High-energy Heavy-ion Collisions Research Articles

Pseudorapidity distribution and decorrelation of anisotropic flow within the open-computing-language implementation CLVisc hydrodynamics

Studies of fluctuations and correlations of soft hadrons and hard and electromagnetic probes of the dense and strongly interacting medium require event-by-event hydrodynamic simulations of high-energy heavy-ion collisions that are computing intensive. We develop a (3+1)D viscous hydrodynamic model -- CLVisc that is parallelized on Graphics Processing Unit (GPU) using Open Computing Language (OpenCL) with 60 times performance increase for space-time evolution and more than 120 times for the Cooper-Frye particlization relative to that without GPU parallelization. The pseudo-rapidity dependence of anisotropic flow $v_n(\eta)$ are then computed in CLVisc with initial conditions given by the A Multi-Phase Transport (AMPT) model, with energy density fluctuations both in the transverse plane and along the longitudinal direction. Although the magnitude of $v_n(\eta)$ and the ratios between $v_2(\eta)$ and $v_3(\eta)$ are sensitive to the effective shear viscosity over entropy density ratio $\eta_v/s$, the shape of the $v_{n}(\eta)$ distributions in $\eta$ do not depend on the value of $\eta_v/s$. The decorrelation of $v_n$ along the pseudo-rapidity direction due to the twist and fluctuation of the event-planes in the initial parton density distributions is also studied. The decorrelation observable $r_n(\eta^a, \eta^b)$ between $v_n\{-\eta^a\}$ and $v_n\{\eta^a\}$ with the auxiliary reference window $\eta^b$ is found not sensitive to $\eta_v/s$ when there is no initial fluid velocity. For small $\eta_v/s$, the initial fluid velocity from mini-jet partons introduces sizable splitting of $r_n(\eta^a, \eta^b)$ between the two reference rapidity windows $\eta^b \in [3, 4]$ and $\eta^b \in [4.4, 5.0]$, as has been observed in experiment. The implementation of CLVisc and guidelines on how to efficiently parallelize scientific programs on GPUs are also provided.

Multiple jets and γ-jet correlation in high-energy heavy-ion collisions

γ-Jet production is considered one of the best probes of the hot quark–gluon plasma in high-energy heavy-ion collisions since the direct γ can be used to gauge the initial energy and momentum of the associated jet. This is investigated within the Linear Boltzmann Transport (LBT) model for jet propagation and jet-induced medium excitation. With both parton energy loss and medium response from jet-medium interaction included, LBT can describe experimental data well on γ-jet correlation in Pb+Pb collisions at the Large Hadron Collider. Multiple jets associated with direct γ production are found to contribute significantly to γ-jet correlation at small pTjet<pTγ and large azimuthal angle relative to the opposite direction of γ. Jet medium interaction not only suppresses the leading jet at large pTjet but also sub-leading jets at large azimuthal angle. This effectively leads to the narrowing of γ-jet correlation in azimuthal angle instead of broadening due to jet-medium interaction. The γ-jet profile on the other hand will be broadened due to jet-medium interaction and jet-induced medium response. Energy flow measurements relative to the direct photon is illustrated to reflect well the broadening and jet-induced medium response.

Correlation measurements between flow harmonics in Au+Au collisions at RHIC

Flow harmonics (vn) in the Fourier expansion of the azimuthal distribution of particles are widely used to quantify the anisotropy in particle emission in high-energy heavy-ion collisions. The symmetric cumulants, SC(m,n), are used to measure the correlations between different orders of flow harmonics. These correlations are used to constrain the initial conditions and the transport properties of the medium in theoretical models. In this Letter, we present the first measurements of the four-particle symmetric cumulants in Au+Au collisions at sNN=39 and 200 GeV from data collected by the STAR experiment at RHIC. We observe that v2 and v3 are anti-correlated in all centrality intervals with similar correlation strengths from 39 GeV Au+Au to 2.76 TeV Pb+Pb (measured by the ALICE experiment). The v2–v4 correlation seems to be stronger at 39 GeV than at higher collision energies. The initial-stage anti-correlations between second and third order eccentricities are sufficient to describe the measured correlations between v2 and v3. The best description of v2–v4 correlations at sNN=200GeV is obtained with inclusion of the system's nonlinear response to initial eccentricities accompanied by the viscous effect with η/s>0.08. Theoretical calculations using different initial conditions, equations of state and viscous coefficients need to be further explored to extract η/s of the medium created at RHIC.

