CENTRALITY EVOLUTION OF pt AND yt SPECTRA FROM Au–Au COLLISIONS AT $\sqrt{s_{NN}} = 200\, {\rm GeV}$

Abstract

A two-component analysis of spectra to pt = 12 GeV/c for identified pions and protons from 200 GeV Au – Au collisions is presented. The method is similar to an analysis of the nch dependence of pt spectra from p–p collisions at 200 GeV, but applied to Au – Au centrality dependence. The soft-component reference is a Lévy distribution on transverse mass mt. The hard-component reference is a Gaussian on transverse rapidity yt with exponential (pt power-law) tail. Deviations of data from the reference are described by hard-component ratio rAA, which generalizes nuclear modification factor RAA. The analysis suggests that centrality evolution of pion and proton spectra is dominated by changes in parton fragmentation. The structure of rAA suggests that parton energy loss produces a negative boost Δyt of a large fraction (but not all) of the minimum-bias fragment distribution, and that lower-energy partons suffer relatively less energy loss, possibly due to color screening. The analysis also suggests that the anomalous p/π ratio may be due to differences in the parton energy-loss process experienced by the two hadron species. This analysis provides no evidence for radial flow.