Medium Density and Parton Energy Loss in Gold--Gold Collisions from the Recombination Model

The pion production at mid-rapidity in gold--gold collisions at √s=200GeV is investigated by the parton recombination model. The density of the hot medium produced in the collisions and the hard
parton energy loss effect are obtained by fitting the experimental data. Both of them are found to have power-law dependence on the number of binary collisions. The implication of such dependence on the nuclear modification factor is discussed.