30 Chapter 2: Motivation heavy ion collisions is under theoretical control. (a) (b) Figure 2.7: (a) d + A results from STAR [218] show that high- p$_{T}$ suppression is not due to initial state effects; figure adapted from [86]. (b) R γ $_{AA}$( N$_{part}$ ) ∼ 1 extends the results of (a) to show that jet suppression is not due to initial state effects in full A + A collisions. NLO prompt photon predictions [219–222] compare well to the PHENIX data [223]; figure adapted from [223]. The high- p$_{T}$ conclusions based on the d + Au control-run were indepen- dently verified and extended by the measurement of direct photons [223]. As electromagnetic probes are only weakly coupled ( α$_{E&M}$ ≈ 1 / 137) to the medium they do not suffer appreciable final state energy loss, even in A + A collisions. Fig. 2.7 (b) compares trivially scaled pQCD predictions for direct photons from p + p reactions [219–222] to those measured in Au + Au . The striking consistency shows that even in A + A collisions the initial state pro- duction is well understood; the variation in nuclear PDFs from p + p to d + A to A + A do not affect high- p$_{T}$ observables much. Since the observed jet suppression cannot come from other sources, it must