Existing data on the prescission neutron multiplicities have been analyzed with a new method combining time-dependent statistical cascade calculations with Feldmeier’s model of the dynamics of nucleus-nucleus collisions. The analysis resulted in the determination of the time scale ${\mathrm{\tau }}_{\mathit{f}}$ of fusion-fission reactions for different composite systems with excitation energies ranging from 80 to 240 MeV. The deduced values of ${\mathrm{\tau }}_{\mathit{f}}$ range from 5×${10}^{\mathrm{-}20}$ to ${10}^{\mathrm{-}18}$ s. These time scales are approximately 10 times longer than those deduced for the same set of data with the ‘‘static’’ model used by Hinde and collaborators.

A relativistic finite nucleus calculation using a Dirac optical potential is used to investigate the importance of final state interactions (FSI) at large momentum transfers in inclusive quasielastic electronuclear reactions. The optical potential is derived from first-order multiple scattering theory and then is used to calculate the FSI in a nonspectral Green’s function doorway approach. At intermediate momentum transfers excellent predictions of the quasielastic ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$(e,e’) experimental data for the longitudinal response function are obtained. In comparisons with recent measurements at ‖q→‖=1.14 GeV/c the theoretical calculations of ${\mathit{R}}_{\mathit{L}}$ give good agreement for the quasielastic peak shape and amplitude, but place the position of the peak at an energy transfer of about 40 MeV higher than the data.

Versions of the M3Y effective nucleon-nucleon interaction, with different prescriptions for the knock-on exchange contributions and density dependence, have been used in a study of the energy dependence of the nucleon optical potential in nuclear matter as well as in a systematic folding model analysis of the elastic ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${+}^{12}$C scattering data at ${\mathit{E}}_{\mathrm{lab}}$=60–318 MeV to study the energy dependence of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${+}^{12}$C optical potential. Contributions from the (breakup) dynamical polarization potential to the real part of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${+}^{12}$C optical potential are simulated by a surface correction using splines added to the real folded potential. This is shown to be strongest at ${\mathit{E}}_{\mathrm{lab}}$=99 MeV. The correction needed to fit the data is found to be qualitatively similar to that predicted theoretically for breakup of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. The optical model analysis of the refractive ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${+}^{12}$C scattering data, using different types of real folded potential, shows that the most successful is the folded potential built upon density-dependent interactions, which have parameters chosen to reproduce the saturation properties of nuclear matter and which predict a nuclear incompressibility K around 200 MeV.

Total and elemental charge-changing cross sections for reactions ΔZ=+1, -1, -2, -3 were measured for 10 GeV/nucleon ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ ions colliding with ${\mathrm{CH}}_2$, C, Al, Cu, Ag, and Pb targets. Contributions to the total cross sections by the process of electromagnetic dissociation are discussed.

Consideration of various time scales in A-A collisions suggests that it is relevant to determine qq¯ pair production rate at finite temperature. We have evaluated this rate using standard techniques of finite temperature field theory. At high temperature the rate is proportional to ${\mathit{T}}^2$ and has a value higher than the rate at T=0. It is also observed that the rate of pair production decreases as one moves away from the Lorentz contracted nuclei, towards the center in the c.m. coordinate system.

We predict the formation of highly dense baryon-rich resonance matter in Au+Au collisions at the Brookhaven Alternating Gradient Synchrotron (AGS) energies. The final pion yields show observable signs for resonance matter. The ${\mathrm{\Delta }}_{1232}$ resonance is predicted to be the dominant source for pions of small transverse momenta. Rescattering effects—consecutive excitation and deexcitation of Δ’s—lead to a long apparent lifetime (>10 fm/c) and rather large volumina (several 100 ${\mathrm{fm}}^3$) of the Δ-matter state. Heavier baryon resonances prove to be crucial for reaction dynamics and particle production at AGS.

