Relativistic Heavy Ion Collider Energies Research Articles

Toward an understanding of the RAA and v2 puzzle for heavy quarks

One of the primary aims of the ongoing nuclear collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is to create a Quark Gluon Plasma (QGP). The Heavy Quarks (HQ) constitutes a unique probe of the QGP properties. Both at RHIC and LHC energies a puzzling relation between the nuclear modification factor RAA(pT) and the elliptic flow v2(pT) related to heavy quark has been observed which challenged all the existing models.We discuss how the temperature dependence of the heavy quark drag coefficient can address for a large part of such a puzzle. We have considered four different models to evaluate the temperature dependence of drag and diffusion coefficients propagating through a quark gluon plasma (QGP). All the four different models are set to reproduce the same RAA(pT) experimentally observed at RHIC energy. We have found that for the same RAA(pT) one can generate 2–3 times more v2 depending on the temperature dependence of the heavy quark drag coefficient.

Kinetic freeze-out temperature and flow velocity extracted from transverse momentum spectra of final-state light flavor particles produced in collisions at RHIC and LHC

The transverse momentum spectra of final-state light flavor particles produced in proton-proton (p-p), copper-copper (Cu-Cu), gold-gold (Au-Au), lead-lead (Pb-Pb), and proton-lead (p-Pb) collisions for different centralities at relativistic heavy ion collider (RHIC) and large hadron collider (LHC) energies are studied in the framework of a multisource thermal model. The experimental data measured by the STAR, CMS, and ALICE Collaborations are consistent with the results calculated by the multi-component Erlang distribution and Tsallis Statistics. The effective temperature and real temperature (kinetic freeze-out temperature) of interacting system at the stage of kinetic freeze-out, the mean transverse flow velocity and mean flow velocity of particles, and the relationships between them are extracted. The dependences of effective temperature and mean (transverse) momentum on rest mass, moving mass, centrality, and center-of-mass energy, and the dependences of kinetic freeze-out temperature and mean (transverse) flow velocity on centrality, center-of-mass energy, and system size are obtained.

Effect of inelastic scattering process of gluons on dilepton productions of quark-gluon plasma

Dileptons have large mean free paths due to their small cross sections for electromagnetic interaction in plasma. Therefore they are considered to be an important probe for the formation and evolution of the quark matter. In this work, we calculate the dilepton production of quark-gluon plasma (QGP) produced in Au197+ Au197 central collisions at relativistic heavy ion collider (RHIC) energy based on the evolution model of a chemically equilibrating viscous QGP. The evolution of the QGP system is described by a set of coupled relaxation equations containing the master equations of partons, the equation of baryon number conservation and equation of energy-momentum conservation. Solving the set of evolution equations, one can obtain the evolution of temperature T, quark chemical potential q, fugacities q for quarks and g for gluons. To discuss the shear viscosity of QGP, the contributions of the elastic scattering of quarks qqqqand gluons gggg, as well as the inelastic scattering process of gluons ggggg are included. Based on the evolution model including the viscosity, we perform a complete calculation of the dilepton production, including the processes of quark-antiquark annihilation qqll, next-order annihilation qqgll, Compton-like scattering qg qll, qg qll, multiple scattering of quarks, as well as gluon fusion gg cc, annihilation qqcc. It is found that the spectra from the quark-antiquark annihilations qqll and qqgll are dominated. The contributions from multiple scattering cannot be neglected. We also find that the dilepton yields remarkably decrease with considering an additional gluon inelastic process in the calculation compared with the results with considering only elastic scatterings of quarks and gluons. This indicates that the evolution of QGP system is accelerated and the evolution time is shortened by the inelastic scatterings of gluons.

