This four-week program, to be held at the Institute for Nuclear Theory from August 3 to August 28, 2015, aims at discussing the general topic of equilibration in gauge field theories, with emphasis on the thermalization process of the quark gluon plasma created in heavy ion collision experiments at RHIC and the LHC. We would like to bring together theorists working on a broad range of approaches, ranging from weak coupling and lattice field theory techniques to the AdS/CFT conjecture and phenomenological heavy ion physics. In addition, we wish to emphasize connections with heavy ion experiments as well as other fields, such as post-inflationary cosmology, condensed matter theory, and cold atomic gas physics.

Background and context

During the past decade, the heavy ion programs at RHIC and the LHC have provided strong evidence for the creation of the deconfined phase of QCD, the quark-gluon plasma (QGP). These experimental efforts have been paralleled by the development of theoretical tools in order to describe the complicated dynamics of the matter formed in these collisions, characterized by the existence of several widely different momentum scales and a strong time dependence. Despite this complexity, it was realized from RHIC data (and confirmed at the LHC) that the bulk evolution of the system is very well described by almost ideal hydrodynamics, which suggests a nearly perfect fluid-like behavior with extremely small viscosities. In turn, this observation led to the conjecture that the QGP may be considered a strongly coupled liquid, and triggered an active development of novel field theory tools going beyond traditional weak coupling methods, including most prominently applications of the gauge/gravity duality.

In recent years, the theory of high energy heavy ion collisions has evolved towards a kind of "Standard Model of heavy ion physics", where each stage of the collision process is described via some type of effective theory or model. These include most importantly the Color Glass Condensate effective theory for the densely occupied initial state before the collision and the very early dynamics immediately after it, viscous hydrodynamics simulations for the bulk expansion of the nearly thermalized plasma, and the Hadron Resonance Gas model for the final hadronic stages and freeze-out of the system. However, the precise connections between these three different epochs are not yet under full theoretical control.

From the point of view of first principles theoretical calculations, one of the most fundamental caveats in the above picture is the evolution of the system from the initial glasma color fields to a strongly interacting QGP, which is amenable to a hydrodynamic description. The details of this "thermalization" process are still very much debated, and in fact it is not even clear whether the system truly thermalizes or whether one may have hydrodynamical behavior without thermalization. A lot of effort has subsequently gone into the description of this early dynamics, with interesting partial results emerging both from detailed weak coupling calculations, from the gauge/gravity duality, as well as from classical lattice field theory simulations.

Interestingly, these works on thermalization in heavy ion collisions are starting to influence also other fields. Particularly strong connections have emerged within the study of strongly interacting off-equilibrium systems, with holographic studies of turbulence as well as quantum quenches in low-dimensional condensed matter systems. Similarly, the imprint that initial state fluctuations may have on the final thermalized fields in heavy ion collisions has connections with post-inflationary cosmology, where similar problems are encountered at radically different physical scales.

Program details

The format of the program will consist of one "official" seminar every morning plus an informal group discussion session in the afternoon. For the morning seminars, we may direct the speakers to give a presentation aimed at a broader audience, while the more specialized and technical discussions will typically be held in the afternoons. We will not a priori enforce a specific topic to be discussed during each week. We rather expect this to be decided based on the list of participants present at a given time.

We foresee the following key questions to be among the ones addressed during the program:

• Thermalization at weak coupling: Relevance of different mechanisms of thermalization. Comparison between theory approaches.
  
  
• Thermalization at strong coupling: hydrodynamization vs thermalization. Comparison of different gauge/gravity duality approaches to off-equilibrium dynamics.
  
  
• How to bridge the gap between weak and strong coupling thermalization studies? Hybrid models of thermalization. Thermalization at realistic couplings.
  
  
• Jets as a nonequilibrium system: relationship between thermalization and jet quenching.
  
  
• Thermalization and heavy ion phenomenology: how sensitive are experimental measurements to the thermalization process?
  
  
• What can thermalization studies in heavy ion physics learn from or teach to other fields, such as condensed matter physics and cosmology?