A modified linear $\Sigma$ model that allows for $g_A=1.26\mathrm{}$ by addition of vector and pseudovector $\pi N$ coupling terms was discussed by Bjorken and Nauenberg and by Lee. In this extended linear $\Sigma$ model the elastic $\pi N$ scattering amplitudes satisfy the relevant chiral low-energy theorems, such as the Weinberg-Tomozawa relation for the isovector $\pi N$ scattering length and in some cases Adler’s “consistency condition.” The agreement of the isospin symmetric $\pi N$ scattering length with experiment is substantially improved in this extended $\Sigma$ model as compared with the original linear one. We show that the nucleon sigma term $\left({\Sigma }_N\right)$ in the linear and the extended $\Sigma$ models with three different kinds of chiral symmetry breaking terms are identical. Within the tree approximation the formal operator expression for the ${\Sigma }_N$ term and the value extracted from the $\pi N$ scattering matrix coincide. Large values of ${\Sigma }_N$ are easily obtained without any $s\overline{s}$ content of the nucleon. Using chiral rotations the Lagrangian of this extended Σ model reproduces the lowest-order $\pi N$ chiral perturbation theory Lagrangian.

We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the $\gamma \gamma$ mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at the Relativistic Heavy Ion Collider are also given.

The relationship between the lower limit on the nuclear stability lifetime as derived from the nondisappearance of “stable” nuclei ${(T}_d\gtrsim 5.4\times {10}^{31}$ yr), and the lower limit thus implied on the oscillation time $\left({\tau }_{n\mathrm{n̄}}\right)$ of a possibly underlying neutron-antineutron oscillation process, is clarified by studying the time evolution of the nuclear decay within a simple model which respects unitarity. The order-of-magnitude result ${\tau }_{n\mathrm{n̄}}\approx {2(T}_d/{\Gamma }_{\mathrm{n̄}}{)}^{1/2}>2\mathrm{}\times {10}^8$ s, where ${\Gamma }_{\mathrm{n̄}}$ is a typical $\mathrm{n̄}$ nuclear annihilation width, agrees as expected with the limit on ${\tau }_{n\mathrm{n̄}}$ established by several detailed nuclear physics calculations, but disagreeing by 15 orders of magnitude with a claim published recently [V. I. Nazaruk, Phys. Rev. C 58, R1884 (1998)].

We have calculated the inclusive quasielastic ${\nu }_e$ and ${\nu }_{\mu }$ cross sections in ${}^{40}\mathrm{Ar}$ relevant for the atmospheric neutrino experiments. The effects of the nuclear medium on the cross sections have been studied. These calculations are done in the local-density approximation and various nuclear effects, such as Pauli blocking, Fermi motion, and strong interaction renormalization of the weak vertex due to the presence of nucleons in the nucleus, are taken into account. Numerical results for the charged lepton yields are presented.

Levels in ${}^{40}\mathrm{Sc}$ below 2.5 MeV excitation energy have been populated in a high-resolution study of the ${}^{40}\mathrm{Ca}{(}^3\mathrm{He}{,t)}^{40}\mathrm{Sc}$ reaction. Three new states have been observed at energies $E_x=1703,$ 1871, and 1925 keV. Correspondence of the observed ${}^{40}\mathrm{Sc}$ levels with known $T=1\mathrm{}$ states in ${}^{40}\mathrm{K}$ and ${}^{40}\mathrm{Ca}$ are based on predictions provided by the isobaric multiplet mass equation. Our results confirm recently estimated stellar reaction rates for proton capture on ${}^{39}\mathrm{Ca}.$

The effect of tensor nucleon-nucleon correlations upon exclusive and semiexclusive electronuclear reactions is studied. Differential cross sections for the semiexclusive ${}^{16}\mathrm{O}{(e,e}^{\prime }p)$ and exclusive ${}^{16}\mathrm{O}{(e,e}^{\prime }pn)$ processes are computed by explicitly evaluating the dynamical electromagnetic coupling to a tensor correlated nucleon pair. In both reaction channels the tensor correlations contribute in a very substantial way. Tensor correlations are found to generate more electronuclear strength than central Jastrow correlations do.

The spherical semimagic ${}^{116,118,120,122,124}\mathrm{Sn}$ nuclei have been investigated intensively using nuclear resonance fluorescence techniques. The measurement of the photon scattering cross sections, angular distributions, and linear polarization or azimuthal asymmetries of the resonantly scattered (unpolarized and polarized) photons enabled a model independent determination of reduced transition probabilities, level spins, and parities. Besides the enhanced dipole excitations to the well-known ${(2\mathrm{}}_1^{+}\otimes 3_1^{-}{)}_{1^{-}}$ two-phonon states several electric quadrupole transitions were detected in the investigated energy region below 4 MeV. Quasiparticle phonon model calculations reveal several collective and noncollective $2^{+}$ states in this energy region. In contrast to the known two-phonon ${(2\mathrm{}}_1^{+}\otimes 3_1^{-}{)}_{1^{-}}$ states, the wave functions of the observed $2^{+}$ states are dominated by one-phonon components. However, the fragmentation of the $B\left(E2\mathrm{}\right)\mathrm{}\uparrow$ strength is influenced by two-phonon $2^{+}$ admixtures.

We have calculated the pion spectra of $(p,{\pi }^{-})$ reactions from near threshold to the $\Delta$ region for medium-heavy nuclei with the zero-range plane-wave approximation supplemented with the cutoff of a radial integral simulating the distortion effect. The