On jet quenching parameters in strongly coupled non-conformal gauge theories

TL;DR

The paper extends the holographic computation of the jet quenching parameter $\hat{q}$ to strongly coupled non-conformal plasmas by studying a cascading gauge theory and its gravity dual. It shows that $\hat{q}$ is not universal across gauge theories and that its value increases with the effective adjoint degrees of freedom, approaching the conformal ${\cal N}=4$ result as temperature rises. The authors derive a quantitative expression for the cascade-to-conformal ratio $\rho(T)$, with $\rho(T)=1+\frac{P^2}{K_*}\chi+\cdots$ and $\chi\approx -1.388$, indicating a gradual approach to conformality with increasing $T$. They propose a phenomenological link to QCD via the speed of sound and offer a framework to translate holographic jet quenching predictions to QCD-like plasmas, highlighting the potential model-dependence of the coefficient $\chi$. Overall, the work clarifies non-universality in $\hat{q}$ and provides a pathway to connect holographic results with real-world QCD phenomenology.

Abstract

Recently Liu, Rajagopal and Wiedemann (LRW) [hep-ph/0605178] proposed a first principle, nonperturbative quantum field theoretic definition of ``jet quenching parameter'' \hat{q} used in models of medium-induced radiative parton energy loss in nucleus-nucleus collisions at RHIC. Relating \hat{q} to a short-distance behavior of a certain light-like Wilson loop, they used gauge theory-string theory correspondence to evaluate \hat{q} for the strongly coupled N=4 SU(N_c) gauge theory plasma. We generalize analysis of LRW to strongly coupled non-conformal gauge theory plasma. We find that a jet quenching parameter is gauge theory specific (not universal). Furthermore, it appears it's value increases as the number of effective adjoint degrees of freedom of a gauge theory plasma increases.