  The PHENIX experiment has established jet quenching at a center-of-mass
energy of 200 GeV in Au+Au collisions. Recent measurements of Cu+Cu collisions
at the same center-of-mass energy support a parton energy loss scenario.
Furthermore, the onset of jet quenching was studied in Cu+Cu collisions for
three center-of-mass energies 22.4, 62.4 and 200 GeV.
