Evolution Of Quark-gluon Plasma Research Articles

Probing parton propagation in heavy-ion collisions with ALICE heavy-flavour measurements

Heavy quarks (charm and beauty) are valuable probes for investigating the properties of the quark–gluon plasma (QGP) formed in ultra-relativistic heavy-ion collisions, as they are mainly produced through hard-scattering processes prior to the formation of the QGP, and their number is conserved during the subsequent QGP evolution. Measurements of the nuclear modification factor RAA of charm and beauty hadrons allow us to characterise the in-medium energy loss of heavy quarks while traversing the QGP. Information on their diffusion and degree of participation in the collective motion of the medium can be obtained by measuring the elliptic-flow coefficient v2 of heavy-flavour particles. Complementary insights into heavy-quark fragmentation and energy redistribution can be obtained by measuring angular correlations involving heavy-flavour particles. In this contribution, the recently published results on the non-prompt v2 coefficient of D0 mesons are presented, comparing them with those of other particle species in Pb–Pb collisions at √SNN = 5.02 TeV. Additionally, we report the recent findings on the RAA of prompt D mesons and Λc+ baryons in Pb–Pb collisions at the same energy. Furthermore, we discuss the new Pb–Pb results on angular correlations between heavy-flavour decay electrons and charged particles in the same collision system.

Impact of vorticity and viscosity on the hydrodynamic evolution of hot QCD medium

The strongly interacting transient state of quark-gluon plasma (QGP) medium created in ultra-relativistic collisions survives for a duration of a few fm/c. The spacetime evolution of QGP crucially depends on the equation of state (EoS), vorticity, viscosity, and external magnetic field. In the present study, we obtain the lifetime of a vortical QGP fluid within the ambit of relativistic second-order viscous hydrodynamics. We observe that the coupling of vorticity and viscosity significantly increases the lifetime of vortical QGP. The inclusion of a static magnetic field, vorticity, and viscosity makes the evolution slower. However, the static magnetic field slightly decreases the QGP lifetime by accelerating the evolution process for a non-rotating medium. We also report the rate of change of vorticity in the QGP, which will be helpful in studying the behavior of the medium in detail.

Importance of higher harmonics and v4 puzzle in quark-gluon plasma tomography

Quark-gluon plasma (QGP) tomography aims to constrain the parameters characterizing the properties and evolution of QGP formed in heavy-ion collisions, by exploiting low- and high-${p}_{\ensuremath{\perp}}$ theory and data. Higher-order harmonics ${v}_{n}$ ($n>2$) are an important---but seldom explored---part of this approach. However, to take full advantage of them, several issues have to be addressed: (i) consistency of different methods for calculating ${v}_{n}$, (ii) importance of event-by-event fluctuations to high-${p}_{\ensuremath{\perp}}\phantom{\rule{4pt}{0ex}}{R}_{AA}$ and ${v}_{2}$ predictions, and (iii) sensitivity of higher harmonics to the initial state of fluid-dynamical evolution. We obtain that (i) several methods for calculating harmonics are compatible with each other, (ii) event-by-event calculations are important in mid-central collisions, and (iii) various initializations of the evolution of the medium lead to quantitatively and qualitatively different predictions, likely to be distinguished by future measurements. We also find that the present high-${p}_{\ensuremath{\perp}}\phantom{\rule{4pt}{0ex}}{v}_{4}$ data cannot be reproduced using initial states for fluid-dynamical evolution given by state-of-the-art models. We call this discrepancy the high-${p}_{\ensuremath{\perp}}{v}_{4}$ puzzle at the Large Hadron Collider.

Quark-Gluon Plasma Fireball Formation in the Environment of Strong Magnetic Field

Using a theoretical model based on quasi-particle, we calculated the free energy evolution of quark-gluon plasma (QGP) in the presence of strong magnetic field generated in the high energy heavy ion collisions. The finite quark mass is used under the assumption of vanishing chemical potential. We found that the size of QGP fireball enhanced and more stable in the presence of strong magnetic field. This indicates that magnetic field plays an important role to describe the dynamics of QGP fireball under suitable conditions. Current results are useful in order to create a finite size of large QGP droplet. We noticed that the current results are also improved as compared to our earlier work where the contribution of magnetic field was neglected at RHIC and LHC. Thus, our results with quasi-particle model are of relevance in connection with the relativistic heavy ion collisions as well as for cosmological quark-hadron phase transition. In near future, experimentalists may provide the data for the existence of QGP fireball at RHIC, LHC and FAIR experiments.

