Two proton correlation functions have been measured at 30 MeV/nucleon for various systems ${(}^{20}$Ne${+}^{12}$C${,}^{27}$Al${,}^{59}$Co${,}^{197}$Au) in the forward hemisphere. Source radii extracted by comparisons to the final state interaction model of Koonin turn out to be independent of the mean emission angle of the protons as well as of the size of the studied system, and little affected by energy cuts on the detected proton pairs. Typical size (∼5.5–6 fm) exceeds the radius of the compound nucleus for almost all studied systems, suggesting significant time delays between proton emissions. However, similarity of transverse and longitudinal correlation functions seems to eliminate the existence of a long lifetime. Due to the poor sensitivity of the proton-proton correlation function, this method seems not well suited for source size measurements in this energy domain.

A simple derivation is given of the nonrelativistic Ward-Takahashi identity, which has been used in nearly every derivation of low-energy theorems for electromagnetic processes in nuclei. The low-energy theorem for Compton scattering is derived without assuming that identity. It is shown that gauge invariance alone leads to the correct form for the Compton amplitude, except that the magnetic recoil term does not have its conventional form. Use of Siegert’s theorem, however, leads to that form.

The relativistic microscopic optical potential, Schrödinger-equivalent potential, and mean free paths of a nucleon at finite temperature in nuclear matter and finite nuclei are studied based on Walecka’s meson-nucleon model and thermofield dynamics. We set the Hartree-Fock self-energy of a nucleon to represent the real part of the microscopic optical potential and the fourth order of meson exchange diagrams; i.e., the polarization and correlation represent the imaginary part of microscopic optical potential in nuclear matter. The calculated results are expected to be valid to nucleon energies between -40 and 300 MeV relative to the Fermi energy.

We have evaluated the mechanism analogous to pion absorption which contributes to the pion double-charge-exchange amplitude leading to double isobar analog states. We find appreciable corrections at low energies around ${\mathit{T}}_{\mathrm{\pi }}$=50 MeV. Around the resonance region the corrections are small and the most visible effect is a small shift of the minimum to smaller angles, which is however not big enough to reproduce the experimental data.

The availability of polarized nuclear targets for nuclear reaction studies offers new research directions. In this paper we present the distorted wave impulse approximation formalism and calculations for two-nucleon pion absorption on polarized nuclear targets. We assume that the pion absorbs on a ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ n-p pair and use phenomenological amplitudes to describe the πNN vertex. The effects of the distortions on the incoming pion and outgoing protons, in contrast to the elementary πNN vertex, are delineated for certain special cases in which the formalism simplifies. Predictions of cross sections and vector and tensor analyzing powers are made for polarized target nuclei ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ at pion incident energies of 115, 165, and 255 MeV.

Observables in two-body reactions such as cross sections, polarizations, and spin correlations are often of simple form at threshold, since only a few partial waves contribute. Spin observables near threshold for meson photoproduction reactions (with emphasis on the γp→${\mathit{K}}^{+}$Λ case) are studied in this paper. The expected nodal structure of all sixteen observables near threshold, their evolution with momentum, the role of the centrifugal barrier on threshold multipole amplitudes, and the role of resonance effects on spin observables are discussed.

The effects of higher configurations in the ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ wave function are studied in a variety of reactions, such as ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$(e,e${)}^{15}$N, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$(γ,${\mathrm{\pi }}^{\mathrm{-}}$${)}^{15}$${\mathrm{O}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}$, and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$(${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^0$${)}^{15}$${\mathrm{O}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}$. A (0+2)ħω wave function for the A=15 system that includes important contributions from the (2s,1d) as well as (2p,1f) orbitals modifies mostly the Fermi transitions and provides an improved description of the pion single charge exchange data on ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$. The (γ,${\mathrm{\pi }}^{\mathrm{-}}$) data require a wave function that can simultaneously fit the M1 form factor.

We investigate the meson exchange current (MEC) contribution to ${\mathit{K}}^{+}$-nucleus elastic scattering. Starting with a model of the off-shell Kπ scattering amplitude and a calculation of the excess pion distribution in finite nuclei, we proceed to derive both real and imaginary forward scattering amplitudes for the MEC contribution. The resulting amplitudes interfere constructively with the optical model amplitudes, and, therefore, lead to an increase in both calculated total and differential cross sections. This increase brings about a better agreement with the data, in particular with regard to the energy dependence of the total cross section. This last feature follows from the fact that the calculated imaginary MEC amplitude has a threshold at incident momentum p∼400 MeV/c, which distinguishes our MEC model from recently proposed nucleon ‘‘swelling’’ models. We also examine the sources of uncertainties for our results.

We extend the theory of particle-induced precompound reactions by including gamma decay. We use the Brink-Axel hypothesis and consider the gamma emission of giant-dipole resonances built on the ground state and on the excited states of the composite system. The latter are modeled as multiparticle multihole excitations. In this way, we combine the statistical ansatz and the chaining hypothesis typical for precompound reaction theories, with the collective aspects of gamma decay. Formulas for average S matrix and average cross section are derived in this framework.

