The total cross section of the ${}^4\mathrm{He}({\pi }^{+},{\pi }^{-})$ reaction was measured for ${\pi }^{+}$ kinetic energies ranging from 70 to 130 MeV using the CHAOS spectrometer at TRIUMF and a liquid ${}^4\mathrm{He}$ target. Around $T_{\pi }=90\mathrm{}\mathrm{MeV},$ total cross sections exceed conventional model predictions by a factor of 3, whereas at $T_{\pi }=70\mathrm{}\mathrm{MeV}$ and for $T_{\pi }>130\mathrm{}\mathrm{MeV}$ the data are consistent with these calculations. An attempt is made to understand this behavior by assuming the production of the hypothetical $d^{\prime }$ dibaryon.

Double differential spectra of neutrons emitted from the reaction ${}^{40}{\mathrm{A}\mathrm{r}+}^{180}\mathrm{Hf}$ at $E_{\mathrm{lab}}=180,$ 190, 216, and 249 MeV were measured in coincidence with fission fragments using a multidetector array. Averaged prescission and postscission neutron multiplicites were extracted from the spectra together with temperature parameters. These values were analyzed in the framework of a modified version of the statistical model with inclusion of dynamical hindrance of fission, which depends on nuclear dissipation. From comparison of the experimental and calculated neutron multiplicities and temperatures it was deduced that nuclear viscosity is strong at excitation energies higher than $\sim 40\mathrm{MeV}$ and decreases with the decrease of excitation energy during evaporation cascade. The extracted fission time scale changes from $40\times {10}^{-20}\mathrm{s}$ to $17\times {10}^{-20}\mathrm{s}$ in the investigated projectile energy interval.

A detailed derivation of the collisional widths of collective vibrations is presented in both quantal and semiclassical frameworks by considering the linearized limits of the extended time-dependent Hartree-Fock and the Boltzmann-Uehling-Uhlenbeck model with a non-Markovian binary collision term. Damping widths of giant dipole and giant quadrupole excitations are calculated by employing an effective Skyrme force, and the results are compared with giant dipole resonance measurements in lead and tin nuclei at finite temperature.

A recent interpretation of the caloric curve based on the expansion of the abraded spectator nucleus is reanalyzed in the framework of the time-dependent Hartree-Fock (TDHF) evolution. It is shown that the TDHF dynamics is more complex than a single monopolar collective motion at moderate energy. The inclusion of other important collective degrees of freedom may lead to the dynamical creation of hollow structure. Then, low density regions could be locally reached after a long time by the creation of these exotic density profiles. In particular the systematic of the minimum density reached during the expansion (the so-called turning points) appears to be different.

A detailed study of the elementary ${}^2\mathrm{H}{(p,p}^{\prime }{\pi }^{+})n$ reaction is presented using the $\Delta$ isobar model. In this model, in the first step one of the two protons in the initial state gets excited to $\Delta .$ This, in the second step, decays into a nucleon and a pion. For the $p\overrightarrow{p}N\Delta$ step the parametrized form of the distorted-wave Born approximation t matrix of Jain and Santra, which reproduces most of the available data on $p\overrightarrow{p}n{\Delta }^{++},$ is used. The cross sections studied include the outgoing proton momentum spectra in coincidence with the pion, the outgoing pion momentum spectra, and the integrated total cross section. We find that all the calculated numbers are in good agreement with the corresponding measured cross sections.

We present a systematic study of the dependence of the fragment production on the nucleon-nucleon cross section using the quantum molecular dynamics model as an event generator. We employ constant energy-dependent and in-medium cross sections. In our analysis, fragments are formed using a minimum spanning tree description. The fragment distribution with different cross sections at low energy is nearly the same. At higher energies, different nucleon-nucleon (NN) cross sections have small effects on fragment production for central collisions, whereas the fragment production is strongly influenced at semicentral and peripheral collisions. A larger NN cross section results in more intermediate mass fragments at peripheral collisions with a shift of the peak towards larger impact parameters. As a result, the rapidity distribution of fragments and the fragment flow is also affected by the choice of the cross section. This influence is reduced if fragments are constructed with a sophisticated algorithm such as the simulated annealing clusterization algorithm which searches for the most bound configuration of nucleons and fragments.

