Matching the Nonequilibrium Initial Stage of Heavy Ion Collisions to Hydrodynamics with QCD Kinetic Theory
Aleksi Kurkela (Theoretical Physics Department, CERN, Geneva, Switzerland and Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway); Aleksas Mazeliauskas (Institut für Theoretische Physik, Universität Heidelberg, 69120 Heidelberg, Germany and Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA); Jean-François Paquet (Department of Physics, Duke University, Durham, North Carolina 27708, USA and Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA); Sören Schlichting (Fakultät für Physik, Universität Bielefeld, D-33615 Bielefeld, Germany and Department of Physics, University of Washington, Seattle, Washington 98195-1560, USA); Derek Teaney (Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA)
High-energy nuclear collisions produce a nonequilibrium plasma of quarks and gluons which thermalizes and exhibits hydrodynamic flow. There are currently no practical frameworks to connect the early particle production in classical field simulations to the subsequent hydrodynamic evolution. We build such a framework using nonequilibrium Green’s functions, calculated in QCD kinetic theory, to propagate the initial energy-momentum tensor to the hydrodynamic phase. We demonstrate that this approach can be easily incorporated into existing hydrodynamic simulations, leading to stronger constraints on the energy density at early times and the transport properties of the QCD medium. Based on (conformal) scaling properties of the Green’s functions, we further obtain pragmatic bounds for the applicability of hydrodynamics in nuclear collisions.