Elliptic flow from Coulomb interaction and low density elastic scattering

In high energy heavy ion collisions and interacting cold atom systems, large elliptic flow anisotropies have been observed. For the large opacity ($\rho\sigma L\sim 10^{3}$) of the latter hydrodynamics is a natural consequence, but for the small opacity ($\rho\sigma L\sim 1$) of the former hydrodynamic description is questionable. To shed light onto the situation, we simulate the expansion of a low density Argon ion (or atom) system, initially trapped in an elliptical region, under the Coulomb interaction (or elastic scattering). Significant elliptic anisotropy is found in both cases, and the anisotropy depends on the initial spatial eccentricity and the density of the system. The results may provide insights into the physics of anisotropic flow in high energy heavy ion collisions and its role in the study of quantum chromodynamics.

A flow paradigm in heavy-ion collisions**Supported by Natural Sciences and Engineering Research Council of Canada

The success of hydrodynamics in high energy heavy-ion collisions leads to a flow paradigm, to understand the observed features of harmonic flow in terms of the medium collective expansion with respect to initial state geometrical properties. In this review, we present some essential ingredients in the flow paradigm, including the hydrodynamic modeling, the characterization of initial state geometry and the medium response relations. The extension of the flow paradigm to small colliding systems is also discussed.

Strange particle production and hypernucleus formation in medium and high energy heavy-ion collisions

The nuclear matter at supra-saturation densities might be created in medium and high energy heavy-ion collisions. The strangeness ingredient in dense nuclear matter impacts the equation of state and maybe exists in the neutron stars. On the other hand, heavy-ion collisions provide a unique way for producing the neutron-rich hypernuclides. In this article, the new progress on strangeness physics in heavy-ion collisions is reviewed. Possible experiments at the high-intensity heavy-ion accelerator facility (HIAF) are to be discussed, in particular on the topics of the high-density nuclear equation of state and hypernuclide in the neutron-rich domain. Within the framework of the Lanzhou quantum molecular dynamics (LQMD) model, the strangeness production and high-density nuclear equation of state in heavy-ion collisions near threshold energies has been investigated, i.e., for the reactions of $^{58}$Ni+$^{58}$Ni and $^{197}$Au+$^{197}$Au. The formation mechanism of $\Lambda$-hyperfragments in heavy-ion collisions is to be investigated. A coalescence approach is developed for constructing the primary fragments in phase space. The secondary decay process of the fragments is described by the well-known statistical code. The impacts of nuclear equation of state on the hyperfragment formation are discussed. The mass, charge and kinetic energy spectra of hypernuclides produced in heavy-ion collisions are thoroughly analyzed. It is found that the kaon yields in the high kinetic energy domain are useful for extracting the high-density nuclear equation of state. The $K^{+}$ yields are strongly suppressed with a hard equation of state. However, the hyperon production is weakly influenced by the incompressibility of nuclear matter. The $\Lambda$-nucleon potential impacts the hypernuclide formation, i.e., the attractive potential enhancing the hypernuclide yields. The heavy-ion collisions are available to produce the light mass neutron-rich hypernuclides and the mid-central collisions are favorable for the medium mass hypernuclide formation. Free hyperons are captured by nucleonic fragments to form hypernuclides in the domain of lower kinetic energies and narrower rapidities. A soft equation of state is available for producing the hyperfragments. The results are very valuable for the forthcoming hypernuclide physics at HIAF.

