Projectile Nucleons Research Articles

Charged-particle pseudorapidity (rapidity) distribution in nucleus–nucleus collisions at alternating-gradient synchrotron energy

The charged-particle pseudorapidity (rapidity) distributions in nucleus–nucleus collisions at the alternating-gradient synchrotron energy (11–15A GeV) have been described by the thermalized cylinder picture. The calculated results are in agreement with the experimental data of the reactions 28Si + Au, Ag, Cu, and Al at 14.6A GeV/c bombarding energy. It is shown that the target stopping power (the leading participant target nucleon rapidity shift with respect to the projectile) increases with the target size, and the projectile penetrating power (the leading participant projectile nucleon rapidity shift with respect to the target) decreases with the increasing of target size. PACS Nos.: 25.75-q, 25.75Dw, and 24.10pa

Reaction mechanisms and multifragmentation processes in64Zn+58Niat35A–79A MeV

Reaction mechanisms and multifragmentation processes have been studied for Zn-64 + Ni-58 collisions at intermediate energies with the help of antisymmetrized molecular dynamics (AMD-V) model calculations. Experimental energy spectra, angular distributions, charge distributions, and isotope distributions, classified by their associated charged particle multiplicities, are compared with the results of the AMD-V calculations. In general the experimental results are reasonably well reproduced by the calculations. The multifragmentation observed experimentally at all incident energies is also reproduced by the AMD-V calculations. A detailed study of AMD-V events reveals that, in nucleon transport, the reaction shows some transparency, whereas in energy transport the reaction is much less transparent at all incident energies studied here. The transparency in the nucleon transport indicates that. even for central collisions, about 75% of the projectile nucleons appear in the forward direction. In energy transport about 80% of the initial kinetic energy of the projectile in the center-of-mass frame is dissipated. The detailed study of AMD-V events also elucidates the: dynamics of the multifragmentation process. The study suggests that, at 35A MeV, the semitransparency and thermal expansion are the dominant mechanisms for the multifragmentation process, whereas at 49A MeV and higher incident energies a nuclear compression occurs at an early stage of the reaction and plays an important role in the multifragmentation process in addition to that of the thermal expansion and the semitransparency.

Analyzing the features of π− mesons and protons from AC interactions at a momentum of p=4.2 GeV/c per projectile nucleon on the basis of the FRITIOF model

The mean multiplicities of π− mesons and protons originating from pC, dC, αC, and CC interactions at a momentum of p=4.2 GeV/c per projectile nucleon and the distributions of these particles in kinematical variables are presented. These experimental distributions are compared with the corresponding predictions obtained on the basis of the FRITIOF model. It is shown that the FRITIOF version used in the present analysis describes satisfactorily our experimental data.

Separating the contributions of interaction mechanisms in the transverse-momentum distributions of light fragments

Transverse-momentum distributions of light spectator fragments are analyzed by representing them as superpositions of contributions from various interaction mechanisms. The temperatures and the collective transverse momenta of light projectile fragments formed in Ne + Em and Si + Em interactions at momenta of 4.1–4.5 GeV/c per projectile nucleon are estimated under the assumption that these spectators are compressed as the participant projectile nucleons traverse the target nucleus. This analysis reveals the important role of sound waves in the transfer of collective momenta to spectator fragments.

Isospin tracing: A probe of nonequilibrium in central heavy-Ion collisions

Four different combinations of 9644Ru and 9640Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400A MeV using a 4pi detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the tritium to 3He yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium ( n,n) cross section used in microscopic transport calculations.

Analysis of inelastic scattering of 3He on 13C at 37.9 MeV

The differential cross section of elastic and inelastic scattering of 3He ions on the 13C nucleus has been measured at an energy of 37.9 MeV. By fitting the shape of the measured angular distribution of the elastic scattering the parameters of the optical model have been found. These parameters have been used for the DWBA calculations of angular distributions corresponding to excitations of the 13C levels 3.09(1/2+) MeV, 3.68(3/2−) MeV, 3.85(5/2+) MeV, 7.55(5/2−) MeV, 8.84(1/2−) MeV and 11.85(7/2+) MeV. Several sets of transition densities calculated with different effective nucleon-nucleon interactions and six different M3Y interactions between a projectile and target nucleon have been tested. Analysis of the inelastic scattering of 135 MeV protons on 13C is also presented, which enabled us to obtain additional information about the tested structure models.

