The charge distributions of fission products in 24 MeV proton-induced fission of ${}^{238}\mathrm{U}$ were measured by the use of an ion-guide isotope separator on line. The most probable charge ${(Z}_p)$ of the charge distribution was discussed in view of the charge polarization in the fission process. It was found that $Z_p$ mainly lies on the proton-rich side in the light mass region and on the proton-deficient side in the heavy mass region compared with the postulate of the unchanged charge distribution. The charge polarization was examined with respect to production $Q$ values.

The influence of the energy dependence of the free $\mathrm{NN}$ $t$ matrix on the optical potential of nucleon-nucleus elastic scattering is investigated within the context of a full-folding model based on the impulse approximation. The treatment of the pole structure of the $\mathrm{NN}$ $t$ matrix, which has to be taken into account when integrating to negative energies, is described in detail. We calculate proton-nucleus elastic scattering observables for ${}^{16}$O, ${}^{40}$Ca, and ${}^{208}$Pb between 65 and 200 MeV laboratory energy and study the effect of the energy dependence of the $\mathrm{NN}$ $t$ matrix. We compare this result with experiment and with calculations where the center-of-mass energy of the $\mathrm{NN}$ $t$ matrix is fixed at half the projectile energy. It is found that around 200 MeV the fixed energy approximation is a very good representation of the full calculation; however, deviations occur when going to lower energies (65 MeV).

New theoretical calculations have been done that compare π- and ρ-exchange effects in exclusive coherent production of pions for ${}^{12}{\mathrm{C}+}^{12}{\overrightarrow{C}}^{12}{\mathrm{C}+}^{12}{\mathrm{C}+\mathrm{\pi }}^0$ below and above the pion threshold at incident energies of 100, 250, and 400 MeV/nucleon. Besides the important enhancement effects to the pion distributions due to the constructive coherence of Δ-hole and particle-hole terms describing the coherent, intermediate nuclear states of both projectile and target, additional effects have been observed due to the ρ-exchange transition interaction. Calculations of the pion angular distributions, which are sensitive to the ρ-exchange effect, are done that compare the $(\pi +\rho )$-exchange effects to the π-exchange effect alone. It is shown that at extreme angles, where the projectile scatters forward $({\Theta }_P=0\mathrm{}°),$ and the target scatters backward $({\Theta }_T=180\mathrm{}°),$ and the pion scattering forward or backward (${\theta }_{\pi }=0\mathrm{}°$ or 180°), that the ρ amplitude is zero under the Born approximation. The insensitivity of the pion-energy distributions to ρ exchange is also explained. It is shown that the ρ-exchange effect is a rather mild one which affects only the midregion of the pion-angular distributions for the particular physical channels and incident energies considered here.

A dispersive optical potential for the interaction between low-energy neutrons and ${}^{12}\mathrm{C}$ nuclei is derived from a dispersion relation based on the Feshbach generalized optical model. The potential reproduces completely neutron total cross sections below 1.0 MeV and substantially reproduces the energy of the $3090\mathrm{keV}{(1/2\mathrm{}}^{+})$ level in ${}^{13}\mathrm{C}$ which is of nearly pure ${2s}_{1/2}$ single-particle character. It is found that direct-capture model calculations with this potential explain quite successfully the observed off-resonance capture transitions to the ground ${(1/2\mathrm{}}^{-}),$ $3090\mathrm{keV}{(1/2\mathrm{}}^{+}),$ $3685\mathrm{keV}{(3/2\mathrm{}}^{-}),$ and $3854\mathrm{keV}{(5/2\mathrm{}}^{+})$ levels in ${}^{13}\mathrm{C}$ at neutron energies of 20–600 keV. Special emphasis is laid on the fact that in these model analyses, account should be taken of the spatial nonlocality of the neutron-nucleus interaction potential, in particular for negative energies.

The chaos weighted wall formula developed earlier for systems with partially chaotic single-particle motion is applied to large amplitude collective motions similar to those in nuclear fission. Considering an ideal gas in a cavity undergoing fissionlike shape evolutions, the irreversible energy transfer to the gas is dynamically calculated and compared with the prediction of the chaos weighted wall formula. We conclude that the chaos weighted wall formula provides a fairly accurate description of one-body dissipation in dynamical systems similar to fissioning nuclei. We also find a qualitative similarity between the phenomenological friction in nuclear fission and the chaos weighted wall formula. This provides further evidence for the one-body nature of the dissipative force acting in a fissioning nucleus.

