Figure. The < Nbinary> -scaled ratio of hadron yields in central Au+Au to peripheral Au+Au collisions, RCP, for two centrality classes as a function of pT. The curves are theoretical model comparisons.

The < N_binary> -scaled ratio of particle yields in central relative to peripheral collisions, R_CP is similar to R_AB and also exhibits large suppression of inclusive hadron production in central collisions. The observed factor of 5 suppression in central collisions was shown by these high precision measurements to be independent of pT for 6 < pT < 12 GeV/c.

We compared the STAR measurements to the results of three theoretical calculations: two models based on pQCD incorporating shadowing, the Cronin effect and jet quenching in dense matter (pQCD-I, pQCD-II) and a model incorporating gluon saturation in the incoming Au nuclei. The pQCD-based calculations contain one free parameter, the energy density for central collisions, which is fit to the data, yielding an initial density 30-50 times that of cold nuclear matter. These calculations then successfully describe the pT and centrality dependence of the inclusive suppression for pT > 5 GeV/c. The saturation model also describes the suppression and its pT-dependence for the 0-5%/40-60% ratio for pT > 5 GeV/c.

These precision STAR data on inclusive charged hadron suppression, covering wide centrality and kinematic range, are well-described by widely differing models which attribute the suppression either to jet quenching in dense matter in the final state or to gluon saturation in the initial state. The resolution of this ambiguity requires the isolation of initial state nuclear effects in d+Au collisions.