  Measurements of the fractional momentum loss ($S_{\rm
loss}\equiv{\delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in
heavy ion collisions are presented. Using $\pi^0$ in Au$+$Au and Cu$+$Cu
collisions at $\sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX
experiment at the Relativistic Heavy Ion Collider and and charged hadrons in
Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron
Collider, we studied the scaling properties of $S_{\rm loss}$ as a function of
a number of variables: the number of participants, $N_{\rm part}$, the number
of quark participants, $N_{\rm qp}$, the charged-particle density, $dN_{\rm
ch}/d\eta$, and the Bjorken energy density times the equilibration time,
$\varepsilon_{\rm Bj}\tau_{0}$. We find that the $p_T$ where $S_{\rm loss}$ has
its maximum, varies both with centrality and collision energy. Above the
maximum, $S_{\rm loss}$ tends to follow a power-law function with all four
scaling variables. The data at $\sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for
sufficiently high particle densities, have a common scaling of $S_{\rm loss}$
with $dN_{\rm ch}/d\eta$ and $\varepsilon_{\rm Bj}\tau_{0}$, lending insight on
the physics of parton energy loss.
