Diffusion or bounce back in relativistic heavy-ion collisions?

The time evolution of pseudorapidity distributions of produced charged hadrons in d + Au collisions at = 200 GeV is investigated. Results of a nonequilibrium statistical relativistic diffusion model with three sources are compared with a macroscopic “bounce-back” model that does not allow for statistical equilibration at large times, but instead leads to motion reversal. When compared to the data, the results of the diffusion approach are more precise, thus emphasizing that the system is observed to be on its way to thermal equilibrium.     

d′ production in heavy-ion collisions

The production of d′ dibaryons in heavy-ion collisions due to the elementary process NN → d′π is considered. The NN → d′π cross section is estimated using the vacuum d′ width Γ_d′ ≈ 0.5 MeV extracted from data on the double charge exchange reactions on nuclei. The d′ production rate per single collision of heavy ions is estimated at an incident beam energy of 1 A GeV within the framework of the Quantum Molecular Dynamics transport model. We suggest to analyse the invariant mass spectrum of the NNπ system in order to search for an abundance of events with the invariant mass of the d′ dibaryon. The d′ peak is found to exceed the statistical fluctuations of the background at a 6σ level for 2 × 10⁵ · A central collisions of heavy ions with the atomic number A.     

Can CP violation be observed in heavy-ion collisions?

We demonstrate that, at least at present, there is no convincing way to detect CP violation in heavy-ion collisions.     

Soft photons in hadron-hadron collisions: synchrotron radiation from the QCD vacuum?

We discuss the production of soft photons in high energy hadron-hadron collisions. We present a model where quarks and antiquarks in the hadrons emit synchrotron light when being deflected by the chromomagnetic fields of the QCD vacuum, which we assume to have a nonperturbative structure. This gives a source of prompt soft photons with frequencies <~300 mev=" in=" the=" c.m.=" system=" of=" the=" collision=" in=" addition=" to=" hadronic=" bremsstrahlung.=" in=" comparing=" the=" frequency=" spectrum=" and=" rate=" of=">synchrotron photons to experimental results we find some supporting evidence for their existence. We make an exclusive-inclusive connection argument to deduce from the synchrotron effect a behaviour of the neutron electric formfactorG _E ⁿ (G ²) proportional to (Q ²)^1/6 forQ ²20 fm^–2. We find this to be consistent with available data. In our view, soft photon production in high energy hadron-hadron and lepton-hadron collisions as well as the behaviour of electromagnetic hadron formafactors for lowQ ² are thus sensitive probles of the nonperturbative structure of the QCD vacuum.     

Double-λ hypernuclear production in high-energy heavy-ion collisions

The production of double- hypernuclei was studied regarding three different mechanisms: (1) coalescence of a nuclear fragment from peripheral collisions and two A particles, (2) coalescence of two A particles, neutrons and protons due to central collisions, and (3) conversion of a hypernucleus. The production probability is estimated for several projectile-target combinations at 5 and 14.5 GeV/nucleon.     

High energy γ-ray emission in heavy-ion collisions

We calculate double-differential cross sections for energetic photon production in intermediate energy nucleus-nucleus collisions. The production mechanism is assumed to be either that of collective bremsstrahlung or that of electromagnetic transitions between time-dependent single-particle states as emerging from a self-consistent treatment of the heavy-ion dynamics in the one-body limit (TDHF). Effects from residual nucleon-nucleon collisions on the collective current are further taken into account via a relaxation ansatz for the single-particle occupation numbers. The total yields for energetic photon production above 50 MeV in the simple limit considered indicate that up to 10% of the experimental cross sections can be attributed to collective bremsstrahlung and electromagnetic transitions.     

Pre-equilibrium γ-ray emission in dissipative heavy-ion collisions

-rays are measured in coincidence with the dissipative complex fragments from the³⁵Cl +⁶⁴Ni reaction at 7.7 A MeV. Theoretical -ray coincidence spectra are determined through a fitting procedure using the contribution of the excited fragment statistical -decay calculated by means of the CASCADE code. Whereas the expected statistical spectra fit very well the data in the energy regionE =2 to 8 MeV, above this energy another emission mechanism in addition to the statistical decay has to be considered in order to reproduce the experimental spectra. Simulations based on the Vlasov equation suggest that this emission could come from the decay of dipole strength excited in the intermediate dinuclear system.The authors are greatly indebted with A. Boiano, A. Buccheri, M. Cipriano, F. Pagano and R. Rocco for their assistance during the set up of the experimental apparatus. Dr. J. Girard and the technical staff of the CEA/DAPNIA-DPhN are acknowledged for the valuable operation of the accelerator.     

