Central Relativistic Heavy Ion Collisions Research Articles

Local equilibration of fermions and bosons

Local kinetic equilibration is a prerequisite for hydrodynamics to be valid. Here it is described through a nonlinear diffusion equation for finite systems of fermions and bosons. The model is solved exactly for constant transport coefficients in both cases. It has the proper Fermi-Dirac and Bose-Einstein equilibrium limits and can replace the relaxation-time approximation (RTA). The microscopic transport coefficients are determined through the macroscopic variables temperature and local equilibration time. Applications to the transverse energy of quarks and gluons in the initial stages of central relativistic heavy-ion collisions, and to bosonic and fermionic atoms at low energies appropriate for cold quantum gases are discussed.

Justification of Landau Hydrodynamic-Tube model in central relativistic heavy-ion collisions

The statistical event-by-event analysis of inelastic interactions of 16O and 32S nuclei in emulsion at 60 A GeV/c and 200 A GeV/c reveals the existence of groups of high multiplicity events belonging to very central nuclear interactions with Gaussian pseudorapidity distributions for produced particles as suggested by the original hydrodynamic-tube model. Characteristics of these events are presented. The experimental observations are interpreted as a result of quark-gluon plasma formation in the course of central nuclear interactions.

Effects of initial-state fluctuations on jet-energy loss

The effect of initial state fluctuations on jet energy loss in relativistic heavy-ion collisions is studied in a 2+1 dimension ideal hydrodynamic model. Within the next-to-leading order perturbative QCD description of hard scatterings, we find that a jet loses slightly more energy in the expanding quark-gluon plasma if the latter is described by the hydrodynamic evolution with fluctuating initial conditions compared to the case with smooth initial conditions. A detailed analysis indicates that this is mainly due to the positive correlation between the fluctuation in the production probability of parton jets from initial nucleon-nucleon hard collisions and the fluctuation in the medium density along the path traversed by the jet. This effect is larger in non-central than in central relativistic heavy ion collisions and also for jet energy loss that has a linear than a quadratic dependence on its path length in the medium.

Effects of initial state fluctuations on jet quenching

Within the next-to-leading order perturbative QCD description of hard scatterings and using a 2+1 dimension ideal hydrodynamic model, we have studied the effect of initial state fluctuations on jet energy loss in relativistic heavy-ion collisions. We find that a jet loses slightly more energy in the expanding quark-gluon plasma if the latter is described by the hydrodynamic evolution with fluctuating initial conditions compared to the case with smooth initial conditions. A detailed analysis indicates that this is mainly due to the positive correlation between the fluctuation in the production probability of parton jets from initial nucleon-nucleon hard collisions and the fluctuation in the medium density along the path traversed by the jet. This effect is larger in non-central than in central relativistic heavy ion collisions and also for a linear than a quadratic dependence of jet energy loss on its path length in the medium.

Pionic freeze-out hypersurfaces in relativistic nucleus-nucleus collisions

The space-time structure of the multipion system created in central relativistic heavy-ion collisions is investigated. Using the microscopic transport model UrQMD we determine the freeze-out hypersurface from the equation on pion density $n(t,\phantom{\rule{0.16em}{0ex}}\mathbf{r})={n}_{c}$. It turns out that for proper value of the critical energy density ${\ensuremath{\epsilon}}_{c}$ equation $\ensuremath{\epsilon}(t,\phantom{\rule{0.16em}{0ex}}\mathbf{r})={\ensuremath{\epsilon}}_{c}$ gives the same freeze-out hypersurface. It is shown that for big enough collision energies ${E}_{\mathrm{kin}}\ensuremath{\ge}40$A GeV ($\sqrt{s}\ensuremath{\ge}8$A GeV) the multipion system at a time moment $\ensuremath{\tau}$ ceases to be one connected unit but splits up into two separate spatial parts (drops), which move in opposite directions from one another with velocities which approach the speed of light with an increase of collision energy. This time $\ensuremath{\tau}$ is approximately invariant of the collision energy, and the corresponding $\ensuremath{\tau}\phantom{\rule{0.16em}{0ex}}=$ const. hypersurface can serve as a benchmark for the freeze-out time or the transition time from the hydrostage in hybrid models. The properties of this hypersurface are discussed.