A collective Hamiltonian for a two alpha particles aggregate, which describes the ${}^8\mathrm{Be}$ nucleus, encompassing a collective potential and an inertia function of that system, is obtained and analyzed through the use of a technique — derived from an approach of the generator coordinate method (GCM) — which allows for the extraction of collective information. The nucleon-nucleon interaction considered here is the one proposed by Volkov plus the Coulomb repulsion. It is shown that nonlocal effects appear in those collective functions describing the spontaneously occurring breakup process. Furthermore, the result for the inertia function stands for a microscopically generated evidence supporting a double-folding-based model of the real part of the nucleus-nucleus nonlocal interaction recently proposed.

The processes involving low-energy $\overline{K}N$ and $Y\pi$ interactions (where $Y=\Sigma$ or $\Lambda )$ are studied in the framework of heavy baryon chiral perturbation theory with the $\Lambda \mathrm{}\left(1405\right)\mathrm{}$ resonance appearing as an independent field. The leading and next-to-leading terms in the chiral expansion are taken into account. We show that an approach which explicitly includes the $\Lambda \mathrm{}\left(1405\right)\mathrm{}$ resonance as an elementary quantum field gives reasonable descriptions of both the threshold branching ratios and the energy dependence of total cross sections.

Momenta and masses of heavy projectile fragments $(Z>~8),$ produced in collisions of ${}^{197}\mathrm{Au}$ with C, Al, Cu, and Pb targets at $E/A=600\mathrm{}\hspace{0.5em}\mathrm{MeV},$ were determined with the ALADIN magnetic spectrometer at SIS. Using this information, an analysis of kinematic correlations between the two and three heaviest projectile fragments in their rest frame was performed. The sensitivity of these correlations to the conditions at breakup was verified within the schematic SOS model. For a quantitative investigation, the data were compared to calculations with statistical multifragmentation models and to classical three-body calculations. With classical trajectory calculations, where the charges and masses of the fragments are taken from a Monte Carlo sampling of the experimental events, the dynamical observables can be reproduced. The deduced breakup parameters, however, differ considerably from those assumed in the statistical multifragmentation models which describe the charge correlations. If, on the other hand, the analysis of kinematic and charge correlations is performed for events with two and three heavy fragments produced by statistical multifragmentation codes, good agreement with the data is found with the exception that the fluctuation widths of the intrinsic fragment energies are significantly underestimated. A new version of the multifragmentation code MCFRAG was therefore used to investigate the potential role of angular momentum at the breakup stage. If a mean angular momentum of $0.75ħ/\mathrm{nucleon}$ is added to the system, the energy fluctuations can be reproduced, but at the same time the charge partitions are modified and deviate from the data.

Two-pion correlations from Pb+Pb collisions at 158 $\mathrm{GeV}/c$ per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume.

The bremsstrahlung produced when heavy nuclei collide is estimated for central collisions at the Relativistic Heavy Ion Collider. Soft photons can be used to infer the rapidity distribution of the outgoing charge. An experimental design is outlined.

The strategy and techniques for analyzing anisotropic flow (directed, elliptic, etc.) in relativistic nuclear collisions are presented. The emphasis is on the use of the Fourier expansion of azimuthal distributions. We present formulas relevant for this approach, and in particular, show how the event multiplicity enters into the event plane resolution. We also discuss the role of nonflow correlations and a method for introducing flow into a simulation.

An exact solution of the two center time-dependent Dirac equation for pair production induced by ultrarelativistic heavy ion collisions is presented. Cross sections to specific final states approach those of perturbation theory. Multiplicity rates are reduced from perturbation theory.

The effect of in-medium vector and axial-vector meson masses on photon production is studied. We assume that the effective mass of a vector meson in hot nuclear matter decreases according to a universal scaling law, while that of an axial-vector meson is given by Weinberg’s mass formula. We find that the thermal production rate of photons increases with reduced masses, and is enhanced by an order of magnitude at $T=160\mathrm{}\mathrm{MeV}$ with $m_{\rho }=300\mathrm{}\mathrm{MeV}.$ Assuming a hydrodynamic evolution, we estimate the effect of the reduced masses on photon production in nucleus-nucleus collisions. The result is compared to experimental data from the WA80/WA98 Collaboration.