An Experimental Review on Heavy-Flavorv2in Heavy-Ion Collision

For over a decade now, the primary purpose of relativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) has been to study the properties of QCD matter under extreme conditions—high temperature and high density. The heavy-ion experiments at both RHIC and LHC have recorded a wealth of data in p+p, p+Pb, d+Au, Cu+Cu, Cu+Au, Au+Au, Pb+Pb, and U+U collisions at energies ranging fromsNN=7.7 GeV to 7 TeV. Heavy quarks are considered good probe to study the QCD matter created in relativistic collisions due to their very large mass and other unique properties. A precise measurement of various properties of heavy-flavor hadrons provides an insight into the fundamental properties of the hot and dense medium created in these nucleus-nucleus collisions, such as transport coefficient and thermalization and hadronization mechanisms. The main focus of this paper is to present a review on the measurements of azimuthal anisotropy of heavy-flavor hadrons and to outline the scientific opportunities in this sector due to future detector upgrade. We will mainly discuss the elliptic flow of open charmed meson (D-meson),J/ψ, and leptons from heavy-flavor decay at RHIC and LHC energy.

Hadrons from coalescence plus fragmentation inAAcollisions at energies available at the BNL Relativistic Heavy Ion Collider to the CERN Large Hadron Collider

In a coalescence plus independent fragmentation approach we calculate the ${p}_{T}$ spectra of the main hadrons: $\ensuremath{\pi},K,p,\overline{p},\mathrm{\ensuremath{\Lambda}},\ensuremath{\phi}$ in a wide range of transverse momentum from low ${p}_{T}$ up to about 10 GeV. The approach in its main features was developed several years ago at Relativistic Heavy Ion Collider (RHIC) energy. Augmenting the model with the inclusion of some more main resonance decays, we show that the approach correctly predicts the evolution of the ${p}_{T}$ spectra from RHIC to LHC (Large Hadron Collider) energy and in particular the baryon-to-meson ratios $p/\ensuremath{\pi},\overline{p}/\ensuremath{\pi},\mathrm{\ensuremath{\Lambda}}/K$ that reach a value of the order of unit at ${p}_{T}\ensuremath{\sim}3\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$. This is achieved without any change of the coalescence parameters. The more recent availability of experimental data up to ${p}_{T}\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ for $\mathrm{\ensuremath{\Lambda}}$ spectrum as well as for $p/\ensuremath{\pi}$ and $\mathrm{\ensuremath{\Lambda}}/K$ shows some lack of yield in a limited ${p}_{T}$ range around 6 GeV. This indicates that the baryons ${p}_{T}$ spectra from Albino-Kniehl-Kramer fragmentation functions are too flat at ${p}_{T}\ensuremath{\lesssim}8\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$. We also show that in a coalescence plus fragmentation approach one predicts a nearly ${p}_{T}$ independent $p/\ensuremath{\phi}$ ratio up to ${p}_{T}\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ followed by a significant decrease at higher ${p}_{T}$. Such a behavior is driven by a similar radial flow effect at ${p}_{T}l2\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ and the dominance of fragmentation for $\ensuremath{\phi}$ at larger ${p}_{T}$.

Unified description of charmonium suppression in a quark-gluon plasma medium at RHIC and LHC energies

Recent experimental and theoretical studies suggest that the quarkonium suppression in a thermal QCD medium created in heavy ion collisions is a complex interplay of various physical processes. In this article we put together most of these processes in a unified way to calculate the charmonium survival probability (nuclear modification factor) at energies available at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments. We include shadowing as the dominant cold-nuclear-matter effect. Further, gluonic dissociation and collision damping are included, which provide width to the spectral function of charmonia in a thermal medium and cause the dissociation of charmonium along with the usual color screening. We include color screening by using our recently proposed modified Chu--Matsui model. Furthermore, we incorporate the recombination of uncorrelated charm and anticharm quarks for the regeneration of charmonium over the entire temporal evolution of the QGP medium. Finally, we do a feed-down correction from the excited states to calculate the survival probability of charmonium. We find that our unified model suitably and simultaneously describes the experimental nuclear modification data of $J/\ensuremath{\psi}$ at RHIC and LHC.

