Equilibration of the gluon-minijet plasma at RHIC and LHC

Abstract

We study the production and equilibration of the gluon-minijet plasma expected to be formed in the central region of ultrarelativistic heavy-ion collisions at the BNL-RHIC and the CERN-LHC by solving a self-consistent relativistic transport equation. We compute the minijet production within perturbative QCD. Subsequent collisions among the semi-hard partons are treated by considering the elastic gg → gg processes with screening of the long wavelength modes taken into account. We determine the time τ eq where close to ideal hydrodynamic flow sets in, and find rather similar numbers for central heavy-ion collisions at BNL-RHIC and CERN-LHC energies. The number densities, energy densities and temperatures of the minijet plasma are found to be different at RHIC and LHC, e.g. T (τ eq ) ∼ 220 MeV and 380 MeV, respectively.

Infrared behavior of the gluon propagator in nonequilibrium situations

“…This study also plays a crucial role in determining many of the potential signatures for quark-gluon plasma detection at RHIC. The space-time evolution of the parton gas for this non-equilibrium situation can be be modeled by solving semi classical relativistic transport equations [2,3,4,5,6,7,8,9,10]. Central to solving the transport equations is what goes into the scattering kernels.…”

Partonic scattering cross sections in the QCD medium

“…The relativistic heavy-ion colliders RHIC (Au-Au collisions at √ s = 200 A GeV ) and LHC (Pb-Pb collisions at √ s = 5.5 A T eV ) offer the unique opportunity to study the production of this state of matter in the laboratory [1]. Perturbative QCD (pQCD) estimates that the energy density of jets and minijets produced in these collisions could be larger than about 50 and 1000 GeV /f m 3 at RHIC and LHC respectively [4,5]. However, even if the attained energy density is sufficient to form a quark-gluon plasma, it is not at all clear whether the parton medium will equilibrate and form a thermalized quark-gluon plasma before hadronization.…”

“…The first stage starts just after the initial nuclear collisions, where many hard, semihard, and soft partons are produced. The hard and semihard partons (jets and minijets) formed at RHIC and LHC can be desribed by using pQCD [4,5,12]), and soft gluons may be described by the creation of a coherent chromofield [13][14][15][16][17]. For the sake of simplicity, we restrict our investigation to jets and minijets only.…”

“…u µ represents the flow velocity. The time dependent relaxation time τ c (τ ) can be determined from the transport cross section (σ tr ) and the number density (n) of the plasma via the relation [4,20,22]:…”

“…(27)). In this figure, we use the values of s and α which correspond to a "realistic" situation at RHIC [4]. It can be seen that for the values of s and α considered here, the difference in the two cross sections increases for decreasing values of s. In Fig.3, we plot the transport cross sections for the two cases described above (Eq.…”