Dynamics Of Heavy-ion Collisions Research Articles

Collision-geometry fluctuations and triangular flow in heavy-ion collisions

We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multiphase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal correlation functions. Using two-particle azimuthal correlations at large pseudorapidity separations measured by the PHOBOS and STAR experiments, we show that this Fourier component is also present in data. Ratios of the second and third Fourier coefficients in data exhibit similar trends as a function of centrality and transverse momentum as in AMPT calculations. These findings suggest a significant contribution of triangular flow to the ridge and broad away-side features observed in data. Triangular flow provides a new handle on the initial collision geometry and collective expansion dynamics in heavy-ion collisions.

Early Time Dynamics in Heavy Ion Collisions from CGC and from AdS/CFT

We review two different theoretical approaches to the strong interaction dynamics at the early times immediately following heavy ion collisions. One approach is based on small-coupling physics of the Color Glass Condensate (CGC). The other approach is based on Anti-de Sitter space/Conformal Field Theory (AdS/CFT) correspondence and may be applicable to describing large-coupling QCD interactions. We point out that in terms of theoretical tools the two approaches are somewhat similar: in CGC one deals with classical gluon fields produced in a nuclear shock wave collision, while in AdS/CFT one studies classical gravity in a gravitational shock wave collision. We stress, however, that the resulting physics is different: the classical gluon fields in CGC lead to a free-streaming medium produced in heavy ion collisions, while the classical gravity in the 5-dimensional AdS bulk is likely to lead to ideal hydrodynamics description of the produced medium. Also, the valence quarks in colliding nuclei in CGC continue along their light cone trajectories after the collision with very little recoil, while we show that in AdS the colliding nuclei are likely to lose most of their energy in the collision and stop.

System size, energy and centrality dependence of strange hadron elliptic flow at STAR

The elliptic flow (v2) pattern in terms of hadron mass and transverse momentum pT is qualitatively described for pT< 2 GeV/c by ideal hydrodynamics in Au + Au collisions at the Relativistic Heavy Ion Collider (RHIC). In addition, for pT = 2–6 GeV/c the measured v2 follow a universal scaling by the number of quarks explained by quark coalescence/recombination models. These observations suggest that a partonic collectivity develops in the matter in early stage of heavy ion collisions. Centrality as well as system size and energy dependence of v2 is important to shed light on the underlying collision dynamics in heavy ion collisions. We present the measurements of centrality dependence of v2 at and 62.4 GeV in Au + Au and Cu + Cu collisions for K0S, ϕ, Λ, Ξ and Ω at STAR experiment. We focus on the recent Cu + Cu results and discuss the centrality dependence of v2 as well as the number of quark scaling as a function of transverse kinetic energy at different system size and energies. We also discuss the eccentricity scaled v2 for identified hadrons and implications that ideal hydrodynamical limit has not been reached at the RHIC.

Parton collectivity from the RHIC to the LHC

Anisotropic flow is recognized as one of the main observables providing information on the early dynamics in heavy-ion collisions. The large elliptic flow observed at the RHIC is considered to be evidence for almost perfect liquid behavior of the strongly coupled quark–gluon plasma produced in the collisions. In this report, we review our current understanding of this new state of matter and investigate the predictions for anisotropic flow at the LHC.

Initial energy density and gluon distribution from the glasma in heavy-ion collisions

We estimate the energy density and the gluon distribution associated with the classical fields describing the early-time dynamics of heavy-ion collisions. In the McLerran-Venugopalan model, we first decompose the energy density into the momentum components exactly, with the use of the Wilson line correlators. Then we evolve the energy density with the free-field equation, which is justified by the dominance of the ultraviolet modes near the collision point. We also discuss the improvement that occurs with the inclusion of nonlinear terms into the time evolution. Our numerical results at RHIC energy are fairly consistent with the empirical values.

Isospin dynamics in heavy ion collisions: From Coulomb barrier to quark gluon plasma

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this report, we present a selection of new reaction observables in dissipative collisions particularly sensitive to the symmetry term of the nuclear Equation of State (Iso-EoS). We will first discuss the Isospin Equilibration Dynamics. At low energies, this manifests via the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation, with the symmetry term acting as a restoring force. At higher beam energies, Iso-EoS effects will be seen in Imbalance Ratio Measurements, in particular from the correlations with the total kinetic energy loss. For fragmentation reactions in central events, we suggest to look at the coupling between isospin distillation and radial flow. In Neck Fragmentation reactions, important Iso-EoS information can be obtained from the correlation between isospin content and alignment. The high density symmetry term can be probed from isospin effects on heavy ion reactions at relativistic energies (few A GeV range). Rather, isospin sensitive observables are proposed from nucleon/cluster emissions, collective flows and meson production. The possibility to shed light on the controversial neutron/proton effective mass splitting in asymmetric matter is also suggested. A large symmetry repulsion at high baryon density will also lead to an “earlier” hadron-deconfinement transition in n -rich matter. A suitable treatment of the isovector interaction in the partonic EoS appears very relevant.

