Thermalization, isotropization, and elliptic flow from nonequilibrium initial conditions with a saturation scale

In this article we report on our results about the computation of the elliptic flow of the quark-gluon plasma produced in relativistic heavy-ion collisions, simulating the expansion of the fireball by solving the relativistic Boltzmann equation for the parton distribution function tuned at a fixed shear-viscosity to entropy-density ratio η/s. Our main goal is to put emphasis on the role of a saturation scale in the initial gluon spectrum, which makes the initial distribution far from a thermalized one. We find that the presence of the saturation scale reduces the efficiency in building up the elliptic flow, even if the thermalization process is quite fast τₜₕₑᵣₘ≈0.84.pt0exfm/c and the pressure isotropization is even faster τᵢₛₒₜᵣ≈0.54.pt0exfm/c. The impact of the nonequilibrium implied by the saturation scale manifests for noncentral collisions and can modify the estimate of the viscosity with respect to the assumption of full thermalization in pT space. We find that the estimate of η/s is modified from η/s≈2/4π to η/s≈1/4π at the Relativistic Heavy-Ion Collider and from η/s≈3/4π to η/s≈2/4π at the Large Hadron Collider. We complete our investigation with a study of the thermalization and isotropization times of the fireball for different initial conditions and values of η/s showing how the latter affects both isotropization and thermalization. Last, we have seen that the range of values explored by the phase-space distribution function f is such that at pT<0.50.16em0exGeV the inner part of the fireball stays with occupation number significantly larger than unity despite the fast longitudinal expansion, which might suggest the possibility of the formation of a transient Bose-Einstein condensate.