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 √s_NN=200  GeV Au-Au collisions at BNL RHIC in terms of a single “jet quenching parameter” q̂. We observe that q̂ 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 q̂ in hot N=4 supersymmetric QCD, finding q̂_SYM=26.69√α_SYMN_cT³ in the limit in which both N_c and 4πα_SYMN_c are large. Thus, at strong coupling q̂ is not proportional to the entropy density s, or to some “number density of scatterers” since, unlike the number of degrees of freedom, q̂ does not grow like N_c².