Baryon Stopping as a New Probe of Geometric Scaling

We suggest using net-baryon rapidity distributions in central relativistic heavy-ion collisions at energies reached at the CERN Super Proton Synchrotron, BNL Relativistic Heavy-Ion Collider (RHIC), and CERN LHC in order to probe saturation physics. Within the color glass condensate framework based on small-coupling QCD, net-baryon rapidity distributions are shown to exhibit geometric scaling. In a comparison with RHIC data in Au+Au collisions at sqrt[sNN=62.4 GeV and 200 GeV the gradual approach to the gluon saturation regime is investigated. Predictions for net-baryon rapidity spectra and the mean rapidity loss in central Pb+Pb collisions at LHC energies of sqrt[sNN=5.52 TeV are made.

Evolution of the bulk properties of the hot dense matter in relativistic heavy ion collisions

Within the framework of a multi-phase transport model, we study the energy density, pressure and pressure anisotropy of the hot dense matter produced in central relativistic heavy ion collisions. We find that their dynamical evolutions depend strongly on the non-equilibrium expansion dynamics in the collisions as well as the hadronization scheme and scattering cross sections used in the model. Furthermore, our results indicate that only partial thermalization is achieved in the produced matter as a result of its fast expansion.

PHOTON-HADRON CORRELATIONS IN $\sqrt{^SNN} = 200\ {\rm GeV/c}$ COLLISIONS WITH THE PHENIX DETECTOR

The yield of charged hadrons at intermediate pT (2-5 GeV/c) on the away-side of a high pT hadron trigger is known to be suppressed in central relativistic heavy-ion collisions. The mechanism by which the jet opposite the trigger has its redistributed is not yet fully understood. Although rarer than dijets, direct photon-jet pairs may provide additional constraints on parton energy loss models. The benefits of direct photon triggers are two-fold. In contrast to dijet systems where the jet energy is known only after full jet reconstruction, the direct photon carries approximately the full energy of the jet. Also, the trigger photons do not suffer from the same geometrical bias that causes high pT hadrons from jets to be produced predominately near the surface of the collision region. This poster discusses the method by which the direct photon correlation signal can be separated from the decay photon correlation signal. We present the results of two particle correlation studies of charged hadrons associated with high pT photon triggers in p − p collisions. The results are compared to data from Au−Au collisions and expectations from the PYTHIA event generator.

Causal viscous hydrodynamics for central heavy-ion collisions

We study causal viscous hydrodynamics in the context of central relativistic heavy-ion collisions and provide details of a straightforward numerical algorithm to solve the hydrodynamic equations. It is shown that correlation functions of fluctuations provide stringent test cases for any such numerical algorithm. Passing these tests, we study the effects of viscosity on the temperature profile in central heavy-ion collisions. Also, we find that it is possible to counter-act the effects of viscosity to some extent by re-adjusting the initial conditions. However, viscous corrections are strongest for high-mass particles, signaling the breakdown of hydrodynamic descriptions for large eta/s.

Pentaquark baryon production at the relativistic heavy ion collider

Production of pentaquark Θ + baryons in central relativistic heavy ion collisions is studied in a kinetic model. Assuming that a quark–gluon plasma is produced in the collisions, we first determine the number of Θ + produced from the quark–gluon plasma using a parton coalescence model, and then take into consideration its production and absorption in subsequent hadronic matter via the reactions K N ↔ Θ , K N ↔ π Θ and π N ↔ K ¯ Θ . We find that although the final Θ + number is affected by hadronic interactions, it remains sensitive to the initial number of Θ + produced from the quark–gluon plasma, particularly in the case of a small Θ + width as imposed by the K + N and K + d scattering data. Because of small baryon chemical potential in the hot dense matter produced in these collisions, the number of produced anti- Θ is only slightly smaller than that of Θ + .

