Evolution Of Quark-gluon Plasma Research Articles

"Chemical" composition of the Quark Gluon Plasma

In this article we discuss the issue of the quark to gluon ratio in the Quark Gluon Plasma (QGP). Our model to describe the QGP evolution is based on transport theory including the mean field dynamics described by a quasi-particle model.The last is able to take into account for the lattice QCD thermodynamics and implies a "chemical" equilibrium ratio between quarks and gluons strongly increasing as T approaches to the temperature of the phase transition Tc. We present first the tests performed in a fixed box to check that our code is able to reproduce the equilibrium ratio and then the results obtained for the simulations of ultra-Relativistic Heavy Ion Collisions (uRHIC's) at RHIC and LHC energies. We observe a rapid evolution from a gluon dominated initial state to a quark dominated plasma and we see that near Tc almost 80% of the particles composing the plasma are quarks. This has potentially a strong impact on several quantitative aspects of QGP probes and furnishes a justification to the coalescence hadronization model.

Can we observe the elliptic flow in pp collisions at LHC?

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages of reaction. Now, with the start of Large Hadron Collider (LHC), the collective effects in proton-proton collisions are also of great interest. In this work we analyze the ability of different methods to extract elliptic flow in pp collisions at LHC. The analysis is based on Monte Carlo simulations of proton-proton collisions with PYTHIA, PHOJET and EPOS event generators at energies = 900 GeV and 7TeV.

Elliptic flow studies using the CMS detector

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages. The nuclear reaction plane can be determined independently by different detector subsystems and using different analysis methods. This paper reports the capability of the CMS detector at the LHC to reconstruct the reaction plane of the collision and to me asure elliptic flow with calorimetry and a tracking system. The analysis is based on a full CMS detector simulation of Pb + Pb events with the HYDJET event generator.

Pre-equilibrium dilepton production from an anisotropic quark-gluon plasma

We calculate leading-order dilepton yields from a quark-gluon plasma that has a time-dependent anisotropy in momentum space. Such anisotropies can arise during the earliest stages of quark-gluon plasma evolution due to the rapid longitudinal expansion of the created matter. Two phenomenological models for the proper-time dependence of the parton hard momentum scale, ${p}_{\mathrm{hard}}$, and the plasma anisotropy parameter, \ensuremath{\xi}, are constructed that describe the transition of the plasma from its initial nonequilibrium state to an isotropic thermalized state. The first model constructed interpolates between 1+1 dimensional free streaming at early times and 1+1 dimensional ideal hydrodynamical expansion at late times. In the second model we include the effect of collisional broadening of the parton distribution functions in the early-time pre-equilibrium stage of plasma evolution. We find for both cases that for fixed initial conditions high-energy dilepton production is enhanced by pre-equilibrium emission. When the models are constrained to fixed final pion multiplicity the dependence of the resulting spectra on the assumed plasma isotropization time is reduced. Using our most realistic collisionally broadened model we find that high-transverse-momentum dilepton production would be enhanced by at most 40% at the Relativistic Heavy Ion Collider and 50% at the CERN Large Hadron Collider if one assumes an isotropization/thermalization time of 2 fm/$c$. Given sufficiently precise experimental data this enhancement could be used to determine the plasma isotropization time experimentally.

Effect of jet quenching on hydrodynamical evolution of QGP

We study the effects of jet quenching on the hydrodynamical evolution of the quark–gluon plasma (QGP) fluid created in a heavy-ion collision. Assuming that the deposited energy quickly thermalizes, we simulate the subsequent hydrodynamic evolution of the QGP fluid. For partons moving at supersonic speed and sufficiently large energy loss, jet quenching can lead to conical flow.

J/ψProduction in Quark-Gluon Plasma

We study $J/\ensuremath{\psi}$ production at RHIC and LHC energies with both initial production at energies reached and the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) with regeneration. We solve the coupled set of transport equations for the $J/\ensuremath{\psi}$ distribution in phase space and the hydrodynamic equation for evolution of quark-gluon plasma. At RHIC, continuous regeneration is crucial for the $J/\ensuremath{\psi}$ momentum distribution while the elliptic flow is still dominated by initial production. At energies reached at the LHC energy, almost all the initially created $J/\ensuremath{\psi}s$ are dissociated in the medium and regeneration dominates the $J/\ensuremath{\psi}$ properties.

