Charmonium-state production in heavy-ion collisions

An overview of existing experimental data on the production of charmonium states (J/ψ and ψ′) at the superproton synchrotron (SPS, CERN) and the relativistic heavy ion collider (RHIC, Brookhaven National Laboratory, United States) is given. The production of J/ψ mesons shows an anomalous suppression discovered by the NA50 Collaboration (CERN) in collisions of lead nuclei at an energy of 158 GeV per nucleon and confirmed in the NA60 experiment (CERN) in collisions of indium nuclei at the same energy. The suppression of J/ψ production depends on interaction centrality and becomes anomalous at about 122 participant nucleons in PbPb collisions and at about 86 participant nucleons in InIn collisions. The experimental data in question are compared with the predictions of existing theoretical models. None of the models is able to simultaneously describe data on PbPb and InIn collisions. Data obtained in the PHENIX experiment at RHIC from measurements of J/ψ production in AuAu and CuCu collisions at an energy of 200 GeV (in the nucleon-nucleon c.m. frame) indicate that the suppression of J/ψ production at such energies approximately corresponds to the suppression of J/ψ production in PbPb collisions at SPS. Theoretical models that describe SPS data on PbPb collisions predict a stronger suppression of J/ψ production. Models that take into account J/ψ-meson regeneration better describe experimental data obtained at RHIC. Measurement of cross sections for charmonium and bottomonium production at the Large Hadron Collider (LHC, CERN) would make it possible to study the properties of matter and to explore the mechanism of quarkonium production at ultrahigh energy densities and temperature and high transverse momenta, as well as to investigate the effect of the regeneration and suppression of quarkonium production as the energy increases.     

Strangeness production in relativistic heavy-ion collisions

Kaon production in relativistic heavy-ion collisions is studied. Particular attention is paid to situations in which high densities are obtained, such as in the Brookhaven AGS experiments with 14.6 GeV/nucleon Si on Au. Because of the explicit chiral-symmetry breaking terms in chiral Langrangians, kaons acquire an effective mass m _K ^* which goes to zero at the critical baryon density. Well before such densities, m _K ^* is sufficiently reduced to greatly facilitate kaon production through processes like K¯K. Previous expressions for the decreasing kaon mass were arrived at by linear chiral perturbation theory. Whereas we cannot systematically proceed to higher order, we use physical models to suggest how relevant quantities will behave in higher order. We present arguments that m _K ^* effectively goes to zero in the present AGS experiments.Supported in part by the U.S. Department of Energy Grant No. DE-FG02-88ER40388     

Double meson production in ultraperipheral heavy-ion collisions

The double-meson production in ultraperipheral heavy-ions collisions is addressed, focusing on the particular case of from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by-product, we estimate the double production cross section using the pomeron-exchange factorization relations. Received: 20 March 2003 / Published online: 2 June 2003     

Direct hadron production in ultrarelativistic heavy-ion collisions

Hadrons emitted by the pre-surface layer of a quark-gluon plasma (QGP) before the phase transition into a hadronic gas are considered as possible sources of direct information about QGP. It is shown that if QGP is created in ultrarelativistic heavy-ion collisions, then these hadrons strongly contribute at soft p_t at SpS energy and dominate up to an order of magnitude at LHC energy.     

Estimating the particle-antiparticle correlation effect for pion production in heavy-ion collisions

Back-to-back ππ correlations arising owing to the evolution of the pionic field in the course of the pion-production process are estimated for central heavy-ion collisions at moderate energies.     

A particle-hole calculation for pion production in relativistic heavy-ion collisions

A differential cross section for π-meson production in peripheral heavy-ion collisions is formulated within the context of a particle-hole model in the Tamm-Dancoff approximation. This is the first attempt at a fully quantum-mechanical particle-hole calculation for pion production in relativistic heavy-ion collisions. The particular reaction studied is an ¹⁶O projectile colliding with a ¹²C target at rest. In the projectile we form a linear combination of isobar-hole states, with the possibility of a coherent isobar giant resonance. The target can be excited to its giant M1 resonance (J^π = 1⁺, T = 1) at 15.11 MeV, or to its isobar analog neighbours, ¹²B at 13.4 MeV and ¹²N at 17.5 MeV. The theory is compared to recent experimental results.     

Antiproton production in heavy-ion collisions at subthreshold energies

Within the framework of the Lanzhou quantum molecular dynamics model,the deep subthreshold antiproton production in heavy-ion collisions has been investigated thoroughly.The elastic scattering,annihilation and charge exchange reactions associated with antiproton channels are implemented in the model.The attractive antiproton potential extracted from the G-parity transformation of nucleon selfenergies reduces the threshold energies in meson-baryon and baryon-baryon collisions,and consequently enhances the antiproton yields to some extent.The calculated invariant spectra are consistent with the available experimental data.The primordial antiproton yields increase with the mass number of the colliding system.However,annihilation reactions reduce the antiproton production which becomes independent of the colliding partners.Anti-flow phenomena of antiprotons correlated with the mean field potential and annihilation mechanism is found by comparing them with the proton flows.Possible experiments at the high-intensity heavy-ion accelerator facility(HIAF) in China are discussed.     

In-medium effects on charmonium production in heavy-ion collisions

Charmonium production in heavy-ion collisions is investigated within a kinetic theory framework incorporating in-medium properties of open- and hidden-charm states in line with recent QCD lattice calculations. A continuously decreasing open-charm threshold across the phase boundary of hadronic and quark-gluon matter is found to have important implications for the equilibrium abundance of charmonium states. The survival of J/psi resonance states above the transition temperature enables their recreation also in the quark-gluon plasma. Including effects of chemical and thermal off-equilibrium, we compare our model results to available experimental data at CERN SPS and BNL RHIC energies. In particular, earlier found discrepancies in the psi(')/psi ratio can be resolved.     

Photon production in heavy-ion collisions at SPS energies

Single photon spectra in heavy-ion collisions at SPS energies are studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. We separate the total photon spectrum into “background” arising from the radiative decays of π⁰ and η mesons, and the “thermal” one from other sources. For the latter we include contributions from radiative decays of ρ, ω, η′, and a₁, radiative decays of baryon resonances, as well as two-body processes such as ππ → ργ and πρ → πγ. It is found that more than 95% of all photons come from the decays of π⁰ and η mesons, while the thermal photons account for less than 5% of the total photon yield. The thermal photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper bound set by the experimental measurement of the WA80 Collaboration.