127 Chapter 5: pQCD vs. AdS/CFT Figure 5.2: R c $_{AA}$( p$_{T}$ ) and R b $_{AA}$( p$_{T}$ ) predicted for central Pb + Pb at$_{Lhc}$ comparing AdS/CFT Eq. (5.1) and pQCD using the WHDG model [152] convolving elastic and inelastic parton energy loss. Possible initial gluon rapidity densities at Lhc are given by dN$_{g}$/dy = 1750 , from a$_{Phobos}$ [87, 403] extrapolation, or dN$_{g}$/dy = 2900 , from the KLN model of the color glass condensate (CGC) [400]. The top two curves from pQCD increase with p$_{T}$ while the bottom two curves from AdS/CFT slowly decrease with p$_{T}$ . The AdS/CFT parameters here were found using the “obvious” prescription with α$_{SY M}$ = . 05 , τ$_{0}$ = 1 fm/c, giving D = 3 / 2 πT (abbreviated to D = 3 in the figure). Similar trends were seen for the other input parameter possibilities discussed in the text. The “(” and “]” denote momenta after which possible string theoretic corrections may need to be considered; the curves’ increasing transparency from “(” to “]” is meant to additionally emphasize this, see text. giving T ∗ ≃ T $^{QCD}$/ 3 1 / $^{4}$, and to fit the coupling λ = g 2 $_{SY M}$N$_{c}$ ≈ 5 . 5 in order to reproduce the static quark-antiquark forces calculated via lattice QCD. The string theoretic result for the diffusion coefficient used in the Langevin