Transverse energy per charged particle in heavy-ion collisions: Role of collective flow

The ratio of (pseudo)rapidity density of transverse energy and the (pseudo)rapidity density of charged particles, which is a measure of the mean transverse energy per particle, is an important observable in high energy heavy-ion collisions, which reveals about the mechanism of particle production and the freeze-out criteria. Its collision energy and centrality dependence is exactly like the chemical freeze-out temperature till top RHIC energy. The LHC measurement at $\sqrt{s_{NN}}$ = 2.76 TeV brings up new challenges to rule out the mechanisms of gluon saturation or non-equilibrium phenomena being prevalent at high energies, which could contribute to the above observable. The Statistical Hadron Gas Model (SHGM) with a static fireball approximation has been successful in describing both the centrality and energy dependence till top RHIC energies. However, the SHGM predictions for higher energies are highly underestimated by the LHC data. In order to understand this, we have incorporated radial flow effect in an excluded volume SHGM. The hard-core radius of baryons at lower collision energies plays an important role in the description of a hadronic system. In view of this, in order to make a complete energy dependence study from FAIR to LHC energies, we have considered an excluded volume SHGM. Our studies suggest that the collective flow plays an important role in describing $E_{T}/N_{ch}$ and it could be one of the possible parameters to explain the jump observed in $E_{T}/N_{ch}$ from RHIC to LHC energies. Predictions for the LHC measurements at $\sqrt{s_{NN}}$ = 5.02 TeV are given.

Transverse momentum spectra of hadrons in high energy pp and heavy ion collisions

We present a study of transverse momentum (pT) spectra of unidentified charged particles in pp collisions at RHIC and LHC energies from to 13 TeV using Tsallis/Hagedorn function. The power law of Tsallis/Hagedorn form gives very good description of the hadron spectra in pT range from 0.2 to 300 GeV/c. The power index n of the pT distributions is found to follow a function of the type with asymptotic value a = 6.8. The parameter T governing the soft bulk contribution to the spectra remains almost same over wide range of collision energies. We also provide a Tsallis/Hagedorn fit to the pT spectra of hadrons in pPb and different centralities of PbPb collisions at . The data/fit shows deviations from the Tsallis distribution which become more pronounced as the system size increases. We suggest simple modifications in the Tsallis/Hagedorn power law function and show that the above deviations can be attributed to the transverse flow in low pT region and to the in-medium energy loss in high pT region.

Unruh thermal hadronization and the cosmological constant

We use black holes with a negative cosmological constant to investigate aspects of the freeze-out temperature for hadron production in high energy heavy-ion collisions. The two black hole solutions present in the anti-de Sitter geometry have different mass and are compared to the data showing that the small black hole solution is in good agreement. This is a new feature in the literature since the small black hole in general relativity has different thermodynamic behavior from that of the large black hole solution. We find that the inclusion of the cosmological constant (which can be interpreted as the plasma pressure) leads to a lowering of the temperature of the freeze-out curve as a function of the baryochemical potential, improving the description previously suggested by Castorina, Kharzeev, and Satz.

Nuclear dynamics and particle production near threshold energies in heavy-ion collisions

Recent progress of the quantum molecular dynamics model for describing the dynamics of heavy-ion collisions is viewed, in particular the nuclear fragmentation, isospin physics, particle production and in-medium effect, hadron-induced nuclear reactions, hypernucleus etc. The neck fragmentation in Fermi-energy heavy-ion collisions is investigated for extracting the symmetry energy at subsaturation densities. The isospin effects, in-medium properties and the behavior of high-density symmetry energy in medium and high energy heavy-ion collisions are thoroughly discussed. The hypernuclide dynamics formed in heavy-ion collisions and in hadron induced reactions is analyzed and addressed in the future experiments at the High-Intensity heavy-ion Accelerator Facility (HIAF).

