  Recent observation of high-$p_T$ hadron spectra suppression and mono-jet
production in central $Au+Au$ collisions and their absence in $d+Au$ collisions
at RHIC have confirmed the long predicted phenomenon of jet quenching in
high-energy heavy-ion collisions. Detailed analyses of the experimental data
also show parton energy loss as the mechanism for the discovered jet quenching.
Using a pQCD parton model that incorporates medium modified parton
fragmentation functions and comparing to experimental data from deeply
inelastic scattering off nuclei, one can conclude that the initial gluon
(energy) density of the hot matter produced in central $Au+Au$ collisions that
causes jet quenching at RHIC is about 30 (100) times higher than in a cold $Au$
nucleus. Combined with data on bulk and collective properties of the hot
matter, the observed jet quenching provides strong evidence for the formation
of a strongly interacting quark gluon plasma in central $Au+Au$ collisions at
RHIC.
