Empirical constraints on parton energy loss in nucleus-nucleus collisions at RHIC

We present empirical features of parton energy loss in nucleus-nucleus collisions at RHIC through studies of the spectra and nuclear modification factors (RAA) for charged hadrons, neutral pions (π0) and non-photonic electrons. The flat distribution of RAA at high transverse momentum (pT) for a given collision centrality is consistent with a scenario where parton energy loss ΔpT is proportional to pT. The centrality dependence of the parton energy loss indicates the absence of path length dependence in the magnitude of energy loss. The lack of strong path length dependence suggests a dynamical picture where the dense partonic medium undergoes rapid expansion and the density of the medium falls rapidly in the first a few Fermi interval, which may be much shorter than the full path length. Implications of the empirical constraints on the parton energy loss will also be discussed