Enhancement of elliptic flow can signal a first-order phase transition in high-energy heavy-ion collisions

The beam energy dependence of the elliptic flow,$v_2$, is studied in mid-central Au+Au collisions in the energy range of $3\leq \sqrt{s_{NN}} \leq 30$ GeV within the microscopic transport model JAM. The results of three different modes of JAM are compared; cascade-,hadronic mean field-, and a new mode with modified equations of state, with a first order phase transition (1.O.P.T.) and with a crossover transition. The standard hadronic mean field suppresses $v_2$, while the inclusion of the effects of a 1.O.P.T. (and also of a crossover transition) does enhance $v_2$ at $\sqrt{s_{NN}}<30$ GeV. The enhancement or suppression of the scaled energy flow, dubbed "elliptic flow"is understood as being due to out of plane- flow, i.e. $v_2<0$, dubbed out of plane - "squeeze-out", which occurs predominantly in the early, compression stage. Subsequently, the in-plane flow dominates, in the expansion stage, $v_2 > 0$. The directed flow, dubbed "bounce- off", is an independent measure of the pressure, which quickly builds up the transverse momentum transfer in the reaction plane. When the spectator matter leaves the participant fireball region, where the highest compression occurs, a hard expansion leads to larger $v_2$. A combined analysis of the three transverse flow coefficients, radial $v_0$-, directed $v_1$- and elliptic $v_2$- flow, in the beam energy range of $3\leq\sqrt{s_{NN}}\leq10$ GeV, distinguishes the different compression and expansion scenarios: a characteristic dependence on the early stage equation of state is observed. The enhancement of both the elliptic and the transverse radial flow and the simultaneous collapse of the directed flow of nucleons offers a clear signature if 1.O.P.T. is realized at the highest baryon densities created in high energy heavy-ion collisions.

Causal electric charge diffusion and balance functions in relativistic heavy-ion collisions

We study the propagation and diffusion of electric charge fluctuations in high-energy heavy-ion collisions using the Cattaneo form for the dissipative part of the electric current. As opposed to the ordinary diffusion equation this form limits the speed at which charge can propagate. Including the noise term in the current, which arises uniquely from the fluctuation-dissipation theorem, we calculate the balance functions for charged hadrons in a simple 1+1-dimensional Bjorken hydrodynamical model. Limiting the speed of propagation of charge fluctuations increases the height and reduces the width of these balance functions when plotted versus rapidity. We also estimate the numerical value of the associated diffusion time constant from anti--de Sitter-space/conformal-field theory.

An equation-of-state-meter of quantum chromodynamics transition from deep learning

A primordial state of matter consisting of free quarks and gluons that existed in the early universe a few microseconds after the Big Bang is also expected to form in high-energy heavy-ion collisions. Determining the equation of state (EoS) of such a primordial matter is the ultimate goal of high-energy heavy-ion experiments. Here we use supervised learning with a deep convolutional neural network to identify the EoS employed in the relativistic hydrodynamic simulations of heavy ion collisions. High-level correlations of particle spectra in transverse momentum and azimuthal angle learned by the network act as an effective EoS-meter in deciphering the nature of the phase transition in quantum chromodynamics. Such EoS-meter is model-independent and insensitive to other simulation inputs including the initial conditions for hydrodynamic simulations.

Measurements of jets in heavy ion collisions

The Quark Gluon Plasma (QGP) is created in high energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). This medium is transparent to electromagnetic probes but nearly opaque to colored probes. Hard partons produced early in the collision fragment and hadronize into a collimated spray of particles called a jet. The partons lose energy as they traverse the medium, a process called jet quenching. Most of the lost energy is still correlated with the parent parton, contributing to particle production at larger angles and lower momenta relative to the parent parton than in proton-proton collisions. This partonic energy loss can be measured through several observables, each of which give different insights into the degree and mechanism of energy loss. The measurements to date are summarized and the path forward is discussed.