K+ versus Λ flow in relativistic heavy-ion collisions

We study K⁺ and Λ flow in heavy-ion collisions at beam energies of about 2 AGeV. We present our results in both the “traditional” (i.e., in terms of the average transverse momentum in the reaction plane) as well as “modern” (i.e., in terms of coefficients of the Fourier analysis of azimuthal distributions) methods of flow analysis. We find significant differences between the K⁺ and the Λ flow: while the Λ flow is basically similar to that of nucleons, the K⁺ flow almost disappears. This difference is attributed chiefly to their different mean field potentials in dense matter. The comparisons with the experimental data, as well as theoretical results from independent calculations, indicate clearly the pivotal roles of both K⁺ and Λ medium effects. We emphasize that similar experimental data from independent collaborations are essential for the eventual verification of these medium effects.     

Vortex collisions: crossing or recombination?

We investigate the collision of two vortex lines moving with viscous dynamics and driven towards each other by an applied current. Using London theory in the approach phase we observe a nontrivial vortex conformation producing antiparallel segments; their attractive interaction triggers a violent collision. The collision region is analyzed using the time-dependent Ginzburg-Landau equation. While we find that vortices will always recombine through the exchange of segments, a crossing channel appears naturally through a double collision process.     

Observation of Δ+→ pπ0 decay in heavy-ion collisions

Proton-π⁰ coincidences have been measured at the beam energy of 180A MeV in the reaction Ar+Ca studied by TAPS at SIS/GSI. In the proton-π⁰ invariant mass spectrum we observe a significant excess of strength above the background obtained by event mixing. We attribute this signal to the strength distribution N _Δ of the Δ baryonic resonance. No correlation is observed in the case of deuteron-π⁰ coincidences. Assuming isotropic emission of π⁰ and Δ⁺ from a midrapidity thermal source and isospin symmetry, we determined the global N _Δ/N _πratio of 0.79 ±0.30(stat) ±0.2(syst). This value indicates that most pions produced at subthreshold energy in heavy-ion reaction are mediated by the Δ-resonance. Received: 7 April 2000 / Accepted: 29 August 2000     

Mach cone induced by γ-triggered jets in high-energy heavy-ion collisions

Medium excitation by jet shower propagation inside a quark-gluon plasma is studied within a linear Boltzmann transport and a multiphase transport model. Contrary to the naive expectation, it is the deflection of both the jet shower and the Mach-cone-like excitation in an expanding medium that is found to give rise to a double-peak azimuthal particle distribution with respect to the initial jet direction. Such a deflection is the strongest for hadron-triggered jets which are often produced close to the surface of a dense medium due to trigger bias and travel against or tangential to the radial flow. Without such trigger bias, the effect of deflection on γ-jet showers and their medium excitation is weaker. Comparative study of hadron and γ-triggered particle correlations can therefore reveal the dynamics of jet-induced medium excitation in high-energy heavy-ion collisions.     

“Free” constituent quarks and dilepton production in heavy-ion collisions

An approach is suggested that invokes vitally the notion of constituent massive quarks (valons) that can survive and propagate rather than hadrons (except of pions) within hot and dense matter formed below the chiral-transition temperature in the course of heavy-ion collisions at high energies. This approach is shown to be quite good for describing the experimentally observed excess in the dilepton yield at masses 250≤M_ee≤700 MeV over the prompt-resonance-decay mechanism (CERES cocktail) predictions. In certain aspects, it appears to be even more successful than conventional approaches: it seems to match the data somewhat better at dilepton masses below the two-pion threshold and below the ρ-meson peak, as well as at higher dilepton masses (beyond the φ-meson one). The approach implies no specific assumptions on the special features of phase transitions in expanding nuclear matter, and the ideal gas approximation is motivated to be still workable for describing the pion-valonic system under consideration.     

Hypertriton and light nuclei production at Λ-production subthreshold energy in heavy-ion collisions

High-energy heavy-ion collisions produce abundant hyperons and nucleons. A dynamical coalescence model coupled with the ART model is employed to study the production probabilities of light clusters, deuteron (d), triton (t), helion (³He), and hypertriton (_Λ³H) at subthreshold energy of Λ production (≈ 1 GeV per nucleon). We study the dependence on the reaction system size of the coalescence penalty factor per additional nucleon and entropy per nucleon. The Strangeness Population Factor (S₃ / (³He × (Λ/p))) shows an extra suppression of hypertriton comparing to light clusters of the same mass number. This model predicts a hypertriton production cross-section of a few μb in ³⁶Ar+³⁶Ar, ⁴⁰Ca+⁴⁰Ca and ⁵⁶Ni+⁵⁶Ni in 1 A GeV reactions. The production rate is as high as a few hypertritons per million collisions, which shows that the fixed-target heavy-ion collisions at CSR (Lanzhou/China) at Λ subthreshold energy are suitable for breaking new ground in hypernuclear physics.