Stage Of Heavy-ion Collisions Research Articles

Initial condition for quark-gluon plasma evolution

We recently proposed a new approach to high energy nuclear scattering, which treats the initial stage of heavy ion collisions in a sophisticated way. We are able to calculate macroscopic quantities like energy density and velocity flow at the end of this initial stage, after the two nuclei having penetrated each other. In other words, we provide the initial conditions for a macroscopic treatment of the second stage of the collision. We address in particular the question of how to incorporate the soft component properly. We find almost perfect "Bjorken scaling": the rapidity coincides with the space-time rapidity, whereas the transverse flow is practically zero. The distribution of the energy density in the transverse plane shows typically a very "bumpy" structure.

Multiphase transport model for heavy ion collisions at RHIC

Using a multiphase transport model (AMPT) with both partonic and hadronic interactions, we study the multiplicity and transverse momentum distributions of charged particles such as pions, kaons and protons in central Au+Au collisions at RHIC energies. Effects due to nuclear shadowing and jet quenching on these observables are also studied. We further show preliminary results on the production of multistrange baryons from the strangeness-exchange reactions during the hadronic stage of heavy ion collisions.

The role of gluons in dilepton production from the quark–gluon plasma

We study high mass dilepton production from gluon-induced processes, gq→qγ ∗ , g q ̄ → q ̄ γ ∗ , and gg→q q ̄ γ ∗ , in a thermally equilibrated but chemically undersaturated partonic matter that is expected to be created in the initial stage of ultrarelativistic heavy ion collisions. Regulating the divergence in these processes by the thermal quark mass, we find that gluon-induced processes are more important than the leading-order q q ̄ annihilation process as a result of the larger number of gluons than quarks in the partonic matter. The dependence of the thermal dilepton yield from the partonic stage of heavy ion collisions on the initial conditions for the partonic matter is also studied. We further discuss the feasibility of observing thermal dileptons from the quark–gluon plasma in heavy ion experiments.

Entropy production in collisions of relativistic heavy ions — a signal for quark-gluon plasma phase transition?

Entropy production in the compression stage of heavy ion collisions is discussed within three distinct macroscopic models (i.e. generalized RHTA, geometrical overlap model and three-fluid hydrodynamics). We find that within these models ∼80% or more of the experimentally observed final-state entropy is created in the early stage. It is thus likely followed by a nearly isentropic expansion. We employ an equation of state with a first-order phase transition. For low net baryon density, the entropy density exhibits a jump at the phase boundary. However, the excitation function of the specific entropy per net baryon, S A , does not reflect this jump. This is due to the fact that for final states (of the compression) in the mixed phase, the baryon density ϱ B increases with s , but not the temperature T. Calculations within the three-fluid model show that a large fraction of the entropy is produced by nuclear shock waves in the projectile and target. With increasing beam energy, this fraction of S A decreases. At s =20 A GeV it is on the order of the entropy of the newly produced particles around midrapidity. Hadron ratios are calculated for the entropy values produced initially at beam energies from 2 to 200 A GeV.

Flash of photons from the early stage of heavy-ion collisions

The dynamics of partonic cascades may be an important aspect for particle production in relativistic collisions of nuclei at CERN SPS and BNL RHIC energies. Within the parton-cascade model, we estimate the production of single photons from such cascades due to the scattering of quarks and gluons $q\stackrel{\ensuremath{\rightarrow}}{g}q\ensuremath{\gamma},$ quark-antiquark annihilation $q\overline{q}\ensuremath{\rightarrow}g\ensuremath{\gamma}$ or \ensuremath{\gamma}\ensuremath{\gamma}, and electromagnetic bremsstrahlung of quarks $\stackrel{\ensuremath{\rightarrow}}{q}q\ensuremath{\gamma}.$ We find that the latter QED branching process plays the dominant role for photon production, similarly as the QCD branchings $\stackrel{\ensuremath{\rightarrow}}{q}\mathrm{qg}$ and $\stackrel{\ensuremath{\rightarrow}}{g}\mathrm{gg}$ play a crucial role for parton multiplication. We conclude therefore that photons accompanying the parton-cascade evolution during the early stage of heavy-ion collisions shed light on the formation of a partonic plasma.

Neck dynamics at the approach stage of heavy ion collisions

Neck dynamics is investigated at the approach phase of reaction. Using realistic mass parameters and friction coefficients in the classical equations of collective motion, an oscillating behaviour of the neck is obtained. Due to the small characteristic time for neck oscillations, the addition of the neck variable to the collective variable set may be questionable for internuclear distances considered. In the liquid drop model an averaging over the fast neck variable leads to a relative motion of the nuclei that is similar to the motion in a potential obtained in the frozen density approximation. The role of the nucleon flow, which is identified to the small neck, is discussed for the case of sub-barrier fusion.

Mean fields in colliding nuclear matter

Abstract The momentum-space configuration of the participant nucleons in the early stage of heavy-ion collisions can be described by a colliding nuclear matter configuration, i.e. by two Lorentz elongated Fermi ellipsoids. We introduce this configuration in the framework of the Hartree approximation of the σω-model. The configuration and the mean fields are constructed self-consistently, respecting the Pauli principle for interpenetrating ellipsoids at low relative velocity in a covariant and density conserving manner. In a second step we approximately construct mean fields for these configurations in the context of Brueckner theory. To do this we parametrize the real part of the self-energy of relativistic Brueckner calculations for equilibrated nuclear matter (one-Fermi sphere) in a Hartree scheme to obtain momentum- and density-dependent coupling parameters. With these we calculate the scalar and vector self-energy of a nucleon in colliding nuclear matter consistently with the configuration. The mean self-energy components show strong correlation and exchange effects. The results can be used to improve relativistic transport calculations for heavy-ion collisions.

Relativistic hydrodynamics for heavy-ion collisions. II. Compression of nuclear matter and the phase transition to the quark-gluon plasma

We investigate the compression of nuclear matter in relativistic hydrodynamics. Nuclear matter is described by a σ-ω-type model for the hadron matter phase and by the MIT bag model for the quark-gluon plasma, with a first order phase transition between both phases. In the presence of phase transitions, hydrodynamical solutions change qualitatively, for instance, one-dimensional stationary compression is no longer accomplished by a single shock but via a sequence of shock and compressional simple waves. We construct the analytical solution to the “slab-on-slab” collision problem over a range of incident velocities. The performance of numerical algorithms to solve relativistic hydrodynamics is then investigated for this particular test case. Consequences for the early compressional stage in heavy-ion collisions are pointed out.

QCD based space-time description of high energy nuclear collisions

The current understanding of the early stage of heavy ion collisions at collider energies on the basis of the improved parton model and kinetic theory is presented. The space-time evolution of these reactions is investigated in terms of t the dissipative dynamics of (semi)hard quark and gluon interactions within perturbative QCD. The partons' phase-space distributions are evolved in real time according to a transport equation through multiple collisions, emission and absorption processes. The resulting parton cascade picture is applied to study the issues of thermalization, chemical equilibration, and particle production in AA collisions at RHIC and LHC.