Drag Force, Jet Quenching, and AdS/QCD

TL;DR

The work applies a deformed AdS$_5$ (AdS/QCD) background with a nontrivial dilaton to study transport in quark-gluon plasma, focusing on the drag force and jet-quenching parameter. By modeling a heavy quark as a trailing string and using a Wilson-loop computation, the authors derive a velocity- and temperature-dependent drag coefficient $\mu(v,T)$ and a holographic expression for the jet-quenching parameter $\hat{q}$ that incorporates non-conformal effects through the dilaton. They find that the dilaton deformation weakens the static quark free-energy temperature dependence, increases damping relative to the conformal case, and modifies $\hat{q}$ in a way that can be compared to RHIC phenomenology, while noting caveats about the physical interpretation of some string configurations. Overall, the results illustrate how non-conformal holographic backgrounds can influence transport observables and potentially improve agreement with heavy-ion collision data.

Abstract

In this note, two important transport observables in the RHIC experiment, relaxation time constant and jet quenching parameter, are calculated from an AdS/QCD model. A quark moving in the viscous medium such as the Quark-Gluon-Plasma is modelled by an open string whose end point travels on the boundary of a deformed AdS_5 black hole. The correction introduced via the deformed AdS_5 is believed to help us better understand the data which is expected to be measured in the RHIC.

Figures (4)

• Figure 1: Plot of red shift v.s. energy scale $(1/\pi z)$. At a temperature of $200$ MeV introduced by black hole (BH), we plot ${\cal V}(z)f(z)$ for both AdS/CFT and AdS/QCD models. For comparison, we also plot those at zero temperature. Notice their differences in the low energy scale.
• Figure 2: Regulated free energy of charm quark is identified as quark mass $m_c$ in thermal background. Position of flavor brane (relative to the horizon) in each model is adjusted to match experimental data $m_c\simeq 1.4$GeV at $T=318$MeV. $\lambda=10$ is chosen for the CFT model.
• Figure 3: Momentum $p$ v.s. time $t$. Simulation is done with charm quark mass $m_c=1.35$ GeV/c at temperature $200$ MeV, where the initial quark momentum is estimated to be $p_0=10$ GeV/c. The dashed curve is our result for AdS/QCD and the solid one is for usual AdS/CFT with $\lambda=10$.
• Figure 4: Jet quenching parameter. Solid curve is our result for AdS/QCD model with $c=0.23$ GeV$^{2}$ and $R^2/\alpha'=3.63$, and dashed one is for usual AdS/CFT model with $\lambda=6\pi$. Straight dotted line is an evolution-time averaged value of $\hat{q}$ from RHIC data with a range $5$-$15$ GeV$^2$/fm.