Diphoton production rate in relativistic nuclear collisions

In this study, we have made an effort to reveal some information about the space-time evolution of quark gluon plasma (QGP). We deal with one of the important signature of quark gluon plasma from the analysis of the experimental results on electromagnetic probes which are measured at relativistic heavy-ion collider (RHIC) and large hadron collider (LHC). Electromagnetic radiations as diphotons emitted from hot and dense matter are investigated using a phenomenological model with quasiparticle approach at temperatures above critical temperature. In this, we use thermodynamically consistent quasiparticle model composed of quarks and gluons. Due to interactions among the quarks, mass of these particles is generated in highly dense and hot matter of QGP. The mass of these particles is temperature dependent and it is found that the model works well at temperatures above the critical temperature. Thus, this work is carried out using a phenomenological model in heavy-ion collisions in the limit of high temperature and zero chemical potential. The rate of diphoton production is calculated by suitably fitted parametrization factors in quark mass. We found an appreciable enhancement using thermal quark mass as compared to dynamical quark mass in the current results of two photon production rate. The results are compared with earlier estimated diphoton production rates from QGP and hadronic matter. Our results are therefore enhanced in comparison to the other theoretical results. The estimation of diphoton emission anticipates useful insights in the relevant range of mass. So these insights on diphotons can be advantageous tool for spectroscopy and thermometry in high energy heavy-ion collisions at RHIC and LHC.

Chiral magnetic effect and three-point function from AdS/CFT correspondence

The chiral magnetic effect with a fluctuating chiral imbalance is more realistic in the evolution of quark-gluon plasma, which reflects the random gluonic topological transition. Incorporating this dynamics, we calculate the chiral magnetic current in response to space-time dependent axial gauge potential and magnetic field in AdS/CFT correspondence. In contrast to conventional treatment of constant axial chemical potential, the response function here is the AVV three-point function of the mathcal{N} = 4 super Yang-Mills at strong coupling. Through an iterative solution of the nonlinear equations of motion in Schwarzschild-AdS5 background, we are able to express the AVV function in terms of two Heun functions and prove its UV/IR finiteness, as expected for mathcal{N} = 4 super Yang-Mills theory. We found that the dependence of the chiral magnetic current on a non-constant chiral imbalance is non-local, different from hydrodynamic approximation, and demonstrates the subtlety of the infrared limit discovered in field theoretic approach. We expect our results enrich the understanding of the phenomenology of the chiral magnetic effect in the context of relativistic heavy ion collisions.

Electromagnetic fields from the extended Kharzeev-McLerran-Warringa model in relativistic heavy-ion collisions

Based on the Kharzeev-McLerran-Warringa (KMW) model that estimates strong electromagnetic (EM) fields generated in relativistic heavy-ion collisions, we generalize the formulas of EM fields in the vacuum by incorporating the longitudinal position dependence, the generalized charge distributions and retardation correction. We further generalize the formulas of EM fields in the pure quark-gluon plasma (QGP) medium by incorporating a constant Ohm electric conductivity and also during the realistic early-time stages QGP evolution by using a time-dependent electric conductivity. Using the extended KMW model, we observe a slower time evolution and a more reasonable impact parameter b dependence of the magnetic field strength than those from the original KMW model in the vacuum. The inclusion of medium effects by using the lattice data helps to further prolong the time evolution of magnetic field, such that the magnetic field strength during the realistic QGP evolution at thermal freeze-out time can meet the 1σ bound constrained from experimentally measured difference in global polarizations of Λ and Λ¯ hyperons in Au+Au collisions at top RHIC energy. These generalized formulations in the extended KMW model will be potentially useful for many EM fields relevant studies in heavy-ion collisions, especially at lower colliding energies and for various species of colliding nuclei.

From Chiral Kinetic Theory To Spin Hydrodynamics

The total angular momentum is conserved in the evolution of the Quark-Gluon Plasma (QGP) created in heavy-ion collision, and consists of two sectors: the orbital angular momentum (OAM) caused by kinetic motion, and the spin, an intrinsic degree of freedom of quarks and gluons. Microscopic scattering processes allow the conversion between these two components, hence the spin density could eventually have non-trivial influence on the QGP evolution. A hydrodynamic theory, with the spin polarization effect properly taken into account, is required to quantitatively study the polarization rate for observed hadrons, e.g. the Λ-hyperon. In this work, we start with chiral kinetic theory and construct the spin hydrodynamic framework for a chiral spinor system. We obtain the equations of motion of second-order dissipative relativistic fluid dynamics with non-trivial spin polarization density.