Dissipative hydrodynamics in2+1dimensions

In $2+1$ dimensions, we simulated the hydrodynamic evolution of quark-gluon plasma (QGP) fluid with dissipation due to shear viscosity. Comparison of the evolution of ideal and viscous fluids, both initialized under the same conditions, e.g., same equilibration time, energy density and velocity profile, reveals that the dissipative fluid evolves slowly, cooling at a slower rate. Cooling slows even more at higher viscosities. The fluid velocities, however, evolve faster in a dissipative fluid than in an ideal fluid. The transverse expansion is also enhanced in dissipative evolution. For the same decoupling temperature, the freeze-out surface for a dissipative fluid is more extended than that for an ideal fluid. Dissipation produces entropy as a result of which particle production is increased. Particle production is increased as a result of the (i) the extension of the freeze-out surface and (ii) the change of the equilibrium distribution function to a nonequilibrium one, the latter effect being prominent at large transverse momentum. Compared to ideal fluid, transverse momentum distribution of pion production is considerably enhanced. Enhancement is greater at high ${p}_{T}$ than at low ${p}_{T}$. Pion production also increases with viscosity; the greater the viscosity, the greater the pion production. Dissipation also modifies the elliptic flow, which is reduced in viscous dynamics. Also, contrary to ideal dynamics where elliptic flow continues to increase with transverse momentum, in viscous dynamics elliptic flow tends to saturate at large transverse momentum. The analysis suggests that the initial conditions of the hot, dense matter produced in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC), as extracted from ideal fluid analysis, can be changed significantly if the QGP fluid is viscous.

Initial condition for quark-gluon plasma evolution

We recently proposed a new approach to high energy nuclear scattering, which treats the initial stage of heavy ion collisions in a sophisticated way. We are able to calculate macroscopic quantities like energy density and velocity flow at the end of this initial stage, after the two nuclei having penetrated each other. In other words, we provide the initial conditions for a macroscopic treatment of the second stage of the collision. We address in particular the question of how to incorporate the soft component properly. We find almost perfect "Bjorken scaling": the rapidity coincides with the space-time rapidity, whereas the transverse flow is practically zero. The distribution of the energy density in the transverse plane shows typically a very "bumpy" structure.

Preequilibrium evolution of quark-gluon plasma

We study the production and the evolution of quark-gluon plasma expected to be formed in ultrarelativistic heavy-ion collisions, within the color flux-tube model. We introduce the gluonic component in the Boltzmann equation, which we solve in the phase space which is extended to include the SU(3) color degree of freedom. The color degree of freedom is shown to play a decisive role in equilibration, and in fixing the temperature of the plasma. We further find that the soft partons that we study here contribute substantially to the bulk properties. Finally, the model is shown to provide a detailed picture of how the quark-gluon plasma evolves and is driven towards a hydrodynamic flow, which starts occurring around 1 fm.

Formation and evolution of quark-gluon plasma at RHIC and LHC

Initial conditions for quark-gluon plasma formation at τ = 0.1 fm are considered in lowest order perturbative QCD. Chemical composition, thermalization of the formed semihard quark-gluon system and especially implications of the new HERA parton distributions with the enhancement at small x are studied. The plasma at τ = 0.1 fm is shown to be strongly gluon dominated both at RHIC and LHC, and a possibility for rapid thermalization of gluons at LHC is pointed out. Uncertainties in the calculations, particularly shadowing corrections to the parton distributions, are discussed. Free streaming and ideal hydro limits for the evolution of the gluonic plasma with the calculated minijet initial conditions are demonstrated, and a lower limit for final multiplicities obtained for the LHC nuclear collisions.

History of quark-gluon plasma evolution from photon interferometry.

Two-photon intensity interferometry is used to probe the dynamics of quarks and gluons in a high-energy nucleus-nucleus collision. A (1+1)-dimensional expansion of the plasma according to Bjorken hydrodynamics as well as a (3+1)-dimensional expansion, both with a first-order phase transition, are considered. The correlation of high transverse-momentum photons is sensitive to the details of the space-time evolution of the high density QCD plasma. The so-called ``longitudinal'' and ``outward'' correlations are found to be dramatically affected by the transverse expansion of the system.

On the hydrodynamic evolution of quark-gluon plasma produced by ultrarelativistic nuclear collisions

The hydrodynamic evolution of a quark-gluon plasma, produced in the central rapidity region, is studied incorporating the external bag pressure acting on the plasma surface. It is shown that the plasma fluid, which undergoes the scaling longitudinal expansion, generates a non-trivial transverse flow pattern, near the plasma surface, consisting of a rarefaction wave and a compression wave, instead of a simple rarefaction wave in the case of free expansion without a surface boundary condition. We also discuss a possible global phase transition of supercooled plasma, in the interior of the cylinder, to a superheated dense hadron gas through a time-like surface of discontinuity.

Frictional properties of rubber

We discuss properties and evolution of quark-gluon plasma in the early Universe and compare to laboratory heavy ion experiments. We describe how matter and antimatter emerged from a primordial soup of quarks and gluons. We focus our discussion on similarities and differences between the early Universe and the laboratory experiments.