[Paper Review] Jet quenching effects on the direct, elliptic, and triangular flow at RHIC

R. P. G. Andrade, Jorge Noronha+1 more|arXiv (Cornell University)|Mar 7, 2014

Magnetic confinement fusion research3 citations

TL;DR

This study investigates how energy and momentum deposition from partonic dijets into the quark-gluon plasma (QGP) at RHIC affects direct, elliptic, and triangular flow in central Au+Au collisions. Using event-by-event ideal hydrodynamics with boost-invariant, external source terms modeling dijet energy loss, it finds that dijet-induced anisotropic flow becomes significant at intermediate $p_T$ ($1 < p_T < 3$ GeV) when more than 12 GeV is deposited, weakening the geometric correlation of flow coefficients and enabling direct flow ($v_1$) to be probed via dihadron correlations in jet-enriched events.

ABSTRACT

In this paper we investigate how the energy and momentum deposited by partonic dijets in the quark-gluon plasma may affect the direct, elliptic and triangular flow of low (and intermediate) $p_T$ hadrons in central Au+Au collisions at RHIC. The dijets are modeled as external sources in the energy-momentum conservation equations for hydrodynamics, which are solved on an event-by-event basis within the ideal fluid approximation. We focus our investigation at mid-rapidity and solve the hydrodynamic equations imposing boost invariance. Differential anisotropic flow coefficients for $p_T \gtrsim 1$ GeV are found to be significantly enhanced if the dijets deposit on average more than 12 GeV in the QGP (or more than 6 GeV per jet). Because this jet-induced extra anisotropic flow is not related to the fluctuations of the initial geometry of the collision, the correlation between the $v_2$ and $v_3$ coefficients and their corresponding eccentricities is considerably weakened. In addition, we argue that the extra amount of direct flow induced by dijets may be quantified by comparing the azimuthal dependence of dihadron correlations in dijet events with the corresponding quantity obtained in events without dijets. This comparison could be used to give a rough estimate of the magnitude of the effective coupling between the jets and the medium.

Motivation & Objective

• To investigate how energy and momentum deposition from dijets in the QGP modifies direct, elliptic, and triangular flow of low- and intermediate-$p_T$ hadrons.
• To assess the extent to which jet-induced anisotropic flow decouples from initial-state geometry, weakening the correlation between flow coefficients and eccentricities.
• To propose a method for estimating the effective jet-medium coupling strength using dihadron angular correlation functions in jet-enriched vs. jet-free event ensembles.
• To explore the detectability of jet-induced direct flow ($v_1$) through azimuthal correlations in dijet events.

Proposed method

• Model dijets as external 4-current sources $J^ u$ in the ideal hydrodynamic energy-momentum conservation equation $D_ u T^{ ueta} = J^ u$.
• Solve the hydrodynamic equations event-by-event under boost-invariant and transverse expansion assumptions, using a parametrized energy loss rate $dE/dl|_0$.
• Simulate central Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV in the $(0-5)\%$ centrality window with fluctuating initial conditions.
• Compute anisotropic flow coefficients $v_n$ and flow angles $\Psi_n$ using the event plane method and compare results with and without dijets.
• Analyze dihadron angular correlation functions $C(\Delta\phi)$ and background-subtracted $R(\Delta\phi)$ to isolate jet-induced flow effects.
• Use the phase difference $\Delta_1 = \Psi_1^t - \Psi_1^a$ to interpret the sign and structure of direct flow in correlation functions.

Experimental results

Research questions

• RQ1 How does dijet energy deposition in the QGP modify the direct, elliptic, and triangular flow of low- and intermediate-$p_T$ hadrons in central Au+Au collisions at RHIC?
• RQ2 To what extent does jet-induced anisotropic flow decouple from the initial geometry of the collision, weakening the correlation between $v_n$ and the corresponding eccentricity $\varepsilon_{m,n}$?
• RQ3 Can the dihadron angular correlation function be used to extract a rough estimate of the effective jet-medium coupling strength?
• RQ4 What is the role of direct flow ($v_1$) in dihadron correlations, and how is it modulated by the transverse momentum of associated particles?

Key findings

• Dijet-induced anisotropic flow becomes significant only at intermediate $p_T$ ($1 < p_T < 3$ GeV), with $v_2$ and $v_3$ coefficients substantially enhanced when more than 12 GeV of energy is deposited in the QGP.
• The correlation between $v_2$ and $v_3$ and their corresponding initial eccentricities $\varepsilon_{2}$ and $\varepsilon_{3}$ is significantly weakened due to jet-induced flow being independent of initial geometry.
• Direct flow ($v_1$) induced by dijets is clearly observable in the dihadron correlation function, especially in the jet-enriched ensemble, with the sign of the coefficient $c_1$ depending on the $p_T$ range of the associated particles.
• For low-$p_T^a$ associated particles ($p_T^a < 1$ GeV), $\left<\cos\Delta_1\right> < 0$, leading to enhanced away-side peaks ($c_1 < 0$), while for intermediate $p_T^a$ ($2 < p_T^a < 3$ GeV), $\left<\cos\Delta_1\right> > 0$, enhancing the near-side peak ($c_1 > 0$).
• The $v_1$ coefficient is positive at low $p_T$ and negative at intermediate and high $p_T$, consistent with the observed modulation in dihadron correlations.
• Comparing dihadron correlations in jet-enriched and jet-free ensembles provides a viable method to estimate the magnitude of the effective jet-medium coupling, particularly through the behavior of $v_1$.