23 Chapter 2: Motivation proach to jet suppression that the large magnitude of the observed anisotropy at intermediate- p$_{T}$ is a result of deconfinement physics. 2.2.1 Factorization in p + p and A + A Factorization provides the theoretical framework within which pQCD calcula- tions are made and states that reactions with large momentum transfer can be factorized separately into long distance and short distance pieces [153–160]. There are two crucial components to factorization: (1) the (presently) incal- culable nonperturbative low momentum contributions are universal; i.e. they are the same for any QCD process, and (2) asymptotic freedom guarantees that the high momentum contribution can be reliably found using pertubative methods in QCD. A factorization scale, µ$_{F}$ , is introduced separating the short and long distance contributions; it is of order the hard scale of the problem, but is not specified further from within the theory. Following [150] the cross section for producing a hadron of type h in a p + p collision is then where the f s are parton distribution functions (PDFs), D h c is a fragmenta- tion function (FF), and d ˆ σ c ab is the perturbative partonic cross section. Using standard notation the partonic variables are in lower case and the hadronic ones are in upper case. In this way a PDF gives the probability of finding dσ h = ∑ a,b,c ∫ dx$_{a}$dx$_{b}$dz$_{c}$f$_{a/p}$ ( x$_{a}$, µ$_{F}$ ) f$_{b/p}$ ( x$_{b}$, µ$_{F}$ ) D h $_{c}$( z$_{c}$, µ ′ $_{F}$) × d ˆ σ c $_{ab}$( x$_{a}$P$_{A}$, x$_{b}$P$_{B}$, P$_{h}$/z$_{c}$, µ$_{R}$, µ$_{F}$ , µ ′ $_{F}$) , (2.2)