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Waking the Colored Plasma

Jorg Ruppert, Berndt Muller

TL;DR

The paper investigates how a fast color charge perturbs a quark-gluon plasma by applying linear response theory to derive the medium's dielectric response via $\epsilon_L$ and $\epsilon_T$, and to compute the accompanying wake, induced densities, and energy loss. It contrasts a weakly coupled QGP described by HTL perturbation theory with a strongly coupled QGP modeled as a quantum liquid that can support a space-like plasmon branch, e.g., with $\omega_L=\sqrt{u^2 k^2+\omega_p^2}$ and $\epsilon_L=1+\frac{\omega_p^2/2}{u^2 k^2-\omega^2+\omega_p^2/2}$. The HTL regime yields only a screening color cloud with no Cherenkov or Mach-wave structure, whereas the sQGP scenario allows Mach cones with opening angle $\Delta\Phi=\arccos(u/v)$ when $v>u$, producing distinct wake patterns behind the jet. The study argues that observing (or constraining) such Mach-cone signatures in jet-associated particle distributions—as discussed in RHIC data—can discriminate between weakly and strongly coupled QGP and help pin down the plasmon dispersion of the medium.

Abstract

We calculate the wake induced in a hot, dense QCD medium by a fast parton in the framework of linear response theory. We discuss two different scenarios: a weakly coupled quark gluon plasma (pQGP) described by hard-thermal loop (HTL) perturbation theory and a strongly coupled QGP (sQGP), which had the properties of a quantum liquid.

Waking the Colored Plasma

TL;DR

The paper investigates how a fast color charge perturbs a quark-gluon plasma by applying linear response theory to derive the medium's dielectric response via and , and to compute the accompanying wake, induced densities, and energy loss. It contrasts a weakly coupled QGP described by HTL perturbation theory with a strongly coupled QGP modeled as a quantum liquid that can support a space-like plasmon branch, e.g., with and . The HTL regime yields only a screening color cloud with no Cherenkov or Mach-wave structure, whereas the sQGP scenario allows Mach cones with opening angle when , producing distinct wake patterns behind the jet. The study argues that observing (or constraining) such Mach-cone signatures in jet-associated particle distributions—as discussed in RHIC data—can discriminate between weakly and strongly coupled QGP and help pin down the plasmon dispersion of the medium.

Abstract

We calculate the wake induced in a hot, dense QCD medium by a fast parton in the framework of linear response theory. We discuss two different scenarios: a weakly coupled quark gluon plasma (pQGP) described by hard-thermal loop (HTL) perturbation theory and a strongly coupled QGP (sQGP), which had the properties of a quantum liquid.

Paper Structure

This paper contains 6 sections, 21 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Plasma physics
  3. Charge wake in a QGP in the high temperature approximation
  4. Charge wake induced in a strongly coupled QGP
  5. Observable consequences
  6. Conclusions

Figures (4)

  • Figure 1: Spatial distribution of the induced charge density from a jet with fixed color charge $q^a$ in a high temperature plasma where the HTL approximation is applicable. The lower plot shows equi-charge lines in the density distribution. The density profile is a cloud traveling with the color charged jet.
  • Figure 2: Spatial distribution of a induced charge density from a jet with high momentum and a fixed color charge $q^a$ that is traveling with $v=0.55c<u$. The lower plot shows equi-charge lines in the density distribution. The density profile is that of a cloud traveling with the color charged jet.
  • Figure 3: (a) Spatial distribution of the induced charge density from a jet with high momentum and fixed color charge $q^a$ that is traveling with $v=0.99c>u=\sqrt{1/3}c$. (b) Plot showing equi-charge lines in the density distribution for the situation in (a).
  • Figure 4: (a) Spatial distribution of the equi-value lines of the current densitie's component parallel to a jet travelling with $v=0.99c>u=\sqrt{1/3}c$ in a sQGP. (b) Spatial distribution of the equi-value lines of the current densitie's component perpendicular to a jet travelling with $v=0.99c>u=\sqrt{1/3}c$ in a sQGP.