We demonstrate the importance of the Bose-statistical effects for pion production in relativistic heavy-ion collisions. The evolution of the pion phase-space density in central collisions of ultrarelativistic nuclei is studied in a simple kinetic model taking into account the effect of Bose-stimulated pion production by the NN collisions in a dense cloud of mesons. It is assumed that pions are produced within a finite formation time at the stage of the projectile-target interpenetration. The mutual slowing down of nuclei is described schematically by introducing an effective deceleration length. The role of the quasideuteron and Δ-isobar mechanisms of pion absorption is investigated. The model predicts the enhancement of the soft pion yield as compared to inclusive pp data. The effect is sensitive to the pion formation time and is more pronounced at midrapidity and for heavy nuclei. The results are compared with pion spectra observed in central 10.7A GeV Au+Au and 200A GeV S+S collisions. We suggest new experiments sensitive to the Bose enhancement phenomena.

Heavy-ion collisions at intermediate energies are studied by using a new relativistic quantum molecular dynamics (RQMD) code, which is a covariant generalization of the quantum molecular dynamics (QMD) approach. We show that this new implementation is able to produce the same results in the nonrelativistic limit (i.e., 50 MeV/nucleon) as the noncovariant QMD. Such a comparison is not available in the literature. At higher energies (i.e., 1.5 GeV/nucleon and 2 GeV/nucleon) RQMD and QMD give different results in respect to the time evolution of the phase space, for example for the directed transverse flow. These differences show that consequences of a covarient description of heavy-ion reactions within the framework of RQMD exist even at intermediate energies.

In reactions with qq¯ production or qq¯ annihilation, initial- and final-state interactions give rise to large corrections to the lowest-order cross sections. We evaluate the correction factor first for low relative kinetic energies by studying the distortion of the relative wave function. We then follow the procedure of Schwinger to interpolate this result with the well-known perturbative QCD vertex correction factors at high energies, to obtain an explicit semiempirical correction factor applicable to the whole range of energies. The correction factor predicts an enhancement for qq¯ in color-singlet states and a suppression for color-octet states, the effect increasing as the relative velocity decreases. Consequences on dilepton production in the quark-gluon plasma, the Drell-Yan process, and heavy quark production processes are discussed.

The boundary of a boson system plays an important role in determining the momentum distribution of the bosons. For a boson system with a cylindrical boundary, the momentum distribution is enhanced at high transverse momenta but suppressed at low transverse momenta, relative to a Bose-Einstein distribution. The boundary effects on systems of massless gluons and massive pions are studied. For gluons in a quark-gluon plasma, the presence of the boundary may modify the signals for the quark-gluon plasma. For pions in a pion system in heavy-ion collisions, Coulomb final-state interactions with the nuclear participants in the vicinity of the central rapidity region further modify the momentum distribution at low transverse momenta. By including both the boundary effect and the Coulomb final-state interactions we are able to account for the behavior of the ${\mathrm{\pi }}^{\mathrm{-}}$ transverse momentum spectrum observed in many heavy-ion experiments, notably at low transverse momenta.

The values of the pion nucleon (πN) σ term, as determined, on the one hand, from experimental pion nucleon scattering by means of dispersion relations and, on the other hand, from baryon masses by means of chiral perturbation theory, differ by 10 to 15 MeV. The origin of this discrepancy is not yet understood. If the difference between the two values is attributed to the scalar current of strange sea quark pairs within the proton, the contribution to the proton mass would be of the order of 120 MeV. The discrepancy may hint at either theoretical deficiencies or an inadequate πN database. In order to provide reliable experimental data we have measured angular distributions of elastic pion proton scattering at pion energies ${\mathit{T}}_{\mathrm{\pi }}$=32.2 and 44.6 MeV using the magnet spectrometer LEPS located at the Paul-Scherrer-Institute (PSI) in Villigen, Switzerland. From the data covering the region of the Coulomb nuclear interference, the real parts of the isospin-even forward scattering amplitude Re${\mathit{D}}^{+}$(t=0), have been determined as a function of energy. The results have been compared with the predictions of the Karlsruhe-Helsinki phase shift analysis KH80, revealing discrepancies most pronounced for the ${\mathrm{\pi }}^{+}$p data. The experimentally determined values for Re${\mathit{D}}^{+}$(t=0), however, support the KH80 prediction (which is based on πN data available in 1979).

