Hadron production in heavy ion collisions: Fragmentation and recombination from a dense parton phase

We discuss hadron production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). We argue that hadrons at transverse momenta ${P}_{T}l5\phantom{\rule{0.3em}{0ex}}\text{GeV}$ are formed by recombination of partons from the dense parton phase created in central collisions at RHIC. We provide a theoretical description of the recombination process for ${P}_{T}g2\phantom{\rule{0.3em}{0ex}}\text{GeV}$. Below ${P}_{T}=2\phantom{\rule{0.3em}{0ex}}\text{GeV}$ our results smoothly match a purely statistical description. At high transverse momentum hadron production is well described in the language of perturbative QCD by the fragmentation of partons. We give numerical results for a variety of hadron spectra, ratios, and nuclear suppression factors. We also discuss the anisotropic flow ${v}_{2}$ and give results based on a flow in the parton phase. Our results are consistent with the existence of a parton phase at RHIC hadronizing at a temperature of $175\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and a radial flow velocity of $0.55c$.