Model For Heavy-ion Collisions Research Articles

Initial-state anisotropies and their uncertainties in ultrarelativistic heavy-ion collisions from the Monte Carlo Glauber model

In hydrodynamical modeling of heavy-ion collisions, the initial-state spatial anisotropies are translated into momentum anisotropies of the final-state particle distributions. Thus, understanding the origin of the initial-state anisotropies and their uncertainties is important before extracting specific QCD matter properties, such as viscosity, from the experimental data. In this work we review the wounded nucleon approach based on the Monte Carlo Glauber model, charting in particular the uncertainties arising from modeling of the nucleon-nucleon interactions between the colliding nucleon pairs and nucleon-nucleon correlations inside the colliding nuclei. We discuss the differences between the black disk model and a probabilistic profile function approach for the inelastic nucleon-nucleon interactions, and investigate the influence of initial-state correlations using state-of-the-art modeling of these.

Colour field flux tubes and Casimir scaling for various [formula omitted] representations

We investigate the QCD flux tubes linking static colour charges of different SU(3) representations, relevant to the understanding of confinement and of the Lund strings of Heavy Ion Collisions. The colour field densities, the Casimir scaling factors and the widths for the flux tubes of the first five different representations are computed in quenched SU(3) Lattice QCD. This study is relevant to understand the mechanisms of confinement and also the flux tubes utilized in the Lund Model and in other models of Heavy Ion Collisions.

Resonances from AMPT

A multiphase transport (AMPT) model that includes both initial partonic and final hadron rescattering effects in relativistic heavy ion collisions is briefly reviewed. Particular attention is given to the scattering processes in the hadronic phase in order to address resonances production in these collisions. As an example, phi meson production is discussed in detail through both its dikaon and dilepton decay channels. Results from the AMPT model for heavy ion collisions at both SPS and RHIC are compared to the experimental data to extract information on the production mechanism and in-medium properties of the phi meson.

Modeling of heavy ion collisions using radial basis function and generalized regression neural networks

Artificial neural networks (ANNs) have been applied to heavy ion collisions. In the present work, the possibility of using ANN methods for modeling the multiplicity distributions, P(ns), of shower particles produced from p, d, 4He, 6Li, 7Li, 12C, 16O, and 24Mg interactions with light (CNO) as well as heavy (AgBr) emulsions at 4.5 A GeV/c was investigated. Two different ANN approaches, namely radial basis function neural network (RBFNN) and generalized regression neural network (GRNN), were employed to obtain a mathematical formula describing these collisions. The results from RBFNN and GRNN models showed good agreement with the experimental data. GRNN models have a better performance than the RBFNN models. This study showed that the RBFNN and GRNN models are capable of accurately predicting the P(ns) of shower particles in the training and testing phases.

Heavy quarkonia production in p+p, d+Au and Cu+Cu collisions by PHENIX experiment at $\sqrt {s_{NN} } $ = 200 GeV

The PHENIX experiment has measured, J/ψ, ψ′ and γ productions for different collision systems in the forward rapidities 1.2 < | η | < 2.2 at \(\sqrt {S_{NN} } \) = 200 GeV. We have observed significant suppressions of J/ψ production in both Cu+Cu and Au+Au collisions relative to the yield in p+p system. The measurements of higher mass heavy quarkonia states (ψ′ and γ) will help us to constrain various quarkonium suppression models in heavy ion collisions. A first hint of ψ′ and γ productions in 200GeV p+p collisions has been observed at forward and backward rapidities at PHENIX.

