25 Chapter 2: Motivation lations exist [150, 151]; [174, 175] give derivations to NLO for prompt photon production; fixed order next to leading log (FONLL) production spectra of heavy quarks can be found in [176, 177]. In heavy ion collisions Eq. (2.2) is altered in three significant ways. First, the geometry of collisions of large nuclei of mass number A is very different from p + p ; for a review of heavy ion collision geometry, see Appendix A. Large nuclei suffer a number of hard p + p -like reactions as they overlap, which is a function of the nuclei involved and the impact parameter b . Second, the PDFs used must be exchanged from nucleon to nuclear. To lowest order these new distributions should account for the isospin asymmetry of nuclei. In discussing these in detail beyond this simple correction it is useful to define the quantity where we have used the structure functions defined as the sum over all fla- vors of the PDF’s weighted by x and the quark charge squared, F$_{2}$ ( x, Q ) ≡ ∑ f xQ 2 $_{f}$f$_{f}$ ( x, Q ). In the shadowing region x ≲ . 1, R A a < 1 [178–181]. . 1 ≲ x ≲ . 3 is the anti-shadowing region in which R A a > 1 [182]. In the EMC . 3 ≲ x ≲ . 7 region R A a < 1 ([183] and references therein), and in the Fermi motion x ≳ . 7 region R A a > 1 [184–186]. [187–190] give some parameteriza- tions of nuclear structure functions. Finally, and most importantly for this thesis, as the high- p$_{T}$ parton travels through the fireball it interacts with col- ored scattering centers in the medium. This is encapsulated in a probability R A $_{a}$( x, Q ) ≡ F a/A 2 ( x, Q ) F a/p 2 ( x, Q ) , (2.3)