Elliptic Flow and Shear Viscosity within a Transport Approach from RHIC to LHC Energy

We have investigated the build up of anisotropic flows within a parton cascade approach at fixed shear viscosity to entropy density \eta/s to study the generation of collective flows in ultra-relativistic heavy ion collisions. We present a study of the impact of a temperature dependent \eta/s(T) on the generation of the elliptic flow at both RHIC and LHC. Finally we show that the transport approach, thanks to its wide validity range, is able to describe naturally the rise - fall and saturation of the v_2(p_T) observed at LHC.Comment: 6 pages, 3 figures, proceedings of the workshop EPIC@LHC, 6-8 July 2011, Bari, Ital

Shear viscosity and chemical equilibration of the QGP

We have investigated, in the frame work of the transport approach, different aspects of the QGP created in Heavy Ion Collisions at RHIC and LHC energies. The shear viscosity $\eta$ has been calculated by using the Green-Kubo relation at the cascade level. We have compared the numerical results for $\eta$ obtained from the Green-Kubo correlator with the analytical formula in both the Relaxation Time Approximation (RTA) and the Chapman-Enskog approximation (CE). From this comparison we show that in the range of temperature explored in a Heavy Ion collision the RTA underestimates the viscosity by about a factor of 2, while a good agreement is found between the CE approximation and Gree-Kubo relation already at first order of approximation. The agreement with the CE approximation supplies an analytical formula that allows to develop kinetic transport theory at fixed shear viscosity to entropy density ratio, $\eta/s$. We show some results for the build up of anisotropic flows $v_{2}$ in a transport approach at fixed shear viscosity to entropy density ratio, $\eta/s$. We study the impact of a T-dependent $\eta/s(T)$ on the generation of the elliptic flows at both RHIC and LHC. We show that the transport approach provides, in a unified way, a tool able to naturally describe the $v_{2}(p_{T})$ in a wide range of $p_{T}$, including also the description of the rise and fall and saturation of the $v_{2}(p_{T})$ observed at LHC. Finally, we have studied the evolution of the quark-gluon composition employing a Boltzmann-Vlasov transport approach that include: the mean fields dynamics, associated to the quasi-particle model, and the elastic and inelastic collisions for massive quarks and gluons. Following the chemical evolution from an initial gluon dominated plasma we predict a quark dominance close to $T_{C}$ paving the way to an hadronization via quark coalescence.Comment: 15 pages, 10 figures, Invited Talk given by S. Plumari at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS

Anisotropic flows and the shear viscosity of the QGP within an event-by-event massive parton transport approach

Abstract We have developed an event-by-event relativistic kinetic transport approach to study the build up of the anisotropic flows $$v_{n}(p_T)$$ vn(pT) for a system at fixed $$\eta /s(T)$$ η/s(T) . The partonic approach describe the evolution of massless partons which imply $$\epsilon =3p$$ ϵ=3p as Equation of State (EoS). We extend previous studies to finite partonic masses tuned to simulate a system that expand with an EoS close to the recent lQCD results. We study the role of EoS and the effect of $$\eta /s(T)$$ η/s(T) ratio on the build up of $$v_n(p_T)$$ vn(pT) up to $$n=5$$ n=5 for two beam energies: RHIC energies at $$\sqrt{s}=200$$ s=200 GeV and LHC energies at $$\sqrt{s}=2.76$$ s=2.76 TeV. We find that for the two beam energies considered the suppression of the $$v_n(p_T)$$ vn(pT) due to the viscosity of the medium have different contributions coming from the cross over or QGP phase. We shows that in ultra-central collisions (0–0.2%) the $$v_n(p_T)$$ vn(pT) have a stronger sensitivity to the T dependence of $$\eta /s$$ η/s that increases with the order of the harmonic n. Finally, we discuss the results for the integrated flow harmonics $$\langle v_{n} \rangle$$ ⟨vn⟩ in ultra-central collisions pointing-out how the relative strength of $$\langle v_{n} \rangle$$ ⟨vn⟩ depend on the colliding energies as well as on the freeze-out dynamics

Elliptic Flow from Non-equilibrium Initial Condition with a Saturation Scale

A current goal of relativistic heavy ion collisions experiments is the search for a Color Glass Condensate as the limiting state of QCD matter at very high density. In viscous hydrodynamics simulations, a standard Glauber initial condition leads to estimate $4\pi \eta/s \sim 1$, while a Color Glass Condensate modeling leads to at least a factor of 2 larger $\eta/s$. Within a kinetic theory approach based on a relativistic Boltzmann-like transport simulation, we point out that the out-of-equilibrium initial distribution proper of a Color Glass Condensate reduces the efficiency in building-up the elliptic flow. Our main result at RHIC energy is that the available data on $v_2$ are in agreement with a $4\pi \eta/s \sim 1$ also for Color Glass Condensate initial conditions, opening the possibility to describe self-consistently also higher order flow, otherwise significantly underestimated, and to pursue further the search for signatures of the Color Glass Condensate.Comment: 6 pages, 4 figures. // Title changed, some discussion added, main conclusions unchanged. Version accepted for publication on Phys. Lett.

Universal strangeness production and size fluctuactions in small and large systems

Strangeness production in high multiplicity events gives indications on the transverse size fluctuactions in nucleus-nucleus ($AA$), proton-nucleus ($pA$) and proton-proton ($pp$) collisions. In particular the behavior of strange particle hadronization in "small" ($pp,pA$) and "large" ($AA$) initial configurations of the collision can be tested for the specific particle species, for different centralities and for large fluctuations of the transverse size in $pA$ and $pp$ by using the recent ALICE data. A universality of strange hadron production emerges by introducing a dynamical variable proportional to the initial parton density in the transverse plane.Comment: talk at EPS-HEP conference , Venice, 201