Longitudinal scaling of observables in heavy-ion collision models

Longitudinal scaling of pseudorapidity distribution of charged particles (${\mathit{dN}}_{\mathrm{ch}}/d\ensuremath{\eta}$) is observed when presented as a function of pseudorapidity ($\ensuremath{\eta}$) shifted by the beam rapidity ($\ensuremath{\eta}\ensuremath{-}{y}_{\mathrm{beam}}$) for a wide range of collision systems (${e}^{+}+{e}^{\ensuremath{-}}$, $p+p$, $d+\text{A}$, and $\text{A}+\text{A}$) and beam energies. Such a scaling is also observed for the elliptic flow (${v}_{2}$) of charged hadrons in $\text{A}+\text{A}$ collisions. This is a striking observation, as ${v}_{2}$ is expected to be sensitive to the initial conditions, the expansion dynamics, and the degrees of freedom of the system, all of which potentially varies with collision system and colliding energies. We present a study of the longitudinal scalings of ${\mathit{dN}}_{\mathrm{ch}}/d\ensuremath{\eta}$, average transverse momentum ($\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$), and ${v}_{2}$ using transport models UrQMD and AMPT for Au $+$ Au collisions at center of mass energies ($\sqrt{{s}_{\mathrm{NN}}}$) of 19.6, 62.4, and 200 GeV and Pb $+$ Pb collisions at 2760 GeV. Only the AMPT models which includes partonic effects and quark coalescence as a mechanism of hadronization shows longitudinal scaling for ${\mathit{dN}}_{\mathrm{ch}}/d\ensuremath{\eta}$, $\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$, and ${v}_{2}$. Whereas the UrQMD and AMPT default versions show longitudinal scaling only for ${\mathit{dN}}_{\mathrm{ch}}/d\ensuremath{\eta}$ and $\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$. We also discuss the possibility of longitudinal scaling of ${v}_{2}$ within two extreme scenarios of models with hydrodynamic and collisionless limits. We find the longitudinal scaling of bulk observables to be an important test for the underlying physics mechanism in models of particle production.

Energy dependence of elliptic flow from heavy-ion collision models

We have compared the experimental data on charged particle elliptic flow parameter (v2) in Au+Au collisions at midrapidity for \surd sNN = 9.2, 19.6, 62.4 and 200 GeV with results from various models in heavy-ion collisions like UrQMD, AMPT, and HIJING. We observe that the average <v2> from the transport model UrQMD agrees well with the measurements at \surd sNN = 9.2 GeV but increasingly falls short of the experimental <v2> values as the beam energy increases. The difference in <v2> being of the order of 60% at \surd sNN = 200 GeV. The <v2> results from HIJING is consistent with zero, while those from AMPT with default settings, a model based on HIJING with additional initial and final state rescattering effects included, gives a <v2> value of about 4% for all the beam energies studied. This is in contrast to increase in <v2> with beam energy for the experimental data. A different version of the AMPT model, which includes partonic effects and quark coalescence as a mechanism of hadronization, gives higher values of <v2> among the models studied and is in agreement with the measured <v2> values at \surd sNN = 200 GeV. These studies show that the experimental < v2 > has substantial contribution from partonic interactions at \surd sNN = 200 GeV whose magnitude reduces with decrease in beam energy. We also compare the available data on the transverse momentum and pseudorapidity dependence of v2 to those from the above models.

Initial conditions and global event properties from color glass condensate

Perturbative unitarization from non-linear effects is thought to deplete the gluon density for transverse momenta below the saturation scale. Such effects also modify the distribution of gluons produced in heavy-ion collisions in transverse impact parameter space. I discuss some of the consequences for the initial conditions for hydrodynamic models of heavy-ion collisions and for hard “tomographic” probes. Also, I stress the importance of realistic modelling of the fluctuations of the valence sources for the small- x fields in the impact parameter plane. Such models can now be combined with solutions of running–coupling Balitsky–Kovchegov evolution to obtain controlled predictions for initial conditions at the LHC.