Dependence of flow observables in heavy-ion collisions on the symmetry energy and the in-medium nucleon–nucleon interaction

We employ a QMD transport model to study the influence of the isospin dependent part of the nuclear matter equation of state on the dynamics of heavy-ion collisions at intermediate energies. Parametrization of the in-medium dependence of the nucleon–nucleon elastic cross-sections, as predicted by microscopical models, are used. The sensitivity of flow observables to various parametrizations of the isospin dependent part of the equation of state (symmetry energy) is studied. The experimentally observed splitting of the elliptic flow values in Ru+Zr and Zr+Ru at incident energies of 400 MeV is shown to be due to, to an equal extent, density dependence of the microscopic nucleon–nucleon cross-section and symmetry energy.

K* production in heavy ion collisions at RHIC

Study of resonances with their short lifetimes provides useful tools to probe the properties of hot and dense matter produced in relativistic heavy ion collisions. The high density and/or high temperature of the medium can modify resonance properties such as mass and width. Therefore, measurement of these properties can reveal important information about the evolution dynamics in heavy ion collisions. We report the measurements of K* transverse momentum (pT) spectra at mid-rapidity via its hadronic decay channel up to intermediate pT of 2.9 GeV/c using the STAR detector in Au+Au and Cu+Cu collisions at 62.4 GeV and 200 GeV. These results are compared to previously reported K* results from Au+Au collisions at RHIC. Integrated yield ratios of K*/K and K*/ϕ are used to understand the rescattering and regeneration effects on K* production.

The effect of “pre-formed” hadron potentials on the dynamics of heavy ion collisions and the HBT puzzle

The nuclear stopping, the elliptic flow, and the HBT interferometry are calculated by the UrQMD transport model, in which potentials for “pre-formed” particles (string fragments) from color fluxtube fragmentation as well as for confined particles are considered. This description provides stronger pressure at the early stage and describes these observables better than the default cascade mode (where the “pre-formed” particles from string fragmentation are treated to be free-streaming). It should be stressed that the inclusion of potential interactions pushes down the calculated HBT radius RO and pulls up the RS so that the HBT time-related puzzle disappears throughout the energies from AGS, SPS, to RHIC.

SINGLE-EVENT HANBURY-BROWN–TWISS INTERFEROMETRY

Large spatial density fluctuations in high energy heavy-ion collisions can come from many sources: initial transverse density fluctuations, non-central collisions, phase transitions, surface tension, and fragmentations. The common presence of some of these sources in high energy heavy-ion collisions suggests that large scale density fluctuations may often occur. The detection of large density fluctuations by single-event Hanbury-Brown–Twiss interferometry in heavy-ion collisions will provide useful information on density fluctuations and the dynamics of heavy-ion collisions.

Early time dynamics in heavy-ion collisions from AdS/CFT correspondence

We study the matter produced in heavy-ion collisions, assuming that this matter is strongly interacting and employing AdS/CFT correspondence to investigate its dynamics. At late proper times \ensuremath{\tau} we show that Bjorken hydrodynamics solution, obtained recently by Janik and Peschanski using gauge-gravity duality [1], can be singled out by simply requiring that the metric tensor is a real and single-valued function of the coordinates everywhere in the bulk, without imposing any constraints on the curvature invariant. At early proper times we use a similar strategy to show that the energy density $\ensuremath{\epsilon}$ approaches a constant as $\ensuremath{\tau}\ensuremath{\rightarrow}0$. We therefore demonstrate that the strong coupling dynamics incorporates the isotropization transition in heavy-ion collisions. By matching our early-time regime with the late-time one of Janik and Peschanski we estimate the isotropization time at RHIC to be approximately ${\ensuremath{\tau}}_{\mathrm{iso}}\ensuremath{\approx}0.3$ fm/$c$, in good agreement with results of hydrodynamic simulations.

Probing the medium at RHIC by identified particles

In a few years of RHIC running major advances have been made in understanding the dynamics of heavy-ion collisions in this new regime of unprecedented high-energy densities. Ultra-relativistic heavy-ion collisions at RHIC have provided a unique environment to create a nuclear medium that exhibits a variety of novel properties (Arsene et al 2005 Nucl. Phys. A 757 1, Back et al 2005 Nucl. Phys. A 757 28, Adcox et al 2005 Nucl. Phys. A 757 184, Adams et al 2005 Nucl. Phys. A 757 102). Determining whether the quarks and gluons in this matter reach thermal equilibrium with one another before they become confined within hadrons is one of the most important questions for understanding this new form of matter. A set of interesting features indicating relevance of partonic degrees of freedom at hadronization is observed in the identified measurements of nuclear modification factors, elliptical flow (v2) and particle ratios in the intermediate transverse momentum range (2 < pT < 6 GeV/c) at RHIC. Those observations are reviewed in this paper in the context of various theoretical models.