We study the energy dependence of the cross sections for nucleon removal of ${}^8\mathrm{B}$ projectiles. It is shown that the Glauber model calculations with nucleon-nucleon $t$-matrix reproduce well the energy dependence of the breakup cross sections of ${}^8\mathrm{B}.$ A distorted wave Born approximation (DWBA) model for the breakup cross section is also proposed and results are compared with those of the Glauber model. We show that to obtain an agreement between the DWBA calculations, the Glauber formalism, and the experimental data, it is necessary to modify the energy behavior of the effective interaction. In particular, the breakup potential has a quite different energy dependence than the strong absorption potential.

Existing intermediate and high energy $\phi$-photoproduction data is consistent with purely diffractive production (i.e., Pomeron exchange). However, near threshold $(1.574\mathrm{}\hspace{0.5em}\mathrm{GeV}<E_{\gamma }^{\mathrm{lab}}<~2.0\hspace{0.5em}\mathrm{GeV})$ the energy dependence of the diffractive amplitude is not well known and nondiffractive amplitudes such as ${\pi }^0$ and $\eta$ exchange, $s\overline{s}$ knockout, and nonzero $\phi \mathrm{NN}$ couplings can give sizable contributions to the cross section and polarization density matrix elements of the $\overrightarrow{\phi }{K}^{+}{K}^{-}$ decay angular distribution. We stress the importance of measurements with linearly polarized photons near the $\phi$ threshold to separate natural and unnatural parity exchange mechanisms. Approved and planned $\phi$ photoproduction and electroproduction experiments at Jefferson Lab will help establish the relative dynamical contributions near threshold and clarify outstanding theoretical issues related to apparent Okubo-Zweig-Iizuka violations.

A new Monte Carlo precompound decay model is used to calculate double-differential spectra using an approach based on conservation of linear momentum between an incident nucleon and the three quasiparticles activated in the collisions of the equilibration process. The angular distribution theory is presented as it is applied to the individual three-exciton cascades of the precompound model. The importance of multiple precompound decay beyond two nucleons per nucleus is illustrated for ${}^{90}$Zr$\mathrm{}(p,xn)\mathrm{}$ reactions for incident protons up to 256 MeV. These components are shown to be important for extending the useful range of the precompound approach to energies beyond 80 MeV. The contribution to nucleon spectra due to hole conversion processes is illustrated. We compare results of the formulation described with experimental angular distributions for ${}^{90}$Zr$\mathrm{}(p,xn)\mathrm{}$ reactions at incident energies of 45, 80, and 160 MeV, and for the ${}^{90}$Zr$\mathrm{}(p,xp)\mathrm{}$ reactions at incident energies of 80, 160, and 200 MeV. Additionally, ${}^{208}$Pb$\mathrm{}(p,xn)\mathrm{}$ spectra at 7.5°, 30°, 60°, 120°, and 150° are compared with two experimental data sets for 256 MeV incident proton energy, and ${}^{90}$Zr$\mathrm{}(p,xn)\mathrm{}$ spectra at the same five angles are compared with predicted results for 256 MeV incident protons. At 256 MeV incident proton energy, the higher energy emission spectra are seriously overpredicted at 60°. All other results are reproduced quite well, including back-angle yields. The quasielastic peak measured at 7.5° at 256 MeV incident proton energy is shown to be in quite reasonable agreement with experimental results.

The production of charged pions, protons, and deuterons has been studied in central collisions of ${}^{58}\mathrm{Ni}$ on ${}^{58}\mathrm{Ni}$ at incident beam energies of $1.06A$, $1.45A$, and $1.93A$ GeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for between 10A and 200A GeV at the Brookhaven AGS and the CERN SPS accelerators energies is valid down to $1A$ GeV. The degree of nuclear stopping is discussed; quantum molecular dynamics calculations reproduce the measured proton rapidity spectra for the most central events reasonably well, but do not show any sensitivity between the soft and the hard equation of state. A radial flow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures $T$ and radial flow velocities ${\beta }_r$ which increase with beam energy up to $2A$ GeV; in comparison to the existing data of Au on Au over a large range of energies, only ${\beta }_r$ shows a system size dependence.