A Description of Pseudorapidity Distributions of Charged Particles Produced in Au+Au Collisions at RHIC Energies

In heavy ion collisions, charged particles come from two parts: the hot and dense matter and the leading particles. In this paper, the hot and dense matter is assumed to expand according to the hydrodynamic model including phase transition and decouples into particles via the prescription of Cooper-Frye. The leading particles are as usual supposed to have Gaussian rapidity distributions with the number equaling that of participants. The investigations of this paper show that, unlike low energy situations, the leading particles are essential in describing the pseudorapidity distributions of charged particles produced in high energy heavy ion collisions. This might be due to the different transparencies of nuclei at different energies.

Equation of state of quark-gluon plasma using a simple phenomenological model

The equation of state (EoS) of quark-gluon plasma (QGP) using a phenomenological model is studied in which finite value of quark mass is modified as effective mass. The effective mass of these quasiparticle generated due to the interaction of quarks and gluons with the surrounding matter in the medium. The model results provide EoS of QGP which are in good agreement and found almost similar results to the earlier theoretical results. This model is successfully applied to the description of the properties of quark-gluon plasma created in the collision of nucleons. Thus, the effective mass of quark shows the useful information to study the EoS of QGP in high energy heavy-ion collisions.

Phenomenology of anomalous chiral transports in heavy-ion collisions

High-energy Heavy-ion collisions can generate extremely hot quark-gluon matter and also extremely strong magnetic fields and fluid vorticity. Once coupled to chiral anomaly, the magnetic fields and fluid vorticity can induce a variety of novel transport phenomena, including the chiral magnetic effect, chiral vortical effect, etc. Some of them require the environmental violation of parity and thus provide a means to test the possible parity violation in hot strongly interacting matter. We will discuss the underlying mechanism and implications of these anomalous chiral transports in heavy-ion collisions.

Corrigendum to “Charged Particle, Photon Multiplicity, and Transverse Energy Production in High-Energy Heavy-Ion Collisions”

Corrigendum to “Charged Particle, Photon Multiplicity, and Transverse Energy Production in High-Energy Heavy-Ion Collisions”

Reexamining the gluon spectrum in the boost-invariant glasma from a semianalytic approach

In high energy heavy-ion collisions, the degrees of freedom at the very early stage can be effectively represented by strong classical gluonic fields within the Color Glass Condensate framework. As the system expands, the strong gluonic fields eventually become weak such that an equivalent description using the gluonic particle degrees of freedom starts to become valid. We revisit the spectrum of these gluonic particles by solving the classical Yang-Mills equations semi-analytically with the solutions having the form of power series expansions in the proper time. We propose a different formula for the gluon spectrum which is consistent with energy density during the whole time evolution. We find that the chromo-electric fields have larger contributions to the gluon spectrum than the chromo-magnetic fields do. Furthermore, the large momentum modes take less time to reach the weak-field regime while smaller momentum modes take more time. The resulting functional form of the gluon spectrum is exponential in nature and the spectrum is close to a thermal distrubtion with effective temperatures around $0.6$ to $0.9\, Q_s$ late in the Glasma evolution. The sensitiveness of the gluon spectrum to the infrared and the ultraviolet cut-offs are discussed.

Heavy and light flavor jet quenching at RHIC and LHC energies

The Linear Boltzmann Transport (LBT) model coupled to hydrodynamical background is extended to include transport of both light partons and heavy quarks through the quark–gluon plasma (QGP) in high-energy heavy-ion collisions. The LBT model includes both elastic and inelastic medium-interaction of both primary jet shower partons and thermal recoil partons within perturbative QCD (pQCD). It is shown to simultaneously describe the experimental data on heavy and light flavor hadron suppression in high-energy heavy-ion collisions for different centralities at RHIC and LHC energies. More detailed investigations within the LBT model illustrate the importance of both initial parton spectra and the shapes of fragmentation functions on the difference between the nuclear modifications of light and heavy flavor hadrons. The dependence of the jet quenching parameter qˆ on medium temperature and jet flavor is quantitatively extracted.