Hadronization in heavy-ion collisions: recombination and fragmentation of partons.

We argue that the emission of hadrons with transverse momentum up to about 5 GeV/c in central relativistic heavy ion collisions is dominated by recombination, rather than fragmentation of partons. This mechanism provides a natural explanation for the observed constant baryon-to-meson ratio of about one and the apparent lack of a nuclear suppression of the baryon yield in this momentum range. Fragmentation becomes dominant at higher transverse momentum, but the transition point is delayed by the energy loss of fast partons in dense matter.

Strange dibaryons in neutron stars and in heavy-ion collisions

The formation of dibaryons with strangeness are discussed for the interior of neutron stars and for central relativistic heavy-ion collisions. We derive limits for the properties of H-dibaryons from pulsar data. Signals for the formation of possible bound states with hyperons at BNL's Relativistic Heavy-Ion Collider (RHIC) are investigated by studying their weak decay patterns and production rates.

Can we extract lambda-lambda interaction from two-particle momentum correlation?

We analyze the invariant mass spectrum of Λ-Λ in 12C( K −, K + ΛΛ) reaction at P K − = 1.65 GeV/c by using a combined framework of IntraNuclear Cascade (INC) model and the correlation function technique. The observed enhancement at low-invariant masses can be well reproduced with attractive Λ-Λ interactions with the scattering length either in the range a = −6 ∼ −4 fm (no bound state) or a = 7 ∼ 12 fm (with bound state). We also discuss Λ-Λ correlation functions in central relativistic heavy-ion collisions as a possible way to eliminate this discrete ambiguity.

Production of low mass electron pairs due to the photon-photon mechanism in central collisions

We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the gamma-gamma mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at RHIC are also given.

Local thermal and chemical equilibration and the equation of state in relativistic heavy ion collisions

Thermodynamical variables and their time evolution are studied for central relativistic heavy ion collisions from 10.7 to 160 A GeV in the microscopic ultrarelativistic quantum molecular dynamics model (UrQMD). The UrQMD model exhibits drastic deviations from equilibrium during the early high density phase of the collision. Local thermal and chemical equilibration of the hadronic matter seems to be established only at later stages of the quasi-isentropic expansion in the central reaction cell with volume 125 . Baryon energy spectra in this cell are reproduced by Boltzmann distributions at all collision energies for fm/c with a unique rapidly dropping temperature. At these times the equation of state has a simple form: . At SPS energies the strong deviation from chemical equilibrium is found for mesons, especially pions, even at the late stage of the reaction. The final enhancement of pions is supported by experimental data.

Unravelling γγ dileptons in central relativistic heavy ion collisions

Using an impact parameter formulation, differential production probabilities for γγ dileptons in central relativistic heavy ion collisions at RHIC and LHC energies are calculated and compared to Drell-Yan and thermal ones. The angular distributions of the leptons give a handle for the discrimination. Nuclear stopping leads, apart from Bremsstrahlung pair production, to a modification of the pure γγ dilepton production. These modifications are studied in a simple model and are found to be of minor influence.

Collective flow and pion production at diogene

Recent results from the large solid angle detector “DIOGENE” at Saturne (Saclay), for Ne + A and Ar + A reactions at 400, 600 and 800 MeV per nucleon are presented. Several points are selected: 1) The particles azimuthal emission pattern (with respect to the reaction plane) exhibiting preferential out-of-plane emission at c.m. rapidities, 2) The presence of an appreciable amount of collective flow for asymmetric systems and even for (rather) small projectile masses, 3) The first measurements of “flow” and multiplicities for both positive and negative pions, 4) The first measurement of deltas in central relativistic heavy ion collisions.

Lepton-pair production by brensstrahlung in central relativistic heavy-ion collisions

We study the production of lepton pairs by classical bremsstrahlung in central relativistic heavy-ion collisions. For the stopping of the nuclei we assume a simple model of point charges and a deceleration time. Pair-creation probabilities are calculated in first-order perturbation theory.