The production of dielectrons in reactions involving $a_1$ mesons and pions is studied. We compare results obtained with different phenomenological Lagrangians that have been used in connection with hadronic matter and finite nuclei. We insist on the necessity for those interactions to satisfy known empirical properties of the strong interaction. Large off-shell effects in dielectron production are found and some consequences for the interpretation of heavy ion data are outlined. We also compare with results obtained using experimentally extracted spectral functions.

The idea that effective string tension increases as a result of the hard gluon kinks on a string is applied to study the strange particle production in proton-nucleus and nucleus-nucleus collisions. It is found that the effective string tension increases with the increase of centrality and mass of the colliding system as a consequence of the minijet (gluon) production stemming from the collective string-string interaction. This mechanism leads to strangeness enhancement in $\mathrm{pA}$ and $\mathrm{AA}$ collisions through the enhanced production of the strange quark pairs from the color field of strings. We discuss different roles played by this mechanism and rescattering of the final state hadrons in the production of strange particles and compare our results with experimental data.

In noncentral heavy-ion collisions, identical two-particle Hanbury-Brown–Twiss (HBT) correlations $C(\mathbf{K},\mathbf{q})$ depend on the azimuthal direction of the pair momentum K. We investigate the consequences for a harmonic analysis of the corresponding HBT radius parameters $R_{\mathrm{ij}}^2.$ Our discussion includes both, a model-independent analysis of these parameters in the Gaussian approximation, and the study of a class of hydrodynamical models which mimic essential geometrical and dynamical properties of peripheral heavy-ion collisions. Also, we discuss the additional geometrical and dynamical information contained in the harmonic coefficients of $R_{\mathrm{ij}}^2.$ The leading contribution of their first and second harmonics are found to satisfy simple constraints. This allows for a minimal, azimuthally sensitive parametrization of all first and second harmonic coefficients in terms of only two additional fit parameters. We determine to what extent these parameters can be extracted from experimental data despite finite multiplicity fluctuations and the resulting uncertainty in the reconstruction of the reaction plane.

Disoriented chiral condensates or long wavelength pionic oscillations and their interaction with the thermal environment can be a significant source of dileptons. We calculate the yield of such dilepton production within the linear σ model, both in a quantum mean-field treatment and in a semiclassical approximation. We then illustrate the basic features of the dilepton spectrum in a schematic model. We find that dilepton yield with invariant mass near and below ${2m}_{\pi }$ due to the soft pion modes can be up to two orders of magnitude larger than the corresponding equilibrium yield.

The Baldin sum rule for the nucleon has been recalculated at the light of the most recent photoabsorption cross section measurements. The proton value $(\alpha +\beta {)}_p=13.69\mathrm{}\pm 0.14$ is smaller but consistent with the one usually quoted in literature. However, the value for the neutron $(\alpha +\beta {)}_n=14.40\mathrm{}\pm 0.66$ turns out to be three standard deviations away from the previously calculated one.

Several methods for the treatment of pion photoproduction in the region of the Δ(1232) resonance are discussed, in particular the effective Lagrangian approach and the speed plot analysis are compared to a dynamical treatment. As a main topic, we discuss the extraction of the genuine resonance parts of the magnetic dipole and electric quadrupole multipoles of the electromagnetic excitation of the resonance. To this end, we try to relate the various values for the ratio $R_{\mathrm{EM}}$ of the $E2\mathrm{}$ to $M1\mathrm{}$ multipole excitation strengths for the Δ(1232) resonance as extracted by the different methods to corresponding ratios of a dynamical model. Moreover, it is confirmed that all methods for extracting resonance properties suffer from an unitary ambiguity which is due to some phenomenological contributions entering the models.

The nonresonant (seagull) contribution to the nuclear Compton amplitude at low energies is strongly influenced by nucleon correlations arising from meson exchange. We study this problem in a modified Fermi gas model, where nuclear correlation functions are obtained with the help of perturbation theory. The dependence of the mesonic seagull amplitude on the nuclear radius is investigated and the influence of a realistic nuclear density on this amplitude is discussed. We found that different form factors appear for the static part (proportional to the enhancement constant $\kappa )$ of the mesonic seagull amplitude and for the parts, which contain the contribution from electromagnetic polarizabilities.

