How hot glue becomes a nearly perfect fluid: the problem of thermalization in ultra-relativistic heavy ion collisions

The hottest matter on earth is created when gluons (with a sprinkling of quark-antiquark pairs) are liberated in the collisions of ultra-relativistic heavy ions at the RHIC and LHC colliders. A fully ab initio understanding of how this strongly correlated gluon matter thermalizes and flows is lacking. We review progress towards solving this problem and discuss surprising recent numerical results-in particular the discovery of a non-thermal fixed point (typical of weak wave turbulence) in an expanding non-Abelian plasma [1]. Remarkably the self-similar behavior of this fluid is identical to those of over-occupied N component self-interacting scalar theories that model, for instance, the behavior of cold atomic gases [2]. We discuss possible insights into the hottest fluids produced on earth obtained from the coldest fluids--in particular, the possible formation of transient Bose-Einstein condensates. Our discussion is grounded in the empirical constraints from the heavy-ion experiments. As an example, we outline how rare high multiplicity proton-nucleus collisions further test the "unreasonable effectiveness of hydrodynamics" as a description of the tiny and ephemeral droplets of quark-gluon matter created at RHIC and the LHC [3]. References: [1] J. Berges, K. Boguslavski, S. Schlichting and R. Venugopalan,``Turbulent thermalization process in heavy-ion collisions at ultrarelativistic energies,’' Phys. Rev. D89, no. 7, 074011 (2014). [2] J. Berges, K. Boguslavski, S. Schlichting and R. Venugopalan, ``Universality far from equilibrium: From superfluid Bose gases to heavy-ion collisions,’' Phys. Rev. Lett. 114, no. 6, 061601 (2015); ibid., arXiv: 1508.03073 [3] B. Schenke and R. Venugopalan,``Collective effects in light–heavy ion collisions,’' Nucl. Phys. A 931, 1039 (2014) ; T. Lappi, B. Schenke, S. Schlichting and R. Venugopalan,``Tracing the origin of azimuthal gluon correlations in the color glass condensate,’' arXiv:1509.03499 [hep-ph].