We report on measurements of the differential ${\mathrm{\pi }}^{\pm }$p cross section at pion energies ${\mathit{T}}_{\mathrm{\pi }}$=32.7, 45.1, and 68.6 MeV. The measurements, covering the angular range 25°≤${\mathrm{\theta }}_{\mathrm{lab}}$≤123°, have been carried out at the Paul-Scherrer-Institute (PSI) in Villigen, Switzerland, employing the magnet spectrometer LEPS. The absolute normalization of the ${\mathrm{\pi }}^{\pm }$p cross sections have been achieved by relating them to the electromagnetic cross sections of ${\mathrm{\mu }}^{\pm \mathrm{}12}$C scattering. The results are in agreement with those of our preceding measurements at ${\mathit{T}}_{\mathrm{\pi }}$=32.2 and 45.1 MeV insofar as they overlap with the region of the Coulomb nuclear interference investigated there. A comparison with the predictions of the Karlsruhe-Helsinki phase shift analysis KH80, which has formed the basis for the determination of the ‘‘experimental’’ σ term, reveals considerable deviations. These are most pronounced for the ${\mathrm{\pi }}^{+}$p cross sections at ${\mathit{T}}_{\mathrm{\pi }}$=32.7 and 45.1 MeV. Single energy partial wave fits result in S-wave contributions, which are about 1° lower in magnitude then those specified by the KH80 solution. The data at 68.6 MeV are in good agreement with the phase shift analysis.

We calculate thermal photon and neutral pion spectra in ultrarelativistic heavy-ion collisions in the framework of three-fluid hydrodynamics. Both spectra are quite sensitive to the equation of state used. In particular, within our model, recent data for S+Au at 200A GeV can only be understood if a scenario with a phase transition (possibly to a quark-gluon plasma) is assumed. Results for Au+Au at 11A GeV and Pb+Pb at 160A GeV are also presented.

We review the relation between covariant calculations and light-front calculations of pion properties. We describe the results obtained for the pion structure function in both formalisms. Conventional (phenomenological) light-front wave functions yield poor results for the structure function unless these results are evolved to higher energies. Our covariant calculation of the structure function has its basis in the Bethe-Salpeter approach to the solution of the two-body problem. Our study shows how an alternative specification of the light-front wave function of the pion leads to a good fit to the (valence) structure function, while also providing good fits for the pion-decay constant, ${\mathit{f}}_{\mathrm{\pi }}$, and the pion electromagnetic form factor (for ${\mathit{Q}}^2$<0.2 ${\mathrm{GeV}}^2$). We also review the standard scheme for evolving distributions from a scale ${\mathit{Q}}_0^2$ to a scale ${\mathit{Q}}^2$ and contrast such procedures with our methods.

The preequilibrium contribution to open charm production in nuclear collisions at √s =200 A GeV is calculated and shown to be sensitive to the early time evolution of the initial minijet gluon plasma. The role of charm as a probe of the formation zone physics and correlations between momentum and space-time coordinates is emphasized. While ideal (Bjorken) correlation between the rapidity y and space-time rapidity η of minijet gluons suppresses greatly the preequilibrium yield, the minimal correlation with finite geometrical spread enhances the initial fusion rate by a factor ∼2 in the moderate ${\mathit{p}}_{\mathrm{\perp }}$∼2–6 GeV range. That is dominated by fusion of ‘‘semihard’’ ${\mathit{p}}_{\mathrm{\perp }}$>2 GeV minijet gluons and secondary ‘‘soft’’ ${\mathit{p}}_{\mathrm{\perp }}$<2 GeV gluons from initial and final state radiation. The ‘‘intrinsic’’ charm process is negligible in the midrapidity domain.

