An absorption model based on the eikonal approximation of the nuclear optical model is proposed for the transverse and the forward energy distributions in ultrarelativistic heavy-ion collisions. It is shown that the measured distributions can be expressed by their geometric distribution convoluted by a Gaussian distribution with centroid and width determined by the collision geometry and the average properties of $N$-$N$ collisions. This result provides a physical argument for the formula used in the data fitting of the transverse energy distribution in the literature. The forward energy distribution formula is shown to be applicable for the data fitting.

We study the Walecka model in one and two spatial dimensions within a mean-field approach. Inspired by infinite nuclear matter, where this model is usually applied, we compare how the observables behave when their phase space is reduced. We find a closed and unified relativistic expression for the compression modulus as well as the relativistic expansion of the model, both dependent on dimension only. This relativistic expansion is analyzed regarding the saturation and/or collapse of the system.

A complicating factor in using hadrons as early probes for relativistic heavy-ion collisions is that they can undergo strong final-state rescattering with each other. This potentially causes hadronic observables to reflect the condition of the system at a late stage rather than at an early one. The present work uses a rescattering calculation with resonances to unfold these effects and thus uses measured observables to constrain a simple initial-state hadronization model for $\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$ collisions at CERN energies. A model parameter set is found which qualitatively agrees with measured single and two-particle observables for $\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$ collisions.

The multipion correlation effect on the source distribution is studied. It is shown that multipion Bose-Einstein correlations make the average radius of the pion source become smaller. The isospin effect on the pion multiplicity distribution and the source distribution is also discussed.

We use a simple QCD-based model to study particle production in S+Au collisions at 200 GeV/nucleon. A requisite consistency is met for the hadronic observables $({\pi }^0$ and ${\pi }^{-}$ spectra) while pursuing estimates for $e^{+}{e}^{-}$ production. Since radiative decays of initially produced hadrons has accounted for only a portion of the observed dileptons at the CERN SPS, we search for additional mechanisms. By including contributions from prompt secondary hadronic scatterings, $\pi \overrightarrow{\rho }\pi e^{+}{e}^{-}$, and adding to ${\pi }^{+}{\pi }^{-}$ annihilation and hadronic decays, part of the “excess” dilepton signal can possibly be interpreted.

We study the propagation of charm quarks, produced from the initial fusion of partons, in an equilibrating quark-gluon plasma which may be formed in the wake of relativistic collisions of gold nuclei. Initial conditions are taken from a self-screened parton cascade model and the chemical equilibration is assumed to proceed via gluon multiplication and quark production. The energy loss suffered by the charm quarks is obtained by evaluating the drag force generated by the scattering with quarks and gluons in the medium. We find that the charm quarks may lose only about 10% of their initial energy in conditions likely to be attained at the Relativistic Heavy Ion Collider, while they may lose up to 40% of their energy while propagating in the plasma created at the Large Hadron Collider. We discuss the implications for signals of the quark-gluon plasma.

We study the effect of energy loss on charm and bottom quarks in high-energy heavy-ion collisions including longitudinal expansion and partial thermalization. We find that high-$p_{\perp }$ heavy quarks are greatly suppressed, and consequently, high-mass dileptons from heavy quark decays are also suppressed. We consider in detail the detector geometry and single lepton energy cuts of the PHENIX detector at the Relativistic Heavy Ion Collider. Because of the longitudinal expansion, the suppressions of $\mathrm{ee}$, $e\mu$, and $\mu \mu$ pairs resulting from the heavy quark energy loss are very different due to the different rapidity coverages and energy cuts. The assumed energy loss rate, on the order of 1 GeV/fm, results in a large suppression on dielectrons, and dielectrons from heavy quark decays become comparable or even lower than the Drell-Yan yield. It is thus possible to probe the energy loss rate of the medium using dileptons from heavy quark decays.

The deconfinement transition region between hadronic matter and quark-gluon plasma (QGP) is studied for finite volumes. Assuming simple model equations of state and a first order phase transition, we find that fluctuations in finite volumes hinder a sharp separation between the two phases around the critical temperature, leading to a rounding of the phase transition. For reaction volumes expected in heavy ion experiments, the softening of the equation of state is reduced considerably. This is especially true when the requirement of exact color singletness is included in the QGP equation of state.

We present a dynamical study of $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $\rho$-meson spectral function that includes the pion modifications in the nuclear medium as well as the polarization of the $\rho$ meson due to resonant $\rho -N$ scattering. We find that the experimental data from the CERES and HELIOS-3 Collaborations can be described equally well as within the dropping $\rho$-mass scenario. Whereas corresponding dilepton $q_T$ spectra are found to be very similar, the inclusive dilepton yield in the invariant mass range $0.85<~M<~1.0$ GeV should allow us to disentangle the two scenarios experimentally.

The importance of an isospin dependent nuclear mean field (IDMF) in regard to the pion production mechanism is studied for the reaction Au+Au at 1 GeV/nucleon using the quantum molecular dynamics (QMD) model. In particular, the effect of the IDMF on pion spectra and the charged pion ratio are analyzed. It is found that the inclusion of a IDMF considerably suppresses the low-$p_t$ pions, thus leading to a better agreement with the data on pion spectra. Moreover, the rapidity distribution of the charged pion ratio appears to be sensitive to the isospin dependence of the nuclear mean field.

We formulate a QCD sum rule to find the three-momentum (q) dependence of the peak position of the vector meson spectral density in the nuclear medium. For both the longitudinal and transverse polarization direction of the vector meson with respect to q, we find less than a 2% (0.1%) shift of the peak position in nuclear matter density and at $\mathbf{q}=0.5\mathrm{}\mathrm{GeV}/c$ for the ρ,ω (φ) mesons. This justifies neglecting the three-momentum dependence and the polarization effect when implementing the universal scaling laws of the vector mesons in understanding the dilepton spectrum in the $A-A$ and $p-A$ reactions.