Net-baryon number fluctuations with the hadron resonance gas model using Tsallis distribution

We explore a hadron resonance gas (HRG) model using the Tsallis non-extensive distribution to study the energy dependence of the product of the moments, and of the net-proton multiplicity distributions of published STAR data for Au+Au collisions at Relativistic Heavy-Ion Collider energies. While excellent agreement was found between model predictions and measurements of and of most peripheral collisions and of most central collisions, the for most central collisions deviates significantly from the predictions particularly at = 19.6 GeV and 27 GeV. This could be an indication of the presence of dynamical fluctuations, which are not contained in the HRG–Tsallis model.

Jet-dilepton conversion in expanding quark-gluon plasma

We calculate the production of large-mass dileptons from the jet-dilepton conversion in the expanding quark-gluon plasma at Relativistic Heavy Ion Collider and Large Hadron Collider (LHC) energies. The jet-dilepton conversion exceeds the thermal dilepton production and Drell--Yan process in the large-mass region of $3.9\phantom{\rule{4pt}{0ex}}\mathrm{GeV}<M<5.8\phantom{\rule{4.pt}{0ex}}\mathrm{GeV}$ and $6.3\phantom{\rule{4pt}{0ex}}\mathrm{GeV}<M<8.7\phantom{\rule{4.pt}{0ex}}\mathrm{GeV}$ in central $\text{Pb}+\text{Pb}$ collisions at $\sqrt{{s}_{NN}}=2.76$ and 5.5 TeV, respectively. We present the numerical solution of ideal fluid hydrodynamics. We find that the transverse flow leads to a rapid cooling of the fire ball. The suppression due to transverse flow appears from small to large mass, the transverse-flow effect becomes important at LHC energies. The energy loss of jets in the hot and dense medium is also included.

Heavy quark diffusion in the pre-equilibrium stage of heavy ion collisions

The drag and diffusion coefficients of heavy quarks (HQs) have been evaluated in the pre-equilibrium phase of the evolving fireball produced in heavy ion collisions at relativistic heavy ion collider and Large Hadron Collider energies. KLN and classical Yang–Mills spectra have been used for describing the momentum distributions of the gluons produced just after the collisions but before they thermalize. The interaction of the HQs with these gluons has been treated within the framework of perturbative quantum chromodynamics. We have observed that the HQs are dragged almost equally by the kinetically equilibrated and out-of-equilibrium gluonic systems. We have also noticed that the HQ diffusion in the pre-equilibrium gluonic phase is as fast as in the kinetically equilibrated gluons. Moreover, the diffusion is faster in the pre-equilibrium phase than in the chemically equilibrated quark–gluon plasma. These findings may have significant impact on the analysis of experimental results on the elliptic flow and the high momentum suppression of the open charm and beauty hadrons.

Phenomenological analysis of rapidity distribution of negative pions in central 12C+12C collisions at $\sqrt{s_{nn}} = 3.14\, {\rm GeV}$

Various aspects of the simple phenomenological model, the grand combinational model (GCM), proposed earlier for the systematic description of the center-of-mass (cm) rapidity distributions of different particles produced in high energy heavy ion collisions, were analyzed. The values of GCM parameters were extracted from fitting the cm rapidity distributions of the negative pions in 12 C +12 C collisions at [Formula: see text] both in the experiment and using Modified FRITIOF Model. The GCM parameters extracted for the central 12 C +12 C collisions were compared with those obtained in central Pb + Pb collisions at super proton synchrotron (SPS) and alternating gradient synchrotron (AGS) energies between [Formula: see text] and [Formula: see text] and in central Au + Au collisions at Relativistic heavy ion collider (RHIC) energies between [Formula: see text] and [Formula: see text]. The plausible physical interpretations for the GCM parameters were given. The initial assumption that the parameter β of GCM should be zero for symmetric systems with identical colliding nuclei was validated. The parameter γ of GCM was deduced to follow an approximate asymptotic behavior (γ → 0 as [Formula: see text] at very large cm energies, and γ ≅ 0 could possibly be related to complete dehadronization of the whole collision system, along with attaining its maximum possible energy density, in central collisions of identical nuclei. The behavior of cm energy dependence of γ suggested that it could possibly be sensitive to deconfinement phase transition.

