Initial conditions for hydrodynamics: implications for phenomenology

We present a description of the early-stage dynamics in relativistic heavy ion collisions, where the free-streaming (FS) of partons is followed by a sudden equilibration (SE) to a thermalized phase, providing initial conditions for the subsequent hydrodynamic evolution. This FS+SE approximation has been proposed by Kolb, Sollfrank, and Heinz [1]. It has been further discussed in the context of the isotropization problem by Jas and Mrówczyński [2], as well as used to analyze the early development of collective flow by Sinyukov, Gyulassy, Karpenko, and Nazarenko [3, 4, 5]. In this talk we point out the emergence of the initial azimuthally asymmetric flow from FS+SE. The cartoon of the approach is given in Fig. 1. Rather than assuming a gradual transition from an inequilibrated partonic stage to a thermalized system (top panel), in FS+SE one approximates this early stage of evolution with free streaming followed by a sudden equilibration (bottom panel). Note that this is analogous to the standard treatment of the freeze-out at the end of the hydrodynamic evolution, where the continuous decoupling of