Photoproduction of $e^{+}{e}^{-}$ pairs at small angles is investigated as a tool to determine the functions $f_1$ and $f_2$ entering the real-photon forward Compton scattering amplitude. The method is based on an interference of the Bethe-Heitler and the virtual Compton scattering mechanisms, generating an azimuthal asymmetry in the $e^{+}$ versus $e^{-}$ yield. The general case of a circularly polarized beam and a longitudinally polarized target allows one to determine both the real and imaginary parts of $f_1$ as well as $f_2$. The imaginary part of $f_2$ requires target polarization only. We calculate cross sections and asymmetries of the reaction $p\left(\gamma {,e}^{+}{e}^{-}\right)p$, estimate corrections and backgrounds, and propose suitable kinematical regions to perform the experiment. Our investigation shows that photoproduction of $e^{+}{e}^{-}$-pairs off the proton and light nuclei may serve as a rather sensitive test of the validity of the Gerasimov-Drell-Hearn sum rule.

QCD sum rules for decuplet baryon two-point functions are investigated using a comprehensive Monte Carlo based procedure. In this procedure, all uncertainties in the QCD input parameters are incorporated simultaneously, resulting in realistic estimates of the uncertainties in the extracted phenomenological parameters. Correlations between the QCD input parameters and the phenomenological parameters are studied by way of scatter plots. The predicted couplings are useful in evaluating matrix elements of decuplet baryons in the QCD sum rule approach. They are also used to check a cubic scaling law between baryon couplings and masses, as recently found by Dey and co-workers. The results show a significant reduction in the scaling constant and some possible deviations from the cubic law.

We have calculated the nucleon form factors $G_{E,M}^{\mathrm{}(p,n)\mathrm{}}{(q}^2)$ in the linear σ model to one-meson-loop order plus (two-loop) $\gamma \pi \pi \pi$ anomaly. The previously derived $\gamma \pi \pi \pi$ anomaly generally reduces the nucleon radii and produces a shift of the magnetic moments of order 0.1 ${\mu }_N$ or less. We present analytical results for $G_{E,M}^{p,n}\mathrm{}\left(0\right)\mathrm{}$ which display explicitly their dependence on hadron masses and coupling constants. Analytical results for the radii are also given and the chiral singularities they contain $(\ln m_{\pi }$ and $m_{\pi }^{-1})$ are exposed. These come from the $\pi \pi$ intermediate state contribution to the form factors and not from the chiral quark substructure of nucleons or mesons. The leading chiral singularity has a universal strength while the chiral log (next-to-leading singularity) picks up a model (or approximation) dependence in terms of $g_A$ and the threshold behavior of the $\pi N$ amplitude $A^{\left(-\right)}(\nu ,0)$. The chiral singularities appear only in the isovector nucleon radii $〈r_{1,2}^2{〉}^{I=1\mathrm{}}$, the leading $m_{\pi }^{-1}$ term appears only in $〈r_2^2{〉}^{I=1\mathrm{}}$ due to a peculiar cancellation between two independent form factor combinations ${\Gamma }_{1,3}{(q}^2)$. The isoscalar anomaly $\gamma \pi \pi \pi$ is finite for $m_{\pi }\to 0$.

The combined absence of physical realizations of the $U_A\mathrm{}\left(1\right)\mathrm{}$ symmetry possessed by the classical QCD action in the chiral limit and of an isoscalar Goldstone boson associated with its spontaneous breakdown has been dubbed the $U_A\mathrm{}\left(1\right)\mathrm{}$ problem. A formal resolution of this problem proposed by ’t Hooft relies on instantons to provide a mass to the would-be Goldstone boson $\left({\eta }^{\prime }\right).\mathrm{}$ An alternate scheme for the generation of an ${\eta }^{\prime }$ mass proposed by Kogut and Susskind derives from quark annihilation into gluons and a strong infrared singularity in the gluon propagator associated with confinement. We demonstrate here how such diagrams are generated in quark-based effective theories by including a certain class of diagrams which arise from correlated $q\overline{q}$ exchange and are of higher order ${1/N}_c.$ A low-energy energy expansion of this correction is of the form discussed by Witten, di Vecchia, and Veneziano.