Calculating the Jet Quenching Parameter from AdS/CFT

TL;DR

This paper defines the jet quenching parameter $t$ in a model-independent way and calculates it at strong coupling using AdS/CFT, providing insights into the properties of quark-gluon plasma.

Contribution

It introduces a nonperturbative, quantum field theoretic definition of $t$ and computes its value in strongly coupled N=4 SYM using AdS/CFT correspondence.

Findings

t is not proportional to entropy density at strong coupling.

Calculated t in N=4 SYM: 26.69 _{SYM} N_c T^3.

t does not scale with N_c^2 like the number of degrees of freedom.

Abstract

Models of medium-induced radiative parton energy loss account for the strong suppression of high-pT hadron spectra in $\sqrt{s_{NN}}=200$ GeV Au-Au collisions at RHIC in terms of a single "jet quenching parameter'' $\hat q$. The available suite of jet quenching measurements make $\hat q$ one of the experimentally best constrained properties of the hot fluid produced in RHIC collisions. We observe that $\hat q$ can be given a model-independent, nonperturbative, quantum field theoretic definition in terms of the short-distance behavior of a particular light-like Wilson loop. We then use the AdS/CFT correspondence to obtain a strong-coupling calculation of $\hat q$ in hot N=4 supersymmetric QCD, finding $\hat{q}_{SYM} = 26.69 \sqrt{\alpha_{SYM} N_c} T^3$ in the limit in which both $N_c$ and $4\pi\alpha_{SYM} N_c$ are large. We thus learn that at strong coupling $\hat q$ is not proportional to the entropy density $s$, or to some "number density of scatterers'' since, unlike the number of degrees of freedom, $\hat q$ does not grow like $N_c^2$.