Isospin equilibration in relativistic heavy-ion collisions

We study the mixing and the kinetic equilibration of projectile and target nucleons in relativistic heavy-ion collisions in the energy regime between 150~AMeV and 2~AGeV in a coupled-channel BUU (CBUU) approach. We find that equilibrium in the projectile-target degrees of freedom is in general not reached even for large systems at low energy where elastic nucleon-nucleon collisions dominate. Inelastic nucleon excitations are more favorable for equilibration and their relative abundance increases both with energy and mass. Experimentally, the projectile/target admixture can be determined by measuring the degree of isospin equilibration in isospin asymmetric nuclear collisions. For one of the most promising systems currently under investigation, ${}^{96}_{44}{\rm Ru}+{}^{96}_{40}{\rm Zr}$, we investigate the influence of the equation of state and the inelastic in-medium cross section.

Nucleus-nucleus collision as superposition of nucleon-nucleus collisions

Angular distributions of charged particles produced in 16O and 32S collisions with nuclear track emulsion were studied at momenta 4.5 and 200 A GeV/c. Comparison with the angular distributions of charged particles produced in proton-nucleus collisions at the same momentum allows to draw the conclusion, that the angular distributions in nucleus-nucleus collisions can be seen as superposition of the angular distributions in nucleon-nucleus collisions taken at the same impact parameter b NA , that is mean impact parameter between the participating projectile nucleons and the center of the target nucleus

Energy Loss and J/ψ Suppression in Relativistic pA and AA Collisions

When the cross section of J/ψ production is considered as varying with the energy of the nucleon-nucleon interaction, the production of J/ψ in pA and AA collisions has been studied by using FRITIOF Model. The calculation shows that the cross section of J/ψ production "per nucleon-nucleon collision" decreases with the increasing mass number and centrality as a consequence of continuous energy loss of the projectile nucleons to the target nucleons in their successive binary nucleon-nucleon collisions. The model predictions have been compared with the experimental data of J/ψ production.

Radioactive nuclear beam facilities based on projectile fragmentation

The production of radioactive ions by using direct in-flight separation techniques is discussed. The reaction mechanisms used to produce radioactive beams near projectile velocities can be broadly divided into four classes: projectile fragmentation, nucleon transfer, fission, and Coulomb excitation. Radioactive nuclei produced by these reactions have large forward momenta with relatively sharp angular distributions peaked close to zero degrees. Such narrow distributions are suitable for collection with magnetic devices. Further beam purification can be achieved by exploiting atomic energy-loss processes in profiled energy degraders combined with a second magnetic selection. Secondary beam intensities of up to 1%, or so, of the primary beam intensity are possible, although the beam emittance depends on the production mechanism and may be poor. The features of the production reaction mechanisms, separation techniques, and a review of worldwide efforts are presented. Also the relative advantages and disadvan...

Subthreshold K+ production in deuteron and alpha induced nuclear reactions

Abstract Double differential cross sections d 2 σ/dpdΩ have been measured for π + and K + emitted around midrapidity in d + A and α + A collisions at a beam kinetic energy of 1.15 GeV/nucleon. The total π + yield increases by a factor of about 2 when using an alpha projectile instead of a deuteron whereas the K + yield increases by a factor of about 4. According to transport calculations, the K + enhancement depends both on the number of hadron-hadron collisions and on the energy available in those collisions: their center-of-mass energy increases with increasing number of projectile nucleons.

A microscopic calculation of secondary Drell-Yan production in heavy ion collisions

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.

A microscopic calculation of secondary Drell-Yan production in heavy ion collisions

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.

Non-equilibrium fluid-dynamics in the early stage of ultrarelativistic heavy-ion collisions

To describe ultrarelativistic heavy-ion collisions we construct a three-fluid hydrodynamical model. In contrast to one-fluid hydrodynamics, it accounts for the finite stopping power of nuclear matter, i.e. for non-equilibrium effects in the early stage of the reaction. Within this model, we study baryon dynamics in the BNL-AGS energy range. For the system Au+Au we find that kinetic equilibrium between projectile and target nucleons is established only after a time t cm eq ≈ 5 fm c ⋍ 2R Au γ cm . Observables that are sensitive to the early stage of the collision (like e.g. nucleon flow) therefore differ considerably from those calculated in the one-fluid model.

