Thermalization of a color glass condensate and review of the ‘bottom-up’ scenario

Abstract

The thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within microscopical parton cascade BAMPS. Our main focus lies on the detailed comparison of thermalization, observed in BAMPS with that suggested in the "Bottom-Up" scenario. We demonstrate that the tremendous production of soft gluons via gg → ggg, which is shown in the "Bottom-Up" picture as the dominant process during the early preequilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction ggg → gg hinders the absolute particle multiplication. Moreover, contrary to the "Bottom-Up" scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration in BAMPS calculations is of order α −2 s (ln α s ) −2 Q −1 s . After this time the gluon system exhibits nearly hydrodynamical behavior. The shear viscosity to entropy density ratio has a weak dependence on Q s and lies close to the lower bound of the AdS/CFT conjecture.

Viscous effects on elliptic flow and shock waves

“…This has raised the speculation about nonperturbative interactions as well as about super symmetric representations of Yang-Mills theories using the AdS/CFT conjecture. However, in this talk, we demonstrate that the perturbative QCD (pQCD) can still explain a fast thermalization of the initially nonthermal gluon system, the large collective effects of QGP created at RHIC and the smallness of the shear viscosity to entropy ratio in a consistent manner by using a relativistic pQCD-based on-shell parton cascade Boltzmann approach of multiparton scatterings (BAMPS) [2,3,4,5,6,7]. BAMPS is a parton cascade, which solves the Boltzmann transport equation and can be applied to study, on a semi-classical level, the dynamics of gluon matter produced in heavy ion collisions at RHIC energies.…”

“…The fast thermalization happens in a similar way if color glass condensate is chosen as the initial conditions. One of the important messages obtained there is that the hard gluons thermalize at the same time as the soft ones due to the ggg → gg process, which is not included in the so called standard "Bottom Up" scenario of thermalization [4]. The situation is also the same when employing PYTHIA based initial conditions [9].…”

Towards a full solution of the relativistic Boltzmann equation for quark-gluon matter on GPUs

“…Later studies from BAMPS imply that the inverse processes, i.e., the 3 → 2 processes, inhibit the 2 → 3 processes. With both processes included, the timescale of thermal equilibration from the BAMPS is of order α −2 s lnðα s Þ −2 Q −1 s [59]. It has also been argued that the inclusion of all next-to-leading order processes may make the equilibration considerably faster than the simple 2 → 3 processes [60].…”

From microscopic interactions to the dynamics of the fireball

“…The partonic transport model BAMPS (Boltzmann Approach to MultiParton Scatterings) [6] has been developed to provide a unified description of the dynamics of the early QGP stage of heavy-ion collisions (HIC) including perturbative QCD based elastic and inelastic processes. BAMPS has been applied to provide explanations of fast thermalization on a very short time scale ≤ 1 fm/c [7] as well as a small value of η/s ≈ 0.08 − 0.2 for α s = 0.6 − 0.3 [8,9], calculated with the original GunionBertsch (GB) matrix element for inelastic processes. However, with extensive numerical comparisons between the GB approximation and the exact leading order pQCD result we showed that the original Gunion-Bertsch (GB) deviates from the exact result at forward and backward rapidity [10].…”