Calculating the jet quenching parameter

Models of medium-induced radiative parton energy loss account for the strong suppression of high-p(T) hadron spectra in root s(NN)=200 GeV Au-Au collisions at BNL RHIC in terms of a single jet quenching parameter q. We observe that q can be given a model-independent, nonperturbative, quantum field theoretic definition in terms of the short-distance behavior of a particular lightlike Wilson loop. We then use the anti-de Sitter/conformal-field-theory correspondence to obtain a strong-coupling calculation of q in hot N=4 supersymmetric QCD, finding q(SYM)=26.69 root alpha SYMNcT3 in the limit in which both N-c and 4 pi alpha N-SYM(c) are large. Thus, at strong coupling q is not proportional to the entropy density s, or to some number density of scatterers since, unlike the number of degrees of freedom, q does not grow like N-c(2).