Robustness and Infrared Sensitivity of Various Observables in the   Application of AdS/CFT to Heavy Ion Collisions
  We investigate the robustness with respect to nonconformality of five properties of strongly coupled plasmas that have been calculated in N=4 supersymmetric Yang-Mills (SYM) theory at nonzero temperature, motivated by the goal of understanding phenomena in heavy ion collisions. (The properties are the jet quenching parameter, the velocity dependence of screening, and the drag and transverse and longitudinal momentum diffusion coefficients for a heavy quark pulled through the plasma.) We use a toy model in which nonconformality is introduced via a one-parameter deformation of the AdS black hole dual to the hot N=4 SYM plasma. Upon introducing a degree of nonconformality comparable to that seen in lattice calculations of QCD at temperatures a few times that of the crossover to quark-gluon plasma, we find that the jet quenching parameter is affected by the nonconformality by at most 30%, the screening length is affected by at most 20%, but the drag and diffusion coefficients for a slowly moving heavy quark can be modified by as much as 80%. However, four of the five properties that we investigate become completely insensitive to the nonconformality in the limit v -> 1. The exception is the jet quenching parameter, which is "infrared sensitive" even at v=1, where it is defined. It is the only high-velocity observable that we investigate which is sensitive to properties of the medium at infrared energy scales proportional to T, namely the scales where the quark-gluon plasma of QCD can be strongly coupled. The other four quantities all probe only scales that are larger than T by a factor that diverges as v -> 1, namely scales where the N=4 SYM plasma can be strongly coupled but the quark-gluon plasma of QCD is not. 
Hong Liu, Krishna Rajagopal, Yeming Shi