  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.
