pattern of $\pi^0$ discovered by PHENIX  at RHIC compared to previous observation of high $p_T$ enhancement at ISR and SPS energies. The nuclear modification factor $R_{AA}= dN_{AA}/T_{AA}(b)d \sigma_{pp}$ measures the deviation of $AA$ spectra from factorized pQCD. Right shows predictions  of the $\sqrt{s}$ and $p_T$ dependence from SPS, RHIC, LHC based on the GLV theory  of radiative energy loss.[]{data-label="line2fig"}](RAA_RHIC_ISR_SPS_EnterriaKemer.eps){height="0.3\textheight" width="47.00000%"}

Below RHIC energies, the initial state Cronin enhancement of moderately high $p_T$ tails was observed in central $Pb+Pb$ reactions at the SPS. At the ISR a reduced Cronin enhancement in $\alpha+\alpha$ reactions was seen. In contrast, at RHIC a large suppression, by a factor of 4-5, was discovered in central $Au+Au$ that extends beyond 10 GeV for $\pi^0$.

Jet quenching in $A+A$ was proposed in [@Gyulassy:1990bh; @Wang:1992xy] as a way to study the dense matter produced at RHIC energies. As noted before, the pQCD jet production rates finally become large enough to measure yields up to high $p_T > 10$ GeV. Order of magnitude suppression effects were predicted based on simple estimates of induced gluon radiative energy loss. Ordinary, elastic energy loss [@Bjorken:1982tu] was known by that time to be too small to lead to significant attenuation.

As reviewed in 