Central Heavy-ion Reactions Research Articles

Baryon-antibaryon annihilation and reproduction in relativistic heavy-ion collisions

The quark rearrangement model for baryon-antibaryon annihilation and reproduction ($B\bar B\leftrightarrow 3M$) - incorporated in the Parton-Hadron-String Dynamics (PHSD) transport approach - is extended to the strangeness sector. A derivation of the transition probabilities for the three-body processes is presented and a strangeness suppression factor for the invariant matrix element squared is introduced to account for the higher mass of the strange quark compared to the light up and down quarks. In simulations of the baryon-antibaryon annihilation and reformation in a box with periodic boundary conditions we demonstrate that our numerical implementation fulfills detailed balance on a channel-by-channel basis for more than 2000 individual $2 \leftrightarrow 3$ channels. Furthermore, we study central Pb+Pb collisions within PHSD from 11.7$A$ GeV to 158$A$ GeV and investigate the impact of the additionally implemented reaction channels in the strangeness sector. We find that the new reaction channels have a visible impact essentially only on the rapidity spectra of antibaryons. The spectra with the additional channels in the strangeness sector are closer to the experimental data than without for all antihyperons. Due to the chemical redistribution between baryons/antibaryons and mesons we find a slightly larger production of antiprotons thus moderately overestimating the available experimental data. We additionally address the question if the antibaryon spectra (with strangeness) from central heavy-ion reactions at these energies provide further information on the issue of chiral symmetry restoration and deconfinement. However, by comparing transport results with/without partonic phase as well as including/excluding effects from chiral symmetry restoration we find no convincing signals in the strange antibaryon sector for either transition due to the strong final-state interactions.

Thermal and transport properties in central heavy-ion reactions around a few hundred MeV/nucleon

Thermalization process of nuclear matter in central fireball region of heavy-ion collisions is investigated by employing an extension model of Boltzmann-Uehling-Uhlenbeck, namely the Van der Waals Boltzmann-Uehling-Uhlenbeck (VdWBUU) model. Temperature ($T$) is extracted by the quantum Fermion fluctuation approach and other thermodynamic quantities, such as density ($\rho$), entropy density ($s$), shear viscosity ($\eta$), isospin diffusivity ($D_{I}$) and heat conductivity ($\kappa$), are also deduced. The liquid-like and gas-like phase signs are discussed through the behavior of shear viscosity during heavy-ion collisions process with the VdWBUU model.

Constancy of energy partition in central heavy-ion reactions at intermediate energies

Semiclassical transport simulation of nucleus-nucleus collisions for the range of incident energy from about the Fermi energy up to a few hundred MeV per nucleon evidences that the maximal excitation energy put into a nuclear system during the early compact stage of heavy-ion reaction is a constant fraction of the center-of-mass available energy of the system. Analysis of experimental data without presuming reaction mechanism dominating the collision process on the best corroborates the found constancy of energy partition in central heavy-ion reactions.

Method for separating jets and the underlying event in heavy ion collisions at the BNL Relativistic Heavy Ion Collider

Reconstructed jets in heavy ion collisions are a crucial tool for understanding the quark-gluon plasma. The separation of jets from the underlying event is necessary particularly in central heavy ion reactions in order to quantify medium modifications of the parton shower and the response of the surrounding medium itself. There have been many methods proposed and implemented for studying the underlying event substructure in proton-proton and heavy ion collisions. In this paper, we detail a method for understanding underlying event contributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV utilizing the HIJING event generator. This method, extended from previous work by the ATLAS collaboration, provides a well-defined association of "truth jets" from the fragmentation of hard partons with "reconstructed jets" using the anti-$k_T$ algorithm. Results presented here are based on an analysis of 750M minimum bias HIJING events. We find that there is a substantial range of jet energies and radius parameters where jets are well separated from the background fluctuations (often termed "fake jets") that make jet measurements at RHIC a compelling physics program.

Indications of suppressed high [formula omitted] hadron production in nucleus–nucleus collisions at CERN-SPS

Inclusive pion production at high transverse momenta (pT≳2 GeV/c) in nucleus–nucleus (A+A) collisions at CERN-SPS (sNN≈20 GeV) is revisited and systematically compared to all existing proton–proton data in the same range of center-of-mass energies. The ratio of A+A to p + p pion cross-sections (nuclear modification factor) for central Pb + Pb, Pb + Au and S + Au reactions does not show a strong enhancement as a function of pT as previously found, but is consistent with scaling with the number of nucleon–nucleon (NN) collisions. Neutral pion yields per NN collision in head-on Pb + Pb reactions are suppressed, whereas peripheral yields are enhanced. These results together indicate that some amount of “jet quenching” may already be present in central heavy-ion reactions at sNN≈20 GeV.

Anomalous radial expansion in central heavy-ion reactions

The expansion velocity profile in central heavy-ion reactions in the Fermi energy domain is examined. The radial expansion is non-hubblean and in the surface region it scales proportional to a higher exponent ($\alpha > 1$) of the radius. The anomalous expansion velocity profile is accompanied by a power law nucleon density profile in the surface region. Both these features of central heavy-ion reactions disappear at higher energies, and the system follows a uniform Hubble expansion ($\alpha \simeq 1$).