Effective Lagrangians involving nucleons coupled to scalar and vector fields are investigated within the framework of relativistic mean-field theory. The study presents the traditional Walecka model and different kinds of scalar derivative couplings suggested by Zimanyi and Moszkowski. The incompressibility (presented in an analytical form), scalar potential, and vector potential at the saturation point of nuclear matter are compared for these models. The real optical potential for the models are calculated and one of the models fits well the experimental curve from -50 to 400 MeV while also giving a soft equation of state. By varying the coupling constants and keeping the saturation point of nuclear matter approximately fixed, only the Walecka model presents a first order phase transition for finite temperature at zero density.

A procedure is presented to calculate the partial and total conversion widths of Ξ hypernuclear states produced in the (${\mathit{K}}^{\mathrm{-}}$,${\mathit{K}}^{+}$) reaction on typical targets ${(}^{12}$C and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$). The final state after the ${\mathrm{\Xi }}^{\mathrm{-}}$p→ΛΛ conversion is classified into three types according to the bound and/or continuum two Λ’s: double-Λ sticking, single-Λ sticking, and no sticking. The probabilities leading to these final states are evaluated as a function of the Ξ-nucleus potential depth ${\mathit{V}}_0^{\mathrm{\Xi }}$. A possible final state with a neutron emission is also considered. It is confirmed that the interaction between the Ξ particle and a proton in the deep 0s orbit plays an important role to bring about the double-Λ sticking. The calculation shows that the double-Λ and single-Λ sticking probabilities are sensitive to the strength of ${\mathit{V}}_0^{\mathrm{\Xi }}$, which suggests that the observation of these probabilities provides us with a useful indicator for ${\mathrm{\Xi }}^{\mathrm{-}}$ hypernuclear bound states.

We apply the chiral color dielectric model to low- and medium-energy scattering within the coupled πN and πΔ system. Dynamic baryon states in which quarks are confined by the scalar color dielectric field are constructed in Fock space. Spurious motion of the center of mass is eliminated by constructing momentum eigenstates via a Peierls-Yoccoz projection. The relativistic Lippmann-Schwinger equation is solved for the complex energies of the T matrix poles, and the pole positions of the nucleon and delta are used to fix the few parameters of the model. The S- and P-wave phase shifts, the bare N and Δ masses, the renormalized πNN and πNΔ coupling constants, and the πNN and πNΔ vertex functions are predicted. We conclude that the monopole form factors often found in the literature are inadequate for reproducing πN scattering, and that more realistic ones such as those from the color dielectric model are needed. We also conclude that the radius parameter of the form factor is more than twice as large as that preferred by meson exchange potential models. By renormalizing the πN vertex by the pion field, the ratio of the πNΔ to πNN coupling constants is closer to experiment than the SU(6) prediction.

Giant dipole resonance (GDR) γ rays were measured in coincidence with reaction fragments in ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$${+}^{165}$Ho at 368 MeV where deep inelastic scattering and quasifission dominate the reaction. The γ spectrum associated with deep inelastic scattering events is well fitted by statistical cooling of projectile and target-like fragments with close to equal initial energy sharing. The γ spectrum associated with quasifission events is well described by statistical emission from the fission fragments alone, with only weak evidence for GDR emission from the mononucleus. A 1σ limit of τ≤11×${10}^{\mathrm{-}21}$ s is obtained for the mononucleus lifetime which is consistent with the lifetimes obtained from quasifission fragment angular distributions.

High-spin states of ${}_{}{}^{114}{}_{}{}^{}\mathrm{Te}$ have been studied using the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$${(}^{28}$Si,p2n) and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$${(}^{29}$Si,p3n) reactions at beam energies of 106 and 108 MeV, respectively. γ-γ and particle-γ-γ coincidences, directional correlations of oriented states, and excitation functions have been measured using six BGO Compton suppressed Ge detectors and several particle detectors. We established the states of ${}_{}{}^{114}{}_{}{}^{}\mathrm{Te}$ up to 9 MeV excitation energy and found the new two-phonon $2_2^{+}$, $4_2^{+}$, and $6_2^{+}$ states. The properties of the states in ${}_{}{}^{114}{}_{}{}^{}\mathrm{Te}$ are discussed in comparison with the other even-even tellurium isotopes.