We study $K^{-}{/K}^{+}$ ratios as a function of centrality (participant nucleon number), transverse mass ${(m}_t),$ and rapidity, in heavy-ion collisions at beam energies between 1 and $2\hspace{0.5em}A\mathrm{GeV}.$ We use the relativistic transport model that includes explicitly the strangeness degrees of freedom and consider two scenarios for kaon properties in dense matter, one with and one without medium modifications of their properties. In both scenarios, the $K^{-}{/K}^{+}$ ratio does not change very much with the centrality, while the $K/\pi$ and $\overline{K}/\pi$ ratios increase with increasing centrality. Significant differences are predicted, both in magnitudes and shapes, for the $m_t$ spectra and rapidity distributions of $K^{-}{/K}^{+}$ ratio. Experimental measurement of these ratios, currently under investigation by the FOPI, KaoS, E866, and E895 Collaborations, will be useful in revealing the kaon in-medium properties.

We study analytically the medium-induced energy loss of a high energy parton passing through a finite size QCD plasma, which is expanding longitudinally according to Bjorken’s model. We extend the Baier-Dokshitzer-Mueller-Peigné-Schiff formalism already applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and we show that the resulting radiative energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature $T=T\left(L\right)\mathrm{}$, which is reached after the quark propagates a distance $L$.

The mass of the $H$ dibaryon with spin, parity $J^{\pi }{=0\mathrm{}}^{+}$, and isospin $I=0\mathrm{}$ is calculated in the framework of the chiral color dielectric model. The wave function of the $H$ dibaryon is expressed as a product of two color-singlet baryon clusters. Thus the quark wave functions within the cluster are antisymmetric. Appropriate operators are then used to antisymmetrize intercluster quark wave functions. The radial part of the quark wave functions are obtained by solving the quark and dielectric field equations of motion obtained in the color dielectric model. The mass of the $H$ dibaryon is computed by including the color magnetic energy as well as the energy due to meson interaction. The recoil correction to the $H$ dibaryon mass is incorporated by Peierls-Yoccoz technique. We find that the mass of the $H$ dibaryon is smaller than the $\Lambda -\Lambda$ threshold by over 100 MeV. The implications of our results on the present day relativistic heavy ion experiments are discussed.

Cumulant moments of negatively charged particles observed in hadron-nulceus collisions are analyzed by a leading particle cascade model. A modified negative binomial distribution (MNBD) or a negative binomial distribution (NBD) is used for multiplicity distribution from each participant hadron. If multiplicity distributions are truncated, both calculated results with the MNBD and the NBD can explain the oscillation of cumulant moments obtained from the data.

Equilibrium properties of infinite relativistic hadron matter are investigated using the ultrarelativistic quantum molecular dynamics (UrQMD) model. The simulations are performed in a box with periodic boundary conditions. Equilibration times depend critically on energy and baryon densities. Energy spectra of various hadronic species are shown to be isotropic and consistent with a single temperature in equilibrium. The variation of energy density versus temperature shows a Hagedorn-like behavior with a limiting temperature of $130\pm 10$ MeV. Comparison of abundances of different particle species to ideal hadron gas model predictions show good agreement only if detailed balance is implemented for all channels. At low energy densities, high mass resonances are not relevant; however, their importance rises with increasing energy density. The relevance of these different conceptual frameworks for any interpretation of experimental data is questioned.

The dynamics of partonic cascades may be an important aspect for particle production in relativistic collisions of nuclei at CERN SPS and BNL RHIC energies. Within the parton-cascade model, we estimate the production of single photons from such cascades due to the scattering of quarks and gluons $q\overrightarrow{g}q\gamma ,$ quark-antiquark annihilation $q\overline{q}\to g\gamma$ or γγ, and electromagnetic bremsstrahlung of quarks $\overrightarrow{q}q\gamma .$ We find that the latter QED branching process plays the dominant role for photon production, similarly as the QCD branchings $\overrightarrow{q}\mathrm{qg}$ and $\overrightarrow{g}\mathrm{gg}$ play a crucial role for parton multiplication. We conclude therefore that photons accompanying the parton-cascade evolution during the early stage of heavy-ion collisions shed light on the formation of a partonic plasma.