The analytically obtained previous results on capture gamma ray reactions are used for a direct numerical calculation. It turns out that this formulation allows for a parameter-free description of gamma emission in precompound reactions. As an example we choose reactions induced by 14.1 MeV neutrons incident on ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$. The individual contributions of different terms to the total cross section are discussed in detail and a comparison to experimental data is pursued.

The production of the ${\mathrm{\Delta }}^{++}$ resonance in high energy collisions of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ with the carbon nucleus, using a 2 m propane bubble chamber, was investigated. The technique for background estimation is proposed. The mass and the width of the resonance were obtained. The ratio between pion production from ${\mathrm{\Delta }}^{++}$ and direct pion creation was estimated assuming that the NN→NΔ process is dominant.

Cross sections for inclusive electron scattering from aluminum have been measured at both forward and backward angles in the kinematic region near x=1 and 1.75<${\mathit{Q}}^2$<7 (GeV/c${)}^2$. The forward-angle data are in good agreement with a recent calculation. Both the data and the calculation show scaling behavior for ${\mathit{F}}_2$(ξ,${\mathit{Q}}^2$)=ν${\mathit{W}}_2$(ξ,${\mathit{Q}}^2$) at high ${\mathit{Q}}^2$ for fixed values of the Nachtmann variable ξ. The ratio R=${\mathrm{\sigma }}_{\mathit{L}}$/${\mathrm{\sigma }}_{\mathit{T}}$ has been extracted for 1.75<${\mathit{Q}}^2$<5 (GeV/c${)}^2$ and is found to decrease with ${\mathit{Q}}^2$ to a value consistent with zero at the highest ${\mathit{Q}}^2$.

Strangeness production processes p¯p→Y¯Y (Y=Λ,Σ) have been studied near threshold in a full coupled-channel calculation. The transition interactions are based on K- and ${\mathit{K}}^{\mathrm{*}}$-meson exchange. The elastic part of the p¯p and Y¯Y interactions has been derived from a one-boson-exchange version of the Bonn NN potential and a corresponding extension to the hyperon-nucleon case whereas the annihilation part is taken into account by introducing phenomenological optical potentials. First results are presented for the reactions p¯p→Λ¯${\mathrm{\Sigma }}^0$+Σ¯ ${}_{}{}^0{}_{}{}^{}\mathrm{\Lambda }$ as well as p¯p→Σ¯ ${}_{}{}^{\pm }{}_{}{}^{}\mathrm{\Sigma }_{}^{\pm }{}_{}{}^{}$, Σ¯ ${}_{}{}^0{}_{}{}^{}\mathrm{\Sigma }_{}^0{}_{}{}^{}$, which agree with existing data. As in p¯p→Λ¯Λ, tensor and spin-triplet dominance is realized in p¯p→Σ¯ Σ but is somewhat reduced in p¯p→Λ¯ ${\mathrm{\Sigma }}^0$+Σ¯ ${}_{}{}^0{}_{}{}^{}\mathrm{\Lambda }$. Although Σ¯ ${}_{}{}^{\mathrm{-}}{}_{}{}^{}\mathrm{\Sigma }_{}^{\mathrm{-}}{}_{}{}^{}$ cannot be reached from p¯p by a single transition, the corresponding cross section is only slightly smaller than for Σ¯ ${}_{}{}^{+}{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$. Initial-and final-state effects play a decisive role, even in cross section ratios.

Using the assumption of a partially conserved axial-vector current and the conservation law for the electromagnetic current, we rederive a low-energy theorem for the pion electroproduction. In contrast to the traditional approach, we include all effects of off-shell nucleons and pions. We parametrize the axial-vector current in its most general form and apply the minimal substitution prescription to obtain the radiative axial-vector vertex that is required for gauge invariance. We split the full radiative axial-vector vertex into the isolated-pole contribution, the minimal-coupling interaction, and the remainder. The Ward-Takahashi identities are translated into the constraints on the pion electroproduction amplitude. The rigorous low-energy theorem is obtained for the zero four-momentum pion electroproduction. It is found that the off-shell matrix elements of the axial-vector current affect the isolated-pole term and the gauge term so that the low-energy theorem is at variance with the standard one given in the literature.