Correlations and characterization of emitting sources

Dynamical and thermal characterizations of excited nuclear systems produced during the collisions between two heavy ions at intermediate incident energies are presented by means of a review of experimental and theoretical work performed in the last two decades. Intensity interferometry, applied to both charged particles (light particles and intermediate mass fragments) and to uncharged radiation (gamma rays and neutrons) has provided relevant information about the space-time properties of nuclear reactions. The volume, lifetime, density and relative chronology of particle emission from decaying nuclear sources has been extensively explored and has provided valuable information about the dynamics of heavy-ion collisions. Similar correlation techniques applied to coincidences between light particles and complex fragments are also presented as a tool to determine the internal excitation energy of excited primary fragments as it appears in secondary-decay phenomena.

Bulk Dynamics in Heavy Ion Collisions

The features of heavy ion collisions that suggest the relevance of collective dynamics, as opposed to mere superpositions of nucleon-nucleon or even parton-parton collisions, are reviewed. The surprise of these studies is that bulk observables are far simpler than typical dynamical models of nucleus-nucleus collisions would imply. These features are shown to have a natural interpretation in terms of statistical-hydrodynamical models. The relevance of hydrodynamics to heavy ion collisions, coupled with the various similarities of the heavy ion data with that of more elementary collisions, raises very basic questions about its relevance to smaller systems.

Non-Identical Particle Correlations in STAR

Non-identical particle correlations offer new methods of probing the dynamics of heavy-ion collision. In STAR we have performed a correlation analysis of pion-kaon and pion-proton systems for √sNN = 130 AGeV Au+Au collisions. The results show that average emission space-time points of pions, kaons and protons are not the same. These results are studied with the help of heavy-ion collision models. The asymmetries appear as a consequence of space-momentum correlations produced by transverse radial expansion of the system. The effects of emission time differences, coming from the decay of resonances are also investigated, and found to explain only part of the asymmetry.

Stopping, flow and in-medium effects of NN cross sections in heavy ion collisions at SIS energies

The influence of density dependent total and differential elastic cross sections on the collective dynamics in heavy ion collisions is briefly outlined.

Charged and neutral kaon correlations in Au–Au collisions at GeV using the solenoidal tracker at RHIC (STAR)

Two boson intensity interferometry (also known as Hanbury-Brown–Twiss, or HBT (Brown R Hanbury and Twiss R Q 1956 Nature 178 1046)) offers information on the spacetime structure of the particle-emitting source created in heavy-ion collisions. Kaon HBT has been suggested as a promising probe of quark–gluon plasma and reveals strangeness dynamics. Small contributions from resonance decays make kaon HBT an ideal tool for correlation studies. In conjunction with pion correlation measurements, they can also reveal properties of collective dynamics in heavy-ion collisions. We present preliminary results on neutral and charged kaon correlations in GeV Au–Au collisions measured by the STAR Collaboration at the relativistic heavy ion collider.

Relativistic effects in the search for high density symmetry energy

Intermediate energy heavy ion collisions open the unique possibility to explore the equation of state (EOS) of nuclear matter far from saturation, in particular the density dependence of the symmetry energy. Within a relativistic transport model it is shown that the isovector–scalar δ meson, which affects the high density behavior of the symmetry energy density, influences the dynamics of heavy ion collisions in terms of isospin collective flows. The effect is largely enhanced by a relativistic mechanism related to the covariant nature of the fields contributing to the isovector channel. Results for reactions induced by 132Sn radioactive beams are presented. The elliptic flows of nucleons and light isobars appear to be quite sensitive to microscopic structure of the symmetry term, in particular for particles with large transverse momenta, since they represent an earlier emission from a compressed source. Thus future, more exclusive, experiments with relativistic radioactive beams should be able to set stringent constraints on the density dependence of the symmetry energy far from ground state nuclear matter.

Multifragmentation and dynamics in heavy ion collisions

A midrapidity zone formed in heavy-ion collisions has been investigated through special selections of light particles and intermediate mass fragments detected in the reaction 35Cl on 12C at 43 MeV/nucleon and the reactions 58Ni on 12C, 24Mg, and 197Au at 34.5 MeV/nucleon, and of neutron energy spectra measured in the reaction 35Cl on natTa. Properties of the observables have been examined to characterize the neck-like structure formed between the two reaction partners.

Coherent photon bremsstrahlung and dynamics of heavy-ion collisions: Comparison of different models

Differential spectra of coherent photon bremsstrahlung in relativistic heavy ion collisions are calculated within various schematic models of the projectile-target stopping. Two versions of the degradation length model, based on a phenomenological deceleration law, are considered. The simple shock wave model is studied analytically. The predictions of these models agree in the soft photon limit, where the spectrum is determined only by the final velocity distribution of charged particles. The results of these models in the case of central Au+Au collisions at various bombarding energies are compared with the predictions of the microscopic transport model UrQMD. It is shown that at the AGS energy the coherent photon bremsstrahlung exceeds the photon yield from $\pi^0$-decays at photon energies $\omega\loo 50$ MeV.