Nuclear Physics Seminar

Abstract: A crucial question in understanding the onset of hydrodynamic behavior in the quark–gluon plasma is whether hydrodynamics is "unreasonably effective" in describing systems far-from-equilibrium. This is motivated by the observation that many models of expanding systems exhibit an apparent simplification in their description while gradients are large and hydrodynamics is not expected to apply. We provide a new conceptual approach to understanding this simplification based on far-from-equilibrium slow modes. For a class of collision integrals in kinetic theory, these slow modes can be explicitly identified as the instantaneous ground states of an effective Hamiltonian describing the evolution of moments of the distribution function. They are qualitatively distinct from hydrodynamic modes when the system is far from equilibrium, suggesting that the simplification is not directly related to the onset of hydrodynamics. Importantly, these slow modes dominate the evolution when gradients are small compared to the energy gap, which occurs both at early times and in the hydrodynamic limit.

Based on 1910.00021 and work in preparation with Weiyao Ke, Li Yan, and Yi Yin