Mean field calculations of nucleon-nucleus scattering.

Corrections to the first order term of the multiple scattering expansion of the nucleon-nucleus optical potential due to the propagation of the projectile nucleon in the target nucleus mean field are estimated. The effects of nonlocalities of the nucleon-nucleon transition amplitude are included. Calculations are performed for nucleon scattering from $^{16}\mathrm{O}$ and $^{208}\mathrm{Pb}$ at 100, 200, and 400 MeV incident energies. We show that the mean field effect is repulsive and reduces the strength of the local representation of the impulse approximation potential by approximately 25% in the nuclear interior. \textcopyright{} 1996 The American Physical Society.

Off-shell effects in heavy particle production

Off-shell propagation of nucleons is neglected in one-body transport models of heavy-ion collisions, but could be significant in processes that are limited by phase space, such as the threshold production of heavy particles. We estimate the relative magnitude of off-shell production to on-shell production of the N ∗ (1535) resonance in heavy ion collisions. In the region where the on-shell production is dominated by a two-step mechanism with an intermediate Δ, we find that the contribution of off-shell scattering between projectile and target nucleons is indeed small. Beyond the latter contribution, however, correlations in the initial wave function produce off-shell contributions which can exceed those of the on-shell Δ mechanism.

Strangeness enhancement in p+A and S+A interactions at energies near 200A GeV.

The systematics of strangeness enhancement is calculated using the hijing and venus models and compared to recent data on pp, pA, and AA collisions at CERN/SPS energies (200A GeV). The hijing model is used to perform a linear extrapolation from pp to AA. venus is used to estimate the effects of final state cascading and possible nonconventional production mechanisms. This comparison shows that the large enhancement of strangeness observed in S+Au collisions, interpreted previously as possible evidence for quark-gluon plasma formation, has its origins in nonequilibrium dynamics of few nucleon systems. A factor of 2 enhancement of ${\mathrm{\ensuremath{\Lambda}}}^{0}$ at midrapidity is indicated by recent pS data, where on the average one projectile nucleon interacts with only two target nucleons. There appears to be another factor of 2 enhancement in the light ion reaction SS relative to pS, when on the average only two projectile nucleons interact with two target ones.

SOME FEATURES OF INELASTIC INTERACTIONS OF 28Si AND 12C IONS IN NUCLEAR EMULSION AT 4.5A GeV/c

Multiplicity distributions of various types of tracks produced in inelastic interactions of 28 Si and 12 C at 4.5A GeV/c with nuclear emulsion nuclei have been investigated. The correlations of the type <Ni(Nj)> where Ni, Nj=Ng, Nb, Nh or Ns with i≠j have been studied and compared with the results from p-Em interactions at the same momentum per nucleon. Some saturation effects in the values of <Nb> with Ng are observed. Furthermore, some of these correlations show almost similar dependence in nucleus-nucleus and proton-nucleus interactions at same per particle momenta. The values of <Ns> have been obtained by subtracting the contribution of Z=1 fragments to the observed showers. The dependence of <Ns> so obtained on the number of interacting projectile nucleons shows that the nucleus-nucleus collisions at these energies can be interpreted in terms of multiple N-N collisions.

Isolating physical effects in the exclusive (N,N' pi ) reaction.

We propose a series of experimental measurements of the exclusive ([ital N],[ital N][prime][pi]) process on light nuclei. It is suggested that different physical effects can be isolated by the following procedures: (a) varying the incident energy of the projectile nucleon at fixed nuclear energy and momentum transfer; (b) measuring ratios of cross sections to different [ital T][sub [ital z]] substates of the same final nuclear isotopic multiplet; and (c) comparing ratios of cross sections for high spin non-normal parity states to those of low spin normal parity states. We discuss each of these methods for comparing data, and show what physical information can be extracted with each method.

When and how are fragments formed in heavy ion collisions?

Employing the Quantum Molecular Dynamics approach for simulating the fragment formation in nucleus nucleus collisions we test the predictive power of the recently advanced Early Cluster Recognition Algorithm. We find that this algorithm gives almost the same number of fragments as the standard minimum spanning tree procedure. It is applicable not only to the late stage of the reaction. Taking advantage of this fact we find that the final fragmentation pattern is already realized shortly after the initial phase of violent collisions between the projectile and target nucleons, long before they are supposed to be formed in thermal models.