Pre-equilibrium evolution effects on heavy-ion collision observables

To investigate the importance of pre-equilibrium dynamics on relativistic heavy-ion collision observables, we match a highly nonequilibrium early evolution stage, modeled by free-streaming partons generated from the Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) and Monte Carlo Glauber (MC-Glb) models, to a locally approximately thermalized later evolution stage described by viscous hydrodynamics and study the dependence of final hadronic transverse momentum distributions, in particular their underlying radial and anisotropic flows, on the switching time between these stages. Performing a three-parameter fit of the measured values for the average transverse momenta (p perpendicular to) for pions, kaons, and protons, as well as the elliptic and triangular flows of charged hadrons upsilon(ch)(2,3), with the switching time tau(s), the specific shear viscosity eta/s during the hydrodynamic stage, and the kinetic decoupling temperature T-dec as free parameters, we find that the preferred "thermalization" times tau(s) depend strongly on the model of the initial conditions. MC-KLN initial conditions require an earlier transition to hydrodynamic behavior (at tau(s) approximate to 0.13 fm/c), followed by hydrodynamic evolution with a larger specific shear viscosity eta/s approximate to 0.2, than MC-Glb initial conditions, which prefer switching at a later time (tau(s) approximate to 0.6 fm/c) followed by a less viscous hydrodynamic evolution with eta/s approximate to 0.16. These new results including pre-equilibrium evolution are compared to fits without a pre-equilibrium stage where all dynamic evolution before the onset of hydrodynamic behavior is ignored. In each case, the quality of the dynamical descriptions for the optimized parameter sets, as well as the observables which show the strongest constraining power for the thermalization time, are discussed.