Pseudorapidity configurations in collisions between gold nuclei and track-emulsion nuclei

A method of parametrically invariant quantities is developed for studying pseudorapidity configurations in nucleus-nucleus collisions involving a large number of secondary particles. In simple models where the spectrum of pseudorapidities depends on three parameters, the shape of the spectrum may differ strongly from the shape of pseudorapidity configurations in individual events. Pseudorapidity configurations in collisions between gold nuclei of energy 10.6 GeV per nucleon and track-emulsion nuclei are contrasted against those in random stars calculated theoretically. An investigation of pseudorapidity configurations in individual events is an efficient method for verifying theoretical models.     

Two-dimensional wavelet analysis of simulated angular distributions of particles in collisions of relativistic nuclei

A method for analysis of a multiparticle event with a complicated structure is developed on the discrete wavelet transform basis. Two-dimensional histograms corresponding to a given function and the events simulated by a generator of nucleus-nucleus collisions at the center-of-mass energy √s = 5.5 TeV are analyzed. It is shown that the wavelet analysis efficiently removes the background, distinguishes the ring structure of the multiparticle event, and allows the decrease in the threshold transverse jet energy required for the separation of the jet from the background.     

A microscopic calculation of fragment formation in nucleus-nucleus collisions

A microscopic model using effective and free nucleon-nucleon scattering cross sections is used to calculate the yields of projectile-like fragments from nucleus-nucleus collisions from 20 MeV/A to 2 GeV/A. Good agreement with reaction cross-section and fragment cross-section measurements is obtained. The enhanced yields of neutron-rich fragments observed experimentally at low beam energies from collisions of projectiles with heavy targets are reproduced somewhat better by the inclusion of a neutron-rich surface on the heavy-target nuclei. Each fragment mass is produced in a strongly localized region of the distance of closest approach between the colliding nuclei; lighter fragments come from small distances and the heavier ones from more peripheral collisions.     

Onset of deconfinement in nucleus-nucleus collisions

The energy dependence of hadron production in relativistic nucleus-nucleus collisions reveals anomalies-the kink, horn, and step. They were predicted as signals of the deconfinement phase transition and observed by the NA49 Collaboration in central PbPb collisions at the CERN SPS. This indicates the onset of the deconfinement in nucleus-nucleus collisions at about 30 A GeV.     

Fission and fragmentation of 208Pb nuclei in collisions with gold nuclei at an energy of 158 GeV per nucleon

The fission and fragmentation of ultrarelativistic ²⁰⁸Pb nuclei in collisionswith gold nuclei were studied by using a beam from the SPS accelerator at CERN at an energy of 158 GeV per nucleon. The detectors of the target area of the NA45/CERES spectrometer were used in respective measurements. The value obtained for the fission cross section is 301 ± 44 mb, where about 77% of events stem from the electromagnetic interaction of colliding nuclei, while the remaining part is the contribution of peripheral nuclear interactions. The spallation of lead nuclei that involves the formation of heavy fragments occurs only in collisions where the impact parameter satisfies the condition b > 10 fm. A complete disintegration of lead nuclei to intermediate-mass fragments and light particles is observed in some peripheral collisions.     

Gluon densities at small <Emphasis Type="Italic">x</Emphasis> from photoproduction of ψ(2<Emphasis Type="Italic">S</Emphasis>) mesons in ultraperipheral collisions at the LHC

In the framework of the leading order of perturbative QCD, we study exclusive photoproduction of ψ(2S) mesons in ultraperipheral proton-proton and nucleus-nucleus collisions at the LHC. We calculate the charmonium rapidity distributions and show that the analysis of the LHC experimental data allows one to significantly reduce uncertainties in the behavior of the gluon densities of the proton and nuclei at small x.     

Production of exotic nuclei in peripheral nucleus-nucleus collisions below 10 A MeV

Experimental data from the literature concerning nucleus-nucleus collisions at beam energies of few A MeV above the Coulomb barrier are investigated in this work with emphasis on projectile-like fragment distributions. In peripheral collisions at beam energies below 10 A MeV, deep-inelastic transfer is shown to be the dominant reaction mechanism. In addition, the mechanism may involve an extension of the nuclear profile in the window (neck) region, observed primarily in reactions with very heavy target nuclei. Isoscaling observed at these energies can be used as a tool to predict the production rates of exotic nuclei in reactions induced by exotic secondary beams.     

Breakup reactions of two-neutron halo nuclei

Investigation of reactions initiated in collisions of beams of unstable nuclei with nuclear targets has resulted in the discovery of a new type of nuclear structure, the halo, in some light nuclei at the boundary of nucleon stability. A survey of different reactions with two-neutron halo nuclei and their use for gaining information on the structure of exotic nuclei is presented. Kinematically complete breakup reactions, which provide the possibility of obtaining quite reliable information on the structure of the ground state and the continuum of nuclei, are considered in detail. The microscopic four-body model of breakup reactions for two-neutron halo nuclei with account of characteristic specific features of their structure is formulated. The model is based on the distorted wave approximation and is applicable for analysis of low excitations of the continuum near the breakup threshold, the region most sensitive to manifestation of specific features of the halo structure. The described approach enables the calculation of all observables of kinematically complete experiments in nucleus-nucleus collisions at intermediate energies for which one-step processes dominate, and creates the basis for the spectroscopy of continua via successive analysis of various correlation cross sections available in kinematically complete experiments.     

