Conical Flow induced by Quenched QCD Jets

TL;DR

This work proposes that energy lost by quenched QCD jets in a near-perfect sQGP excites a conical, Mach-like flow rather than mere heating. Using linearized relativistic hydrodynamics, it shows that a propagating sound mode yields a Mach cone with an opening angle determined by the time-weighted sound speed, giving about $70^\circ$ at RHIC and resulting in azimuthal correlations at $Δφ=π\pm1.2$ rad. The appearance of the cone depends on the initial deposition: a gradient-driven source excites the sound mode and reveals the cone in high-$p_t$ spectra, while a diffuson-dominated deposition can wash it out. Comparisons with STAR and PHENIX data on away-side correlations support the conical-flow interpretation, and the paper discusses alternative explanations and experimental tests to distinguish them.

Abstract

Quenching is a recently discovered phenomenon in which QCD jets created in heavy ion collisions deposit a large fraction or even all their energy and momentum into the produced matter. At RHIC and higher energies, where that matter is a strongly coupled Quark-Gluon Plasma (sQGP) with very small viscosity, we suggest that this energy/momentum propagate as a collective excitation or ``conical flow''. Similar hydrodynamical phenomena are well known, e.g. the so called sonic booms from supersonic planes. We solve the linearized relativistic hydrodynamic equations to detail the flow picture. We argue that for RHIC collisions the direction of this flow should make a cone at a specific large angle with the jet, of about $70^o$, and thus lead to peaks in particle correlations at the angle $Δφ=π\pm 1.2$ rad relative to the large-$p_t$ trigger. This angle happens to matchperfectly the position of the maximum in the angular distribution of secondaries associated with the trigger recently seen by the STAR and PHENIX collaborations. We also discuss briefly possible alternative explanations and suggest some further tests to clarify the mechanism.

Figures (3)

• Figure 1: A schematic picture of flow created by a jet going through the fireball. The trigger jet is going to the right from the origination point (the black circle at point B) from which sound waves start propagating as spherical waves (the dashed circle). The companion quenched jet is moving to the left, heating the matter and thus creating a cylinder of additional matter (shaded area). The head of the jet is a "nonhydrodynamical core" of the QCD gluonic shower, formed by the original hard parton (black dot A). The solid arrow shows a direction of flow normal to the shock cone at the angle $\theta_M$, the dashed arrows show the direction of the flow after the shocks hit the edge of the fireball.
• Figure 2: Velocity field $v_x$ created by a jet moving along the $\hat{x}$ direction ($c^2_s=1/3$, $\Gamma_s=1/(4\pi T)$, $\sigma=\Gamma_s$). The jet is assumed to disappear at $t=7$ fm while the spectrum calculated at $t=10$ fm. The two figures (a) and (b) are for scenarios 1 and 2, respectively. The values of the parameters are arbitrary. Note that in (a) matter moves preferentially along the $\hat{x}$ direction, while in (b) it is in the Mach direction (\ref{['eqn_Mach']}).
• Figure 3: The normalized spectrum of associated secondaries versus the azimuthal angle $\phi$. Three curves are for different $p_t$ at $y=0$ for $c^2_s=1/3$, $\Gamma_s=1/(4\pi T)$, $\sigma=\Gamma_s$. Note the different scales. The jet disappears completely at $t=7$ fm while the spectrum is calculated at $t=10$ fm. The two figures (a) and (b) are for scenarios 1 and 2, respectively.