### Does AdS/QCD really help us understand physics at RHIC ?

As the number of theoretical manuscripts applying AdS/CFT duality techniques to the physics of relativistic heavy ion collisions (and other QCD phenomena) proliferates, it is still unclear which aspects of the dual models in the strong ‘t Hooft coupling limit shed light on real physics governed by QCD. In a recently published manuscript

Csaki, Reece and Terning argue that the Kaluza-Klein (KK) modes characteristic of strongly coupled supersymmetric gauge theories with a gravity dual in AdS space render the dynamical response of the medium sensitive to ultra-short distance interactions in a way that differs fundamentally from confining, asymptotically free gauge theories like QCD. They also emphasize the gross different event shapes of energetic phenomena, which are generally jet-like in QCD, but generically isotropic in strongly coupled gauge theories with a known gravity dual. See also:

**AdS/CFT duality and hard processes**(VJ entry)**Partons and jets in a strongly-coupled plasma from AdS/CFT**

Chaki *et al.* argue that the problem does not lie in the limit of large number of colors, but in the limit of strong ‘t Hooft coupling λ, which corresponds to the classical limit of the dual gravity theory. They speculate that a more realistic phenomenology will require the incorporation of genuine stringy dynamics into the gravity dual. This requirement may make the dual theory as difficult to solve of the original gauge theory (QCD).

A related problem is that attempts to model heavy ion collisions as collisions between gravitational shock waves in the dual theory fail to reproduce a Bjorken-type final state characterized by approximate boost invariance at midrapidity, but rather lead to a Landau-like scenario with complete stopping of the incident energy and momentum:

**Modeling Heavy Ion Collisions in AdS/CFT****Asymmetric Collision of Two Shock Waves in AdS(5)****Off-center collisions in AdS_5 with applications to multiplicity estimates in heavy-ion collisions**

Whether the AdS/CFT dual models can be practically tweaked to simulate the weakly coupled color glass condensate initial condition thought to be relevant for heavy ion collisions or the perturbative phenomenology of jets in the thermal quark-gluon plasma, remains an open question.

Some interesting steps in the direction of understanding the thermalization process at strong coupling were recently made in four preprints:

**Horizon formation and far-from-equilibrium isotropization in supersymmetric Yang-Mills plasma****Boost invariant flow, black hole formation, and far-from-equilibrium dynamics in N = 4 supersymmetric Yang-Mills theory****Weak Field Black Hole Formation in Asymptotically AdS Spacetimes****Boost-invariant early time dynamics from AdS/CFT**

The authors of the first two of these manuscripts apply a new method, in which they inject energy in the 4-dimensional space-time by temporarily deforming its metric. This procedure models the energy deposition occurring during the first moments of a relativistic heavy ion collision. They then follow the evolution of the bulk AdS metric and analyze the formation of an event horizon, indicating thermalization. The authors of the third manuscript study the equilibration process via the formation of an event horizon due to the collapse of spherical shells of energy in the AdS space. The authors of the fourth manuscript extend their earlier work on the boost invariant hydrodynamic regime in the N=4 super Yang-Mill theory by extending their treatment to earlier times. They find that the approach to equilibrium and isotropy of the stress tensor is sensitive to the precise nature of the initial conditions, and there is no simple scaling solution for the pre-equilibrium period.