27 Chapter 2: Motivation Figure 2.5: An illustration of A + B → h + X in a heavy ion collision. We assume factorization still holds such that the main alteration to p + p collisions, Eq. (2.2), is the addition of a theoretically calculable P ( ϵ ) encapsulating the in-medium energy loss. and the angle with respect to the reaction plane φ . Older, lower statistics data is sometimes reported as R$_{AA}$ ( N$_{part}$ ), although [191] demonstrated its limited utility. R$_{AA}$ is a nice quantity to work with as it displays the effect of nuclear col- lisions at a glance: R$_{AA}$ = 1 means no modification from trivially scaled p + p collisions, R$_{AA}$ > 1 means enhancement, and R$_{AA}$ < 1 means suppression. For this reason, R$_{AA}$ is known as the nuclear modification factor. When the partonic production spectra are power laws of approximately constant power, which is the case at Rhic and $_{Lhc}$, R$_{AA}$ is approximately given by an espe-