Charmonium production in proton-proton collisions and in collisions of lead nuclei at CERN and comparison with Brookhaven data

A review of experimental data on charmoniumproduction that were obtained in fixed-target experiments at the SPS synchrotron and in proton-proton collisions and in collisions of lead nuclei in beams of the Large Hadron Collider (LHC) at CERN (Switzerland) is presented. A comparison with data obtained at the Brookhaven National Laboratory (USA) from experiments at the Relativistic Heavy Ion Collider (RHIC) is performed. Measurement of the suppression of J/ψ-meson production as a possible signal of the production of quark-gluon plasmawas proposed back in 1986 by T. Matsui and H. Satz. An anomalous suppression of J/ψ-meson production was discovered by the NA50 Collaboration at SPS (CERN) in central collisions of lead nuclei at the c.m. collision energy of 158 GeV per nucleon. Data obtained at the c.m. energy of 200 GeV per nucleon in the PHENIX experiment at RHIC indicate that, depending on multiplicity, the suppression of J/ψ-meson production at this energy approximately corresponds to the suppression of J/ψ-meson production in collisions of lead nuclei at the SPS accelerator. Theoretical models that take into account the regeneration of J/ψ mesons describe better RHIC experimental data. The measurement of charmonium production in proton-proton collisions and in collisions of lead nuclei in LHC beams revealed the importance of taking into account the regeneration process. At the LHC energies, it is also necessary to take into account the contribution of B-meson decays. Future measurements of charmonium production at the LHC to a higher statistical precision and over an extended energy region would be of importance for obtaining deeper insight into the mechanism of charmonium production and for studying the properties of matter at high energy density and temperature.     

An “effective-A” dependence of cross sections in ultra-relativistic heavy-ion collisions and its interference with quark-gluon plasma signatures

It is shown that in ultra-relativistic nucleus-nucleus collisions, as a consequence of the geometry of the collision, there is a dependence of the total and differential cross sections on effective atomic mass numbers of the target and beam nuclei. The general expressions of the effective atomic mass numbers are deduced and they are computed for 0+Cu, O+U, S+U, Si+Au and U+U collisions. This particular A-dependence of the cross sections can have a strong effect on the production ratio of two particles of different species. As a consequence it might simulate a signature of quark-gluon plasma which is expected to be given by the yields ratio of two different particles. It is shown that this effect can explain quantitatively the K/π ratios measured in Si+Au and S+W collisions as well as the decrease of the J/ψ production relative to Drell-Yan lepton pairs measured in O+Cu, O+U and S+U collisions. The need of precise measurements of the powers α and α(x_F, P_T) in inclusive nuleon-nuleus interactions for the understanding of particle production in nucleus-nucleus collisions is stressed. Expectations for future experiments are presented.     

Charm and bottom production at RHIC

The current status of charm and bottom production measurements at RHIC is summarized. Heavy-flavor data from p+p collisions at provide a crucial testing ground for perturbative QCD calculations and serve as a baseline for measurements in nucleus-nucleus collisions. Due to their large mass heavy quarks are produced in hard parton scattering processes in the earliest phase of a nucleus-nucleus collisions such that they can be used to probe the hot and dense medium that is formed in the course of Au+Au collisions at RHIC.     

Resolution of theK-factor ambiguity in QCD

The previously presented relations between inclusive cross sections for nucleus-nucleus, nucleon-nucleus and nucleon-nucleon scatterings with largep _T production were generalized to include effects of large momentum tail of nucleons inside the nucleus. Simple numerical estimations show that these effects are likely to dominate the inclusive cross section for nucleus-nucleus collisions at very largep _T.     

Production of baryon resonances in collisions of 4He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon

Under conditions of 4π geometry, Δ⁰-and Δ⁺⁺-resonance production in collisions of ⁴He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon was investigated for the first time. The resonance masses and widths are estimated by analyzing the experimental and background invariant-mass distributions of (pπ ^±) pairs. Contributions to pion production from Δ-resonance decays, as well as from direct pion production, are determined. The results are compared with corresponding data obtained for CC collisions at the same primary momentum of 4.2 GeV/c per nucleon. By and large, the results obtained here comply with the existence of a collective mechanism of delta-isobar excitation in nuclei.     

Higgs-boson production in nucleus-nucleus collisions

Cross-section calculations are presented for the production of intermediate-mass Higgs bosons produced in ultrarelativistic nucleus-nucleus collisions via two-photon fusion. The calculations are performed in position space using Baur's method for folding together the Weizsacker-Williams virtual-photon spectra of the two colliding nuclei. It is found that two-photon fusion in nucleus-nucleus collisions is a plausible way of finding intermediate-mass Higgs bosons at the Superconducting Super Collider or the CERN Large Hadron Collider.     

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number <n _g> of the prompt gluons are calculated perturbatively and the inelastic cross section σ_abs ^Ψg is estimated. The integrated cross sections σ(A B →J/ψX) for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter <n _g>σ_abs ^Ψg. Received: 20 August 1999