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 ($R_{AA}$) for charged hadrons, neutral pions ($\pi^0$) and non-photonic electrons. The flat distribution of $R_{AA}$ at high transverse momentum ($p_{T}$) for a given collision centrality is consistent with a scenario where parton energy loss $\Delta p_{T}$ is proportional to $p_{T}$. 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.