Thermalization and the chromo-Weibel instability

Despite the apparent success of ideal hydrodynamics in describing the elliptic flow data which have been produced at Brookhaven National Lab's Relativistic Heavy Ion Collider, one lingering question remains: is the use of ideal hydrodynamics at times t<1 fm/c justified? In order to justify its use, a method for rapidly producing isotropic thermal matter at RHIC energies is required. One of the chief obstacles to rapid isotropization/thermalization is the rapid longitudinal expansion of the matter during the earliest times after the initial nuclear impact. As a result of this expansion, the parton distribution functions become locally anisotropic in momentum space. In contrast to locally isotropic plasmas, anisotropic plasmas have a spectrum of soft unstable modes which are characterized by an exponential growth of transverse chromo-magnetic/-electric fields at short times. This instability is the QCD analogue of the Weibel instability of QED. Parametrically, the chromo-Weibel instability provides the fastest method for generation of soft background fields and dominates the short-time dynamics of the system.