Photon production and elliptic flow from a momentum-anisotropic quark-gluon plasma

The emission of real photons from a momentum-anisotropic quark-gluon plasma (QGP) is affected by both the collective flow of the radiating medium and the modification of local rest frame emission rate due to the anisotropic momentum distribution of partonic degrees of freedom. In this paper, we first calculate the photon production rate from an ellipsoidally momentum-anisotropic QGP including hard contributions from Compton scattering and quark pair annihilation and soft contribution calculated using the hard thermal loop (HTL) approximation. We introduce a parametrization of the nonequilibrium rate in order to facilitate its further application in yield and flow calculations. We convolve the anisotropic photon rate with the space-time evolution of QGP provided by 3+1d anisotropic hydrodynamics (aHydro) to obtain the yield and the elliptic flow coefficient $v_2$ of photons from QGP generated at Pb-Pb collisions at LHC at 2.76 TeV and Au-Au collisions at RHIC at 200 GeV. We investigate the effects of various parameters on the results. In particular we analyze the sensitivity of results to initial momentum anisotropy.

First order dissipative hydrodynamics and viscous corrections to the entropy four-current from an effective covariant kinetic theory

The first order hydrodynamic evolution equations for the shear stress tensor, the bulk viscous pressure and the charge current have been studied for a system of quarks and gluons, with a non-vanishing quark chemical potential and finite quark mass. The first order transport coefficients have been obtained by solving an effective Boltzmann equation for the grand-canonical ensemble of quasiquarks and quasigluons. We adopted temperature dependent effective fugacity for the quasiparticles to encode the hot QCD medium effects. The non-trivial energy dispersion of the quasiparticles induces mean field contributions to the transport coefficients whose origin could be directly related to the realization of conservation laws from the effective kinetic theory. Both the QCD equation of state and chemical potential are seen to have a significant impact on the quark-gluon plasma evolution. The shear and bulk viscous corrections to the entropy-four current have been investigated in the framework of the effective kinetic theory. The effect of viscous corrections to the entropy density have been quantified in the case of one dimensional boost-invariant expansion of the system. Further, the first order viscous corrections to the time evolution of temperature along with the description of pressure anisotropy and Reynolds number of the system have been explored for the longitudinal boost-invariant expansion.

Fluctuations of collective flows for event-by-event hydrodynamic evolution sources

Through investigating the space-time evolution of the hydrodynamic particle-emitting sources with the fluctuating initial conditions generated by Heavy Ion Jet Interaction Generator (HIJING), we discussed the space-time and velocity evolution of quark gluon plasma (QGP). In order to detect the event-by-event inhomogeneity of the sources, we examine the distribution of the error-inverse-weighted fluctuations, between the two-pion Bose-Einstein correlation functions of single events. We find that the distribution becomes wide for the fluctuating initial sources with velocity v = 0, which means the sources evolution faster than nonzero initial conditions. We also discussed the elliptic flow effect, velocity and temperature evolution in Heavy Ion Reactions, and get the same results.

QGP hydrodynamical study using energy-momentum in-medium deposition by an extended source.

The quark-gluon plasma (QGP) is created under extreme conditions, such as the ones prevailing in heavy ion collisions. The characterization of the QGP can be done using high-pT probes such as the partons that are created through hard scatterings in the fireball. These fast-moving partons lose energy and momentum along their traveled path through the medium. The parton deposition of energy-momentum creates an in-medium disturbance that can be described using approximations within relativistic hydrodynamics in a defined regime of the QGP evolution. Based on earlier research in this field, we study the use of extended sources that depend on the location of the parton-jet in the initial stages of the QGP evolution. We explore this approach as a way to complement the current numerical landscape of hydrodynamical QGP studies and to eventually generate initial conditions that can be used as input of hydrodynamical numerical simulations.

Pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss interferometry in a viscous granular source model**Supported by National Natural Science Foundation of China (11675034, 11275037)

We examine the evolution of quark-gluon plasma (QGP) droplets with viscous hydrodynamics and analyze the pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in a granular source model consisting of viscous QGP droplets. The shear viscosity of the QGP droplet speeds up and slows down the droplet evolution in the central and peripheral regions of the droplet, respectively. The effect of the bulk viscosity on the evolution is negligible. Although there are viscous effects on the droplet evolution, the pion momentum spectrum and elliptic flow change little for granular sources with and without viscosity. On the other hand, the influence of viscosity on HBT radius Rout is considerable. It makes Rout decrease in the granular source model. We determine the model parameters of granular sources using the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii together, and investigate the effects of viscosity on the model parameters. The results indicate that the granular source model may reproduce the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii in heavy-ion collisions of Au-Au at and Pb-Pb at in different centrality intervals. The viscosity of the droplet leads to an increase in the initial droplet radius and a decrease of the source shell parameter in the granular source model.

Flow excited by full jet shower in QGP fluid and its effect on jet shape

We study the nuclear modification of full jet structure in relativistic heavy-ion collisions, with the inclusion of the effect of the medium response. The evolution of full jet shower is described by a set of transport equations, and the space-time evolution of quark-gluon plasma is simulated by solving relativistic hydrodynamic equations coupled with the energy and momentum depositions by jets as the source terms. We study the effect of medium response to full jet shower and present how jet-induced flow contributes to the full jet structure in PbPb collisions at the LHC.

ϒ production in p–Pb and Pb–Pb collisions with ALICE at the LHC

ALICE (A Large Ion Collider Experiment) is devoted to the study of heavy-ion collisions at LHC energies. In such collisions a deconfined state of nuclear matter, the Quark-Gluon Plasma (QGP), is formed. Due to their early production, quarkonium states are good probes to study the QGP evolution. Such states are affected by suppression mechanisms which lead to reduced yields with respect to pp and p-Pb collisions, while regeneration phenomena might lead to an enhancement of their production. The latter effects are expected to be negligible at LHC for bottomonium states. The recent ALICE results on ϒ production in Pb-Pb collisions at sNN=5.02 TeV will be presented and compared with previous measurements at sNN=2.76 TeV. A comparison with theoretical calculations will be performed as well. Results obtained in p-Pb collisions at sNN=5.02 TeV will also be discussed.

Photon radiation from hot and dense matter of quark gluon plasma

We work on photon radiation from hot and dense matter of quark gluon plasma (QGP). The total photon spectra of leading order (LO) process is produced in relativistic massive collisions of two nuclei using various value of quark phenomenological flow parameter dependent on quark chemical potential in quark mass. The total photon measurement is strongly increasing function of quark flow parameter ranging 2γg ≤ γq ≤ 8γg and also find appreciable enhancement at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN as a result of high temperature and chemical potential using quark phenomenological flow parameter. The results obtained are presented on the study of photon production of leading order process. Thus, the study of photon radiation from hot and dense matter provides a valuable information in the formation and evolution of QGP and in the subsequent investigation of its properties. The results are compared with other theoretical work.

Analytical and numerical Gubser solutions of the second-order hydrodynamics

Evolution of quark-gluon plasma (QGP) near equilibrium can be described by the second-order relativistic viscous hydrodynamic equations. Consistent and analytically verifiable numerical solutions are critical for phenomenological studies of the collective behavior of QGP in high-energy heavy-ion collisions. A novel analytical solution based on the conformal Gubser flow which is a boost-invariant solution with transverse fluid velocity is presented. Due to the non-linear nature of the equation, the analytical solution is non-perturbative and exhibits features that are rather distinct from solutions to usual linear hydrodynamic equations. It is used to verify with high precision the numerical solution with a newly developed state-of-the-art $(3+1)$-dimensional second-order viscous hydro code (CLVisc). The perfect agreement between the analytical and numerical solutions demonstrates the reliability of the numerical simulations with the second-order viscous corrections. This lays the foundation for future phenomenological studies that allow one to gain access to the second-order transport coefficients.

Onset of cavitation in the quark-gluon plasma

We study the onset of bubble formation (cavitation) in the quark-gluon plasma as a result of the reduction of the effective pressure from bulk-viscous corrections. By calculating velocity gradients in typical models for quark-gluon plasma evolution in heavy-ion collisions, we obtain results for the critical bulk viscosity above which cavitation occurs. Since present experimental data for heavy-ion collisions seems inconsistent with the presence of bubbles above the phase transition temperature of QCD, our results may be interpreted as an upper limit of the bulk viscosity in nature. Our results indicate that bubble formation is consistent with the expectation of hadronisation in low-temperature QCD.

Connecting QGP-Heavy Ion Physics to the Early Universe

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.

"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.