Elliptic Flow and Nuclear Modification Factor in Ultrarelativistic Heavy-Ion Collisions within a Partonic Transport Model

Abstract

The quark gluon plasma produced in ultra-relativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultra-relativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.In ultra-relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN a hot and dense medium is created that consists of quarks and gluons. Experimental data shows that this quark gluon plasma (QGP) possesses a strong collective behavior and that high-energy partons deposit a sizeable amount of their energy in this medium [1,2].The collective behavior is often quantified by the elliptic flow coefficient v 2 , which is the second harmonic of the Fourier decomposition of the azimuthal angle distribution of particle yields. Comparisons to hydrodynamic calculations reveal that the QGP behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio [3]. However, the microscopic reason for this small ratio is currently not understood.Experimental data of the nuclear modification factor R AA , which is defined as the yield in heavy-ion (A+A) collisions divided by the yield in proton-proton (p+p) collisions scaled with the number of binary collisions,and the momentum imbalance of fully reconstructed jets indicate that high-energy particles are quenched by the created medium and lose lots of their energy [1, 2]. Several calculations based on perturbative QCD (pQCD) energy loss in the QGP can describe the experimental data [4][5][6][7][8][9][10][11]. A simultaneous understanding of collective bulk phenomena and jet quenching on the microscopic level remains a challenge, although several partonic transport models [12][13][14][15][16][17] have been developed to address this issue. In this paper we will present new results on both observables obtained with the partonic transport model Boltzmann Approach to Multi-Parton Scatterings (BAMPS). Based on cross sections calculated in pQCD, soft and ha...

Constraints on the Jet-Medium Coupling from Measurements at RHIC and LHC

“…It includes a running coupling α S (E 2 ) = 1/(c + 9/4π log(E 2 /T 2 c )) with c = 1.05, the Polyakov-loop suppression of the color-electric scattering [15] via χ T = c q L + c g L 2 with pre-factors c q , c g for quarks and gluons, and the Polyakov loop L < 1 as parametrized from lattice QCD, as well as an enhancement of scattering due to the magnetic monopoles near the critical temperature [1,2,3,4] is compared to a pQCD-based energy loss dE/dτ = κ(E 2 , T )E 0 τ 1 e 3/4 ζ 0 v f , including jet-energy loss fluctuations (ζ 0 ) and transverse flow fields (v f ), for a jet-medium coupling that depends on the energy of the jet, the temperature of the medium, and non-equilibrium effects around the phase transition (κ = κ(E 2 , T )) applying the hydrodynamic backgrounds of VISH2+1 [6] (solid lines) and the parton cascade BAMPS [7] (dashed-dotted lines). For comparison, we include the results of CUJET3.0 [14] (dotted lines).…”

“…Fig. 1 demonstrates that there is a surprising similarity between the results that cannot be expected a priori given the fact that the two background media are so different: While the hydrodynamic description of VISH2+1 [6] assumes an equilibrated system, the parton cascade BAMPS [7] also includes non-equilibrium effects in the bulk medium evolution.…”

“…Below, we study the influence of the details of the jet-medium coupling and the medium background on the simultaneous description of the nuclear modification factor (R AA ) and the high-p T elliptic flow (v 2 ) measured at RHIC and LHC [1,2,3,4] for a radiative pQCD energy-loss ansatz [5]. We contrast media determined via the viscous hydrodynamic approach VISH2+1 [6] with the parton-cascade BAMPS [7] as well as a jet-medium coupling depending on the collision energy with a jet-medium coupling influenced by the energy of the jet, the temperature of the medium and non-equilibrium effects around the phase transition.…”

“…On the other hand, the jet-energy loss of the Hybrid AdS energy-loss ansatz [8] is based on falling strings [1,2,3,4] is compared to a pQCD-based energy loss dE/dτ = κ( √ s NN )E 0 τ 1 e 3/4 ζ 0 v f , including jet-energy loss fluctuations (ζ 0 ) and transverse flow fields (v f ), for a jet-medium coupling that depends on the collision energy (κ = κ( √ s NN )) applying the hydrodynamic backgrounds of VISH2+1 [6] (solid lines) and the parton cascade BAMPS [7] (dashed-dotted lines).…”

“…The jets are spread according to a transverse initial profile specified by the bulk flow fields given by the VISH2+1 and BAMPS backgrounds considered [6,7].…”