Centrality evolution of $p_t$ and $y_t$ spectra from Au-Au collisions at $\sqrt{s_{NN}} = 200$ GeV

A two-component analysis of spectra to $p_t = 12$ GeV/c for identified pions and protons from 200 GeV Au-Au collisions is presented. The method is similar to an analysis of the $n_{ch}$ dependence of $p_t$ spectra from p-p collisions at 200 GeV, but applied to Au-Au centrality dependence. The soft-component reference is a L\'evy distribution on transverse mass $m_t$. The hard-component reference is a Gaussian on $y_t$ with exponential ($p_t$ power-law) tail. Deviations of data from the reference are described by hard-component ratio $r_{AA}$ which generalizes nuclear modification factor $R_{AA}$. The analysis suggests that centrality evolution of pion and proton spectra is dominated by changes in parton fragmentation. The structure of $r_{AA}$ suggests that parton energy loss produces a negative boost $\Delta y_t$ 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 $p/\pi$ 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.