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heavy ion collisions is under theoretical control. 

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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 

