content_image/319669.jpg content_image/319668.jpg
of energy loss P ( ϵ ), where p f T = (1 − ϵ ) p i $_{T}$. Although it is called a probability of energy loss there are processes—e.g. 3 → 2 detailed balance and rare elastic events—in which momentum is gained and ϵ < 0 ( ϵ is certainly restricted from above by 1; more on this later). Given this Eq. (2.2) becomes 

<img src='content_image/319669.jpg'>

where P ( ϵ ) in general depends on the position of hard parton production ⃗ x$_{0}$ , its direction of propagation φ , and the properties { p }$_{i}$ of the medium; we will often refer to P ( ϵ ; ⃗ x$_{0}$, φ, { p }$_{i}$ ) in shorthand as P ( ϵ ). See Fig. 2.5 for a schematic picture of Eq. (2.4). 

## 2.2.2 R$_{AA}$ and Final State Suppression 

A common means of reexpressing the spectrum of hadrons produced in a nu- clear collision A + B → h + X is to divide out by the trivially binary scaled proton-proton spectrum: 

<img src='content_image/319668.jpg'>

For symmetric collisions (such as Au + Au or P b + P b ) we will denote the above quantity as R$_{AA}$ . Often R$_{AA}$ is given as a function of transverse momenta p$_{T}$ 