$\pi \pi$ scattering amplitudes inside the subthreshold triangle $(0\mathrm{}<~s<~\Sigma$, $0<~t<~\Sigma$, $0<~u<~\Sigma =4\mathrm{}{\mu }^2)$ are studied using interior dispersion relations. The dispersion integrals are evaluated above a center-of-mass energy $W_{\pi \pi }=0.6\mathrm{}$ GeV by using a standard phase shift analysis and from this value down to threshold by using unitary models consistent with existing low-energy experimental data. It was found that by restricting the $s$-wave scattering lengths to lie on a “universal curve” all crossing properties and appropriated threshold sum rules were reasonably well satisfied. The invariant amplitudes are found to be in good qualitative agreement with the predictions of Weinberg’s chiral model and its corrections derived from chiral perturbation theory.

We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $e\overrightarrow{p}{e}^{\prime }{p}^{\prime }\gamma$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.

We argue that the leading charm effect observed in pion-nucleon interactions indicates the presence of rather strong color fields in hadronic collisions. We show that a flux tube picture which includes such strong fields accounts naturally for the leading charm phenomenon, providing a unified description of the production of charmed and noncharmed leading particles.

We have made the first attempt at determining the ${}^8$Li$(n,\gamma )$${}^9$Li cross section at astrophysical energies. This reaction competes with the ${}^8$Li$(\alpha {,n)}^{11}$B reaction in the inhomogeneous big bang model and with ${}^8$Li $\beta$ decay in the $r$ process. It may affect the primordial abundance and the stellar production of A>12 nuclei. Using a radioactive beam of ${}^9$Li and the Coulomb dissociation method we attempted to measure the cross section of the inverse reaction ${}^9$Li$(\gamma {,n)}^8$Li. We report only an upper limit because we were unable to determine and subtract the nuclear dissociation. Through the detailed balance theorem, this lead to an upper limit for the cross section of interest, ${}^8$Li$(n,\gamma )$${}^9$Li. The limit is 19$\pm$8 b for $E_n$ = 0–500 keV or, when expressed as a reaction rate, 7200 cm${}^3$ s${}^{-1}$ mole${}^{-1}$.

New spectroscopic strength information extracted through ${}^{102}$Ru${\mathrm{}(d,p)\mathrm{}}^{103}$Ru shows the ${3/2}^{+}$ ground state in ${}^{103}$Ru to be poorly populated in both one-neutron transfer reactions, in contrast with former claims. In comparison with ${}^{101}$Ru, particle and hole strengths are significantly reduced in ${}^{103}$Ru and these facts are interpreted as signs of a poor overlap of the odd isotope with its even neighbors plus or minus one neutron. Very-low-lying ${3/2}^{+}$ levels, associated with small one-particle spectroscopic factors, are shown to exist in most odd-neutron nuclei in the region and may represent the lowest state of a coexisting configuration.

Fission fragment angular distributions and fission excitation functions have been measured for ${}^{11}\mathrm{B}$, ${}^{12}\mathrm{C}$, and ${}^{14}\mathrm{N}{+}^{209}\mathrm{Bi}$ systems at near-barrier energies. The measured fission fragment anisotropies are in agreement with the predictions of the standard saddle point statistical model. For these systems, there is no evidence for any “anomalous” behavior of the anisotropies as a function of both energy and entrance channel mass asymmetry. The present results combined with the data available for ${}^{16}\mathrm{O}$, ${}^{19}\mathrm{F}{+}^{208}\mathrm{Pb}$, and ${}^{16}\mathrm{O}{+}^{209}\mathrm{Bi}$ systems and for deformed actinide targets systems are consistent with the conjecture that target deformation plays a strong role in the observed anomalous anisotropies in actinide targets.

By using the complex scaling method, the ${}^{10}\mathrm{C}+p$ resonance parameters are determined in order to investigate the spectroscopic property of the proton-rich nucleus ${}^{11}$N in comparison with the mirror nucleus ${}^{11}$Be which has a typical neutron halo structure. The obtained resonance energy ${(E}_r)$ and width $\left(\Gamma \right)$ of ${}^{11}$N are ${(E}_r,$ Γ)=(1.29 MeV, 1.39 MeV) and (2.12 MeV, 0.95 MeV) for $s_{1/2}$ and $p_{1/2}$ states, respectively. These results are in good agreement with the recent experimental data reported by Axelsson et al. Thomas-Ehrman effect on the ${1/2}^{-}$ and ${1/2}^{+}$ resonance states in ${}^{11}$N is discussed in association with the weakly bound states in ${}^{11}$Be.

A master equation is incorporated in Feshbach, Kerman, and Koonin model calculations of statistical multistep compound (MSC) emission. Damping $X$ and $Y$ functions which describe the particle-hole annihilation process are derived. The MSC cross sections are calculated with the master equation. The effect is found to be large at lower energies for light nuclei, but not significant when the incident energy is above 20 MeV. The difference between a closed form solution and the master equation result is masked by the large multistep direct and evaporation components.

Quantum mechanical optical model methods for calculating isotope production cross sections from the spallation of heavy nuclei by high-energy protons are developed from a modified abrasion-ablation collision formalism. The abrasion step is treated quantum-mechanically as a knockout process which leaves the residual prefragment nucleus in an excited state. In ablation the prefragment deexcites to produce the final fragment. The excitation energies of the prefragments are estimated from a combination of liquid drop and frictional-spectator interaction considerations. Estimates of elemental and isotopic production cross sections are in good agreement with recently published cross section measurements.