In spite of the apparent limitations of the model, in recent years there have been many applications of the Nambu–Jona-Lasinio (NJL) model in the study of hadron structure and in the study of the behavior of nuclear matter at finite temperature and density. A number of researchers have studied a generalized SU(3) version of the NJL model. For example, Vogl, Lutz, Klimt, and Weise [Nucl. Phys. A516 469 (1990)] have performed extensive calculations that include a calculation of a scalar form factor of a constituent quark, ${\mathit{F}}_{\mathit{s}}$(${\mathit{q}}^2$), and a calculation of a quark sigma term ${\mathrm{\sigma }}_{\mathit{q}}$. (In their work, the latter quantity is related to the nucleon sigma term ${\mathrm{\sigma }}_{\mathit{N}}$ as in a constituent quark model: ${\mathrm{\sigma }}_{\mathit{N}}$=3${\mathrm{\sigma }}_{\mathit{q}}$.) These calculations are made in what may be termed a sigma-dominance approximation. In the work reported here, we review the important role played by the nucleon sigma term in understanding the behavior of the quark condensate in the presence of matter. We make use of the original SU(2) version of the NJL model to study how various quark properties are modified when we take into account the dressing of the constituent quarks by the pion, the Goldstone boson of the model. We calculate the quark self-energy arising from emission and absorption of a pion and also show how the calculation of the scalar form factor of the quark and ${\mathrm{\sigma }}_{\mathit{q}}$ are modified due to the coupling of the quark to the pion. The correction terms considered here serve to reduce the value of ${\mathrm{\sigma }}_{\mathit{q}}$ by a small amount relative to the value obtained in the simplest version of the sigma dominance model. For example, for a Euclidean momentum cutoff, Λ=1050 MeV, the uncorrected result is ${\mathrm{\sigma }}_{\mathit{N}}$=54.6 MeV. That value is then reduced to ${\mathrm{\sigma }}_{\mathit{N}}$=51.5 MeV, if the corrections due to the pion ‘‘dressing’’ are included. It is also found that the residue at the quasiparticle pole of the quark propagator Z is about 0.86 when the coupling to the pion field is taken into account.

The assumption that a small pointlike configuration does not interact with nucleons leads to a new set of sum rules that are interpreted as models of the baryon-nucleon interaction. These models are rendered semirealistic by requiring consistency with data for cross section fluctuations in proton-proton diffractive collisions.

A measurement of the muon-neutrino carbon charged-current cross section, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathrm{\mu }}$,${\mathrm{\mu }}^{\mathrm{-}}$)X, was performed using the in-flight pion-decay neutrino source at the LAMPF accelerator. At an average interacting neutrino energy of 202 MeV an inclusive cross section of (15.9±2.6±3.7)×${10}^{\mathrm{-}39}$ ${\mathrm{cm}}^2$ was measured. This value is in best agreement with the Fermi-gas model and is in disagreement with a previous experiment and subsequent calculations that yielded substantially lower cross sections. However, the muon energy spectrum shows a marked depletion of events at high energies compared to the Fermi-gas model. From a small sample of events in which the final-state nucleus was in the ground state of ${}_{}{}^{12}{}_{}{}^{}\mathrm{N}$, a cross section for the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathrm{\mu }}$,${\mathrm{\mu }}^{\mathrm{-}}$${)}^{12}$N(g.s.) of (1.7±0.8±0.3)×${10}^{\mathrm{-}39}$ ${\mathrm{cm}}^2$ or about 11±5 % of the total event rate was obtained.

To incorporate the Δ isobar in the relativistic Dirac-Brueckner model we develop a formalism that treats the Δ analogously to the nucleon. That is, we introduce scalar and vector components of the Δ self-energy and define effective quantities for the Δ similar to the ones for the nucleon. Using this formalism and based on one-boson-exchange interactions, we present a parameter-free calculation of the equation of state of nuclear matter. We list results for the effective quantities of both the nucleon and the Δ. The main result we find is that in our model the properties of the nucleon in nuclear matter seem hardly affected by the inclusion of the Δ. Also the medium dependence of the properties of the Δ turns out to be quite small. Finally we investigate the momentum dependence of the mean field of the Δ.

The nonzero average parity-nonconserving longitudinal analyzing powers for p-wave neutron resonances in compound nuclear systems are analyzed within the giant J=$0^{\mathrm{-}}$ resonance doorway model. Assuming that the parity-nonconservation interaction can be approximated by a one-body operator of the type σ⋅p, we find that the observed fixed sign of the asymmetry arises naturally within the doorway model. The J=$0^{\mathrm{-}}$ giant resonance does not enhance the size of the constant asymmetry. The model prediction for the average symmetry is shown to be similar to that obtained in the single-particle model. In particular, if one retains only the first-order contribution to the analyzing power, a very large single-particle matrix element 〈4${\mathit{p}}_{1/2}$‖${\mathit{V}}_{\mathrm{pv}}$‖5${\mathit{s}}_{1/2}$〉≃75±50 eV is needed to reproduce the observed average asymmetry. This large value is in contradiction with the value of 2 eV estimated from the experimentally measured spreading width of the parity-violating interaction using the doorway model.

There have been suggestions to measure atomic parity nonconservation (PNC) along an isotopic chain, by taking ratios of observables in order to cancel complicated atomic-structure effects. Precise atomic PNC measurements could make a significant contribution to tests of the standard model at the level of one-loop radiative corrections. However, the results also depend upon certain features of nuclear structure, such as the spatial distribution of neutrons in the nucleus. To examine the sensitivity to nuclear structure, we consider the case of Pb isotopes using various recent relativistic and nonrelativistic nuclear model calculations. Contributions from nucleon internal weak structure are included, but found to be fairly negligible. The spread among present models in predicted sizes of nuclear-structure effects may preclude using Pb isotope ratios to test the standard model at better than a 1% level, unless there are adequate independent tests of the nuclear models by various alternative strong and electroweak nuclear probes. On the other hand, sufficiently accurate atomic PNC experiments would provide a unique method to measure neutron distributions in heavy nuclei.