Abstract
A current goal of relativistic heavy ion collisions experiments is to understand the impact of initial non-equilibrium on final observables. A Color Glass Condensate (CGC) as the limiting state of QCD matter at very high density implies initial non-thermal distribution at least for momenta below the saturation scale. In viscous hydrodynamics simulations, a standard Glauber initial condition leads to estimate 4= s 1, while em- ploying the Kharzeev-Levin-Nardi (KLN) modeling of the CGC leads to at least a factor of 2 larger = s. Within a kinetic theory approach based on a relativistic Boltzmann-like transport simulation, our main result is that the out-of-equilibrium initial distribution in p-space reduces the e ciency in building-up the elliptic flow. At RHIC energy we find the available data onv2 are in agreement with a 4= s 1 also for KLN initial conditions.
Highlights
A main discovery of the Relativistic Heavy-Ion Collider (RHIC) confirmed by the Large Hadron Collider (LHC) has been that the QGP has a very small shear viscosity to density entropy [1], η/s, close to the lower bound of 1/4π conjectured for systems at infinite strong coupling
Both viscous hydrodynamics [2, 3, 5, 6], and transport Boltzmann-like approaches [7,8,9,10] agree in indicating an average η/s of the QGP lying in the range 4πη/s ∼ 1 − 3
The experimental data of v2(pT ) at the highest RHIC energy are in agreement with a fluid at 4πη/s ∼ 1, assuming a standard Glauber initial condition, while due to the larger x MC-KLN favors instead a fluid at 4πη/s ∼ 2
Summary
A main discovery of the Relativistic Heavy-Ion Collider (RHIC) confirmed by the Large Hadron Collider (LHC) has been that the QGP has a very small shear viscosity to density entropy [1], η/s, close to the lower bound of 1/4π conjectured for systems at infinite strong coupling. The uRHIC program offers the tantalizing opportunity to explore the existence of an exotic state, namely the Color Glass Condensate (CGC) Such a state of matter would be primarily generated by the very high density of the gluon parton distribution function at low x (parton momentum fraction), which triggers a saturation of the gluon distribution function at a pT below the saturation scale, Qs. a simple geometrical description through the Glauber model predicts a x smaller at least 25-30%. The experimental data of v2(pT ) at the highest RHIC energy are in agreement with a fluid at 4πη/s ∼ 1, assuming a standard Glauber initial condition, while due to the larger x MC-KLN favors instead a fluid at 4πη/s ∼ 2
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