Importance of different energy loss effects in jet suppression at the RHIC and the LHC

Jet suppression is considered to be an excellent probe of quantum chromodynamic (QCD) matter created in ultra-relativistic heavy ion collisions. Our theoretical predictions of jet suppression, which are based on our recently developed dynamical energy loss formalism, show a robust agreement with various experimental data, which spans across different probes, experiments (Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC)) and experimental conditions (i.e. all available centrality regions). This formalism includes several key ingredients, such as the inclusion of dynamical scattering centers, a finite size QCD medium, collisional energy loss, finite magnetic mass and running coupling. While these effects have to be included based on theoretical grounds, it is currently unclear what their individual importance is in accurately interpreting the experimental data; in particular because other approaches to suppression predictions commonly neglect some—or all—of these effects. To address this question, we study the relative importance of these effects in obtaining accurate suppression predictions for D mesons (clear energy loss probe) at top RHIC and LHC energies. We obtain that several different ingredients are responsible for accurate predictions, i.e. robust agreement with the data is a cumulative effect of all the ingredients, though inclusion of the dynamical scattering centers has the largest relative importance.

Effects of viscosity on the mapping of initial to final state in heavy ion collisions

We investigate the correlation between various aspects of the initial geometry of heavy ion collisions at the Relativistic Heavy Ion Collider energies and the final anisotropic flow, using v-USPhydro, a 2+1 event-by-event viscous relativistic hydrodynamical model. We test the extent of which shear and bulk viscosity affect the prediction of the final flow harmonics, $v_n$, from the initial eccentricities, $\varepsilon_n$. We investigate in detail the flow harmonics $v_1$ through $v_5$ where we find that $v_1$, $v_4$, and $v_5$ are dependent on more complicated aspects of the initial geometry that are especially important for the description of peripheral collisions, including a non-linear dependence on eccentricities as well as a dependence on shorter-scale features of the initial density. Furthermore, we compare our results to previous results from NeXSPheRIO, a 3+1 relativistic ideal hydrodynamical model that has a non-zero initial flow contribution, and find that the combined contribution from 3+1 dynamics and non-zero, fluctuating initial flow decreases the predictive ability of the initial eccentricities, in particular for very peripheral collisions, but also disproportionately in central collisions.

On Direct Photon Production at RHIC and LHC-energies: A Theoretical Approach

The Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have recently studied different collisions at extremely high energies and produced a sizable amount of high-precision data. In the present work, we deal with the direct photon production phenomena in p + p, d + Au Au + Au collisions at RHIC energy p sNN = 200 GeV and in Pb + Pb-collisions at LHC energy p sNN = 2.76 T eV on the basis of Sequential Chain Model (SCM). Comparisons of the model-based results with the measured data on some observables are generally found to be modestly satisfactory.

Pion Transverse Momentum Spectrum, Elliptic Flow, and Interferometry in the Granular Source Model for RHIC and LHC Heavy Ion Collisions

We systematically investigate the pion transverse momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in the granular source model for the heavy ion collisions of Au-Au atsNN=200 GeV and Pb-Pb atsNN=2.76 TeV with different centralities. The granular source model can well reproduce the experimental results of the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). We examine the parameters involved in the granular source model. The experimental data of the momentum spectrum, elliptic flow, and HBT radii for the two collision energies and different centralities impose very strict constraints on the model parameters. They exhibit certain regularities for collision centrality and energy. The space-time structure and expansion velocities of the granular sources for the heavy ion collisions at the RHIC and LHC energies with different centralities are investigated.

Anisotropic flow in transport + hydrodynamics hybrid approaches

This contribution to the focus issue covers anisotropic flow in hybrid approaches. The historical development of hybrid approaches and their impact on the interpretation of flow measurements is reviewed. The major ingredients of a hybrid approach and the transition criteria between transport and hydrodynamics are discussed. The results for anisotropic flow in (event-by-event) hybrid approaches are presented. Some hybrid approaches rely on hadronic transport for the late stages for the reaction (so called afterburner) and others employ transport approaches for the early non-equilibrium evolution. In addition, there are ‘full’ hybrid calculations where a fluid evolution is dynamically embedded in a transport simulation. After demonstrating the success of hybrid approaches at high Relativistic Heavy Ion Collider and Large Hadron Collider energies, existing hybrid caluclations for collective flow observables at lower beam energies are discussed and remaining challenges outlined.