Dynamical constraints on phase transitions

The numerical solutions of nonlocal and local Boltzmann kinetic equations for the simulation of central heavy ion reactions are parameterized in terms of time dependent thermodynamical variables in the Fermi liquid sense. This allows one to discuss dynamical trajectories in phase space. The nonequilibrium state is characterized by non-isobaric, non-isochoric etc. conditions, shortly called iso-nothing conditions. Therefore a combination of thermodynamical observables is constructed which allows one to locate instabilities and points of possible phase transition in a dynamical sense. We find two different mechanisms of instability, a short time surface - dominated instability and later a spinodal - dominated volume instability. The latter one occurs only if the incident energies do not exceed significantly the Fermi energy and might be attributed to spinodal decomposition. In contrast the fast surface explosion occurs far outside the spinodal region and pertains also in the cases where the system develops too fast to suffer a spinodal decomposition and where the system approaches equilibrium outside the spinodal region.

On the space-time difference of proton and composite particle emission in central heavy-ion reactions at 400 A· MeV

Small-angle correlations of pairs of nonidentical light charged particles produced in central collisions of heavy ions in the A= 100 mass region at a beam energy of 400 A· MeV are investigated with the FOPI detector system at GSI Darmstadt. The difference of longitudinal correlation functions with the relative velocity parallel and anti-parallel to the center-of-mass velocity of the pair in the central source frame is studied. This method allows extracting the apparent space-time differences of the emission of the charged particles. Comparing the correlations with results of a final-state interaction model delivers quantitative estimates of these asymmetries. Time delays as short as 1~fm/c or – alternatively – source radius differences of a few tenth fm are resolved. The strong collective expansion of the participant zone introduces not only an apparent reduction of the source radius but also a modification of the emission times. After correcting for both effects a complete sequence of the space-time emission of p, d, t, 3He, α particles is presented for the first time.

The sphericity of central heavy-ion reactions.

We have experimentally studied small impact parameter heavy-ion collisions in the (nearly) symmetry entrance channels $^{12}\mathrm{C}$${+}^{12}$C, $^{20}\mathrm{Ne}$${+}^{27}$Al, $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La, each at many intermediate beam energies. The results from a number of analyses based on a projection of the ``shapes'' of the experimental events called the sphericity are presented. Comparisons of the relative efficiencies of various experimental methods for the selection of central events are made. The importance of autocorrelations between the sphericity and the various impact-parameter--dependent variables is evaluated. Searches for beam energy-dependent transitions from sequential binary disassembly to multifragmentation in the central events are described. Comparisons to dynamic and hybrid model code calculations will be discussed. The average sphericities of the intermediate mass fragments (IMF's, for which 3\ensuremath{\le}Z\ensuremath{\lesssim}20), are presented. The possibility that the IMF emission occurs following the formation of transient toroidal or disk-like geometries in the central events is explored. Increases in the average sphericities of the central events for increasing beam energies are observed which is attributed to transitions from sequential binary disassembly to multifragmentation. The transitional beam energies for the central $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La reactions are near \ensuremath{\sim}50, \ensuremath{\sim}40, and \ensuremath{\sim}40 MeV/nucleon, respectively.

Effects of compression and collective expansion on particle emission from central heavy-ion reactions.

Conditions under which compression occurs and collective expansion develops in energetic symmetric reactions of heavy nuclei are analyzed, together with their effects on emitted light baryons and pions. Within transport simulations, it is shown that shock fronts perpendicular to beam axis form in head-on reactions. The fronts separate hot compressed matter from normal matter and propagate into the projectile and target. As the impact parameter increases, the angle of inclination of the fronts relative to beam axis decreases, and in between the fronts a weak tangential discontinuity develops. Hot matter exposed to the vacuum in directions perpendicular to shock motion (and parallel to fronts) starts to expand sideways early within the reactions. Expansion in the direction of shock motion follows after the shocks propagate through nuclei, but due to the delay does not acquire the same strength. Expansion affects angular distributions, mean-energy components, shapes of spectra, and mean energies of different particles emitted into any one direction and further particle yields. Both the anisotropy in the expansion and a collective motion associated with the weak discontinuity affect the magnitude of sideward flow within the reaction plane. Differences in mean particle energy components in and out of the reaction plane in semicentral collisions depend sensitively on the relative magnitude of shock speed in normal matter and speed of sound in hot matter. The missing energy, considered in the past in association with low measured pion multiplicity in central reactions, may be identified with the energy of collective expansion. Relations are established which govern approximately the behavior of density and entropy in the compressed region in reactions with beam energy and impact parameter.

Cold production of fast nucleons in central heavy-ion reactions

The transfer of fast nucleons between two ions is investigated as a possible source of preequilibrium emissions. The transferred intensities are given by the tunnelling of a semi classical nuclear matter distribution through the potential barrier. The calculations predict proton-to-neutron ratios larger than one (1.35 for40Ca+40Ca), linear increases of the multiplicities with the incident energy; the results show too that the flow resulting from the overlap of the frozen density tails cannot account for the characteristics of the tunnelling flow.

Space-Time Development of Nuclear Collisions in Central Heavy Ion Reactions at Very High Energies

We present a clean cut spectator-participant model for particle production in heavy ion reactions at very high energies. The model is an extension of independent particle models for hadron-nucleus interactions.