Matching stages of heavy-ion collision models

Heavy ion reactions and other collective dynamical processes are frequently described by different theoretical approaches for the different stages of the process, like initial equilibration stage, intermediate locally equilibrated fluid dynamical stage and final freeze-out stage. For the last stage the best known is the Cooper-Frye description used to generate the phase space distribution of emitted, non-interacting, particles from a fluid dynamical expansion/explosion, assuming a final ideal gas distribution, or (less frequently) an out of equilibrium distribution. In this work we do not want to replace the Cooper-Frye description, rather clarify the ways how to use it and how to choose the parameters of the distribution, eventually how to choose the form of the phase space distribution used in the Cooper-Frye formula. Moreover, the Cooper-Frye formula is used in connection with the freeze-out problem, while the discussion of transition between different stages of the collision is applicable to other transitions also. More recently hadronization and molecular dynamics models are matched to the end of a fluid dynamical stage to describe hadronization and freeze-out. The stages of the model description can be matched to each other on spacetime hypersurfaces (just like through the frequently used freeze-out hypersurface). This work presents a generalized description of how to match the stages of the description of a reaction to each other, extending the methodology used at freeze-out, in simple covariant form which is easily applicable in its simplest version for most applications.

Lattice QCD and hydro/cascade model of heavy ion collisions

We report here on a recent lattice study of the QCD transition region at finite temperature and zero chemical potential using domain wall fermions (DWF). We also present a parameterization of the QCD equation of state obtained from lattice QCD that is suitable for use in hydrodynamics studies of heavy ion collisions. Finally, we show preliminary results from a multi-stage hydrodynamics/hadron cascade model of a heavy ion collision, in an attempt to understand how well the experimental data (e.g. particle spectra, elliptic flow, and HBT radii) can constrain the inputs (e.g. initial temperature, freezeout temperature, shear viscosity, equation of state) of the theoretical model.

The HotQCD Equation of State

We present results from recent calculations of the QCD equation of state by the HotQCD Collaboration and review the implications for hydrodynamic modeling. The equation of state of QCD at zero baryon density was calculated on a lattice of dimensions 32 3 × 8 with m l = 0.1 m s (corresponding to a pion mass of ∼ 220 MeV ) using two improved staggered fermion actions, p4 and asqtad. Calculations were performed along lines of constant physics using more than 100M cpuhours on BG/L supercomputers at LLNL, NYBlue, and SDSC. We present parameterizations of the equation of state suitable for input into hydrodynamics models of heavy ion collisions.

Hadronization via recombination

The recombination model as a model for hadronization from a quark–gluon plasma has been recently revived since it has advantages in explaining several important features of the final state produced in heavy-ion collisions at the RHIC, such as the constituent quark number scaling of the elliptic coefficient versus the transverse energy of identified hadrons, the bending shape of the pT spectrum of hadrons near 5 GeV/c, and the measured large value of baryon-to-meson ratio (of the order of unity) in the same pT range. We have developed a dynamic simulation model of heavy-ion collisions in which a quark–gluon plasma, starting from a certain initial condition, evolves hydrodynamically until it reaches the phase boundary, and then hadronizes by valence quark recombination. Rescattering after hadronization is described by UrQMD. We discuss some details of the model and report first, preliminary results.

Dilepton spectra in p+p and Au+Au collisions at RHIC

This work presents measurements of electron-positron pairs from p+p collisions at sqrt(s) = 200 GeV collected during the 2005 RHIC run and compares them to results from Au+Au collisions at sqrt(s_NN) = 200 GeV taken in 2004 with the PHENIX detector. The invariant mass distribution of e+e- pairs in p+p is consistent with the expected contributions from Dalitz decays of light hadrons, dielectron decays of vector mesons and correlated charm production, which have been measured in the same experiment. The charm and bottom cross section extracted from the measured dielectron yield are sigma_cc = 544 +/- 39(stat.) +/- 142(syst.) +/- 200(model) microbarns and sigma_bb = 3.9 +/- 2.4(stat.) +3/-2(syst.) microbarns, respectively. In min. bias Au+Au collisions the yield of dielectrons in the low mass region is enhanced by a factor of 4.0 +/- 0.3(stat.) +/- 1.5(syst.) +/- 0.8(model) compared to the known hadronic sources. The excess dominates the yield in the transverse momentum region below 1 GeV/c and shows significantly lower average p_T than the expected sources. The low p_T enhancement is currently not understood by any theoretical model of heavy ion collisions. The enhancement extends to larger transverse momenta where it is also observed in p+p and explained by virtual direct photons. The p+p measurement serves as an important test to pQCD calculations of direct photon production from hard scattering processes in this momentum range. An excess with an inverse slope of T_eff = 221 +/- 23(stat.) +/- 18(syst.) MeV is observed in central Au+Au collisions above the binary scaled direct photon yield in p+p. This can be qualitatively explained by hydrodynamical models including thermal photon radiation with initial temperatures of 300 - 600 MeV and formation times of 0.12 - 0.6 fm/c.