Correlations in the Monte Carlo Glauber model

Event-by-event fluctuations of observables are often modeled using the Monte Carlo Glauber model, in which the energy is initially deposited in sources associated with wounded nucleons. In this paper, we analyze in detail the correlations between these sources in proton-nucleus and nucleus-nucleus collisions. There are correlations arising from nucleon-nucleon correlations within each nucleus, and correlations due to the collision mechanism, which we dub twin correlations. We investigate this new phenomenon in detail. At the Brookhaven Relativistic Heavy Ion Collider and CERN Large Hadron Collider energies, correlations are found to have modest effects on size and eccentricity fluctuations, such that the Glauber model produces to a good approximation a collection of independent sources.

Mach cones in viscous heavy-ion collisions

The formation of Mach cones is studied in a full $(3+1)$-dimensional setup of ultrarelativistic heavy-ion collisions, considering a transverse and longitudinal expanding medium at Relativistic Heavy-Ion Collider energies. For smooth initial conditions and central collisions the jet-medium interaction is investigated using high-energy jets and various values of the ratio of shear viscosity over entropy density, $\ensuremath{\eta}/s$. For small viscosities, the formation of Mach cones is proven, whereas for larger viscosities the characteristic structures smear out and vanish eventually. The formation of a double-peak structure both in a single- and in a multiple-jet event is discussed.

Transverse energy and charged particle production in heavy-ion collisions: From RHIC to LHC

We study the charged particle and transverse energy production mechanism from AGS, SPS, Relativistic Heavy-Ion Collider (RHIC) to Large Hadron Collider (LHC) energies in the framework of nucleon and quark participants. At RHIC and LHC energies, the number of nucleons-normalized charged particle and transverse energy density in pseudorapidity, which shows a monotonic rise with centrality, turns out to be an almost centrality independent scaling behavior when normalized to the number of participant quarks. A universal function which is a combination of logarithmic and power-law, describes well the charged particle and transverse energy production both at nucleon and quark participant level for the whole range of collision energies. Energy dependent production mechanisms are discussed both for nucleonic and partonic level. Predictions are made for the pseudorapidity densities of transverse energy, charged particle multiplicity and their ratio (the barometric observable, [Formula: see text]) at mid-rapidity for Pb + Pb collisions at [Formula: see text]. A comparison with models based on gluon saturation and statistical hadron gas is made for the energy dependence of [Formula: see text].

Chemical Potentials of Quarks Extracted from Particle Transverse Momentum Distributions in Heavy Ion Collisions at RHIC Energies

In the framework of a multisource thermal model, the transverse momentum distributions of charged particles produced in nucleus-nucleus (A-A) and deuteron-nucleus (d-A) collisions at relativistic heavy ion collider (RHIC) energies are investigated by a two-component revised Boltzmann distribution. The calculated results are in agreement with the PHENIX experimental data. It is found that the source temperature increases obviously with increase of the particle mass and incident energy, but it does not show an obvious change with the collision centrality. Then, the values of chemical potentials for up, down, and strange quarks can be obtained from the antiparticle to particle yield ratios in a wide transverse momentum range. The relationship between the chemical potentials of quarks and the transverse momentum with different centralities is investigated, too.

Nuclear Stopping in Central Au+Au Collisions at RHIC Energies

Nuclear stopping in central Au+Au collisions at relativistic heavy-ion collider (RHIC) energies is studied in the framework of a cascade mode and the modified ultrarelativistic quantum molecular dynamics (UrQMD) transport model. In the modified mode, the mean field potentials of both formed and “preformed” hadrons (from string fragmentation) are considered. It is found that the nuclear stopping is increasingly influenced by the mean-field potentials in the projectile and target regions with the increase of the reaction energy. In the central region, the calculations of the cascade model considering the modifying factor can describe the experimental data of the PHOBOS collaboration.