarized Parton Distribution Functions for diquarks based on a soft-wall light front AdS/QCD quark-diquark nucleon model. From a probed model consistent with the Drewll-Yan-West relation and quark counting rule, we have performed a fit of some free parameters using known phenomenological quark PDF data. The model considers the entire set of possible diquarks within the nucleon valence, in the present work we focus on the spin-0 $ud_0$, spin-1 $ud_1$ and spin-1 $uu_1$ diquarks into the valence of protons. The diquark PDFs obtained are able to used in proton-prot…
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.
We explore phenomenological signatures of light quark jet quenching within the AdS/CFT correspondence. Numerical studies of the instantaneous energy loss of light quarks, modeled as falling strings, suggest a linear path dependence. We propose a phenomenological model for the energy loss and use it to compute the nuclear modification factor $R_{AA}$ for light quarks in an expanding plasma with Glauber initial conditions. The results are compared to the light hadron $R_{AA}$ data at the LHC and, although they show qualitative agreement, the quantitative disagreement we found motivated the explo…
We propose a novel definition of a holographic light hadron jet and consider the phenomenological consequences, including the very first fully self-consistent, completely strong-coupling calculation of the jet nuclear modification factor $R_{AA}$, which we find compares surprisingly well with recent preliminary data from LHC. We show that the thermalization distance for light parton jets is an extremely sensitive function of the \emph{a priori} unspecified string initial conditions and that worldsheets corresponding to non-asymptotic energy jets are not well approximated by a collection of nul…
Experiments on high-energy heavy-ion collisions reveal the formation and some intriguing properties of the Quark-Gluon Plasma (QGP), a new phase of matter predicted by Quantum Chromodynamics (QCD), the quantum field theory of strong interactions. The phenomenological success of relativistic hydrodynamic simulations with remarkably weak shear viscosity, modeling QGP as an almost perfect fluid, are in favor of the occurrence of a strongly-coupled QGP expanding and cooling during the reaction. A derivation of these features in QCD at strong coupling is still lacking and represents a very intricat…
The AdS/QCD models are believed to interpolate between low and high energy sectors of QCD. This belief is usually based on observations that many phenomenologically reasonable predictions follow from bounds imposed at high energies although the hypothetical range of applicability of semiclassical bottom-up holographic models is restricted by the gauge/gravity duality to low energies where QCD is strongly coupled. For testing the feasibility of high energy constraints it is interesting to calculate holographically some observable constants at low and high momenta independently and compare. We c…
Bottom-up holographic models of QCD, inspired by the AdS/CFT correspondence, have shown a remarkable degree of phenomenological success. However, they rely on a number of bold assumptions. We investigate the reliability of one of the key assumptions, which involves matching the parameters of these models to QCD at high 4D momentum $q^2$ and renormalization scale $μ^2$. We show that this leads to phenomenological and theoretical inconsistencies for scale-dependent quantities such as $\bra \bar{q}q\ket$
To study drag force on a moving heavy quark through a plasma, we use a deformed AdS space-time, in which deformation parameter $c$ describes non-conformality in AdS/QCD. In this case the quark is mapped to a probe string in the AdS space. Considering probable contribution of deformation parameter in the probe string, we apply a general form of c-dependent string ansatz in the drag force computation. Then we find the acceptable value of this parameter as it satisfies QCD calculations. Using this result, we also discuss diffusion constant which is in agreement with phenomenological result for no…
We analyse the cosmology of a brane world model where a single brane carrying the standard model fields forms the boundary of a 5-dimensional AdS bulk (the Randall-Sundrum II scenario). We focus on the thermal radiation of bulk gravitons, the formation of the bulk black hole, and the holographic AdS/CFT definition of the RSII theory. Our detailed calculation of bulk radiation reduces previous estimates to a phenomenologically acceptable, although potentially visible level. In late cosmology, in which the Friedmann equation depends linearly on the energy density ρ, only about 0.5% of energy den…
The correspondence between theories in anti-de Sitter space and field theories in physical space-time leads to an analytic, semiclassical model for strongly-coupled QCD which has scale invariance at short distances and color confinement at large distances. These equations, for both mesons and baryons, give a very good representation of the observed hadronic spectrum, including a zero mass pion. Light-front holography allows hadronic amplitudes in the AdS fifth dimension to be mapped to frame-independent light-front wavefunctions of hadrons in physical space-time, thus providing a relativistic …
The phenomenological implications of allowing the Higgs to propagate in both AdS${}_5$ and a class of asymptotically AdS spaces are considered. Without tuning, the vacuum expectation value (VEV) of the Higgs is peaked towards the IR tip of the space and hence such a scenario still offers a potential resolution to the gauge-hierarchy problem. When the exponent of the Higgs VEV is approximately two and one assumes order one Yukawa couplings, then the fermion Dirac mass term is found to range from $\sim 10^{-5}$ eV to $\sim 200$ GeV in approximate agreement with the observed fermion masses. Howev…