Correlations and fluctuations of conserved charges in a dynamical recombination approach

We investigate the effect of the dynamical recombination of quarks on the correlations and fluctuations of conserved charges within the quark molecular dynamics (qMD) model. Charged particles ratio fluctuations, charge transfer fluctuations and the baryon number–strangeness correlation coefficient CBS are computed as a function of time from a sample of central Au+Au qMD events at AGeV. We show that all studied observables, initially compatible with the quark phase expectation values, vanish with time to the hadronic result. We trace back the disappearance of the signals to the recombination–hadronization procedure implemented in qMD. Various ratios of susceptibilities (or conversely the ratios of correlations over fluctuations) were recently measured on the lattice and can allow us to distinguish between different models of heavy ion collisions. We show that the qMD model yields a reasonable agreement with these calculations.

Erratum: Comparisons of statistical multifragmentation and evaporation models for heavy-ion collisions

Erratum: Comparisons of statistical multifragmentation and evaporation models for heavy-ion collisions

Comparisons of statistical multifragmentation and evaporation models for heavy-ion collisions

The results from ten statistical multifragmentation models have been compared with each other using selected experimental observables. Even though details in any single observable may differ, the general trends among models are similar. Thus, these models and similar ones are very good in providing important physics insights especially for general properties of the primary fragments and the multifragmentation process. Mean values and ratios of observables are also less sensitive to individual differences in the models. In addition to multifragmentation models, we have compared results from five commonly used evaporation codes. The fluctuations in isotope yield ratios are found to be a good indicator to evaluate the sequential decay implementation in the code. The systems and the observables studied here can be used as benchmarks for the development of statistical multifragmentation models and evaporation codes.

No-recoil approximation to the knock-on exchange potential in the double folding model for heavy-ion collisions

We propose the no-recoil approximation, which is valid for heavy systems, for a double folding nucleus-nucleus potential. With this approximation, the non-local knock-on exchange contribution becomes a local form. We discuss the applicability of this approximation for the elastic scattering of $^6$Li + $^{40}$Ca system. We find that, for this system and heavier, the no-recoil approximation works as good as another widely used local approximation which employs a local plane wave for the relative motion between the colliding nuclei. We also compare the results of the no-recoil calculations with those of the zero-range approximation often used to handle the knock-on exchange effect.

Hydrodynamic Models for Heavy Ion Collisions

▪ Abstract Application of hydrodynamics for modeling of heavy ion collisions is reviewed. We consider several physical observables that can be calculated in this approach and compare them to the experimental measurements.

Fluctuations in the statistical model of relativistic heavy ion collisions

Fluctuations in the statistical model of heavy ion collisions are studied. The role of statistics, relativity, constraints, decaying resonances and branching processes are investigated using this model. Also studied are thermodynamic properties related to these fluctuations with specific concerns centered around the behavior of the specific heat.

Apparent Thermalization due to Plasma Instabilities in the Quark-Gluon Plasma

Hydrodynamical modeling of heavy-ion collisions at RHIC suggests that the quark-gluon plasma (QGP) "thermalizes" in a remarkably short time scale, about 0.6 fm/c. We argue that this should be viewed as indicating fast isotropization, but not necessarily complete thermalization, of the nonequilibrium QGP. Non-Abelian plasma instabilities can drive local isotropization of an anisotropic QGP on a time scale which is faster than ordinary perturbative scattering processes. As a result, we argue that theoretical expectations based on weak-coupling analysis are not necessarily in conflict with hydrodynamic modeling of the early part of RHIC collisions, provided one recognizes the key role of non-Abelian plasma instabilities.