QCD and Gauge Theory Dynamics in the RHIC Era
Coordinators: Dmitri Kharzeev, Emil Mottola, Krishna Rajagopal
By spring 2002, we expect that the Relativistic Heavy Ion Collider(RHIC) at Brookhaven will be providing mature data, and the time will be right for a multi-faceted program to understand the implications of RHIC for QCD and of QCD for RHIC. RHIC offers the potential to yield a deeper understanding of new phases of QCD, of the spin of the proton, and of the dynamics of gauge theories more generally, particularly under extreme and nonequilibrium conditions. For this potential to be realized, an integrated theoretical approach along several directions is required.
Perturbative QCD and small-x physics are necessary ingredients for an understanding of the early stage of nuclear collisions and of various spin asymmetries; nonequilibrium field theory methods, both analytical and numerical, are required in this pre-thermalization stage as well. As the plasma approaches local equilibrium, QCD thermodynamics as studied perturbatively, by Euclidean lattice methods, and by appealing to universality and to models which capture some features of the QCD phase diagram, should become applicable. The final stages of a RHIC collision are best described in terms of hadrons; this guarantees that understanding RHIC phenomena requires input from phenomenological approaches which have been developed to understand the dynamics of, and observables in, lower energy heavy ion collisions. The experimental results from RHIC will require all of these ingredients, and hence, a more comprehensive, integrated theoretical framework firmly based on QCD for their interpretation than has been developed until now.
The renaissance of interest in the dynamics of gauge theories, which the advent of experimental data from RHIC can and should accelerate, provides a good example of recent developments which must become a part of this framework. There has been a growing recognition of the need to develop theoretical methods to study field theories under extreme nonequilibrium conditions, i.e. in real time. While their motivations have been as diverse as the intended applications, ranging from phase transitions in the early universe and electroweak baryogenesis to Bose condensation in gases and vortex dynamics in superconductors, the basic theoretical tools being developed in these apparently unrelated areas are precisely the same as those required to describe the quark-gluon plasma phase of QCD, which will be produced and probed at RHIC. In each of these cases one has to treat in a consistent theoretical framework such dynamical issues as soft collective excitations and the validity of the quasi-particle approximation, transport processes, quantum decoherence and damping effects, thermalization, the approach to equilibrium and the emergence of hydrodynamic behavior from microscopic physics. Many field theorists working on problems such as these, which are of direct relevance to RHIC physics have not been actively engaged in the phenomenology of heavy ion collisions; we expect this to change as RHIC comes on line, and would like to use this ITP program two years from now to serve as a catalyst for that change.
Expertise in nonequilibrium and gauge theory dynamics gleaned in other contexts could be of considerable value in understanding the implications of RHIC phenomena for QCD, but this can only happen if the more theoretically minded keepers of this expertise understand what the interesting phenomenological questions are, while at the same time conveying the power of newly developed methods to those who already have a good sense of the phenomenology. Thus, a major goal of our program will be to provide a sharp focus in the form of the concrete problems posed by the RHIC data for the best theoretical methods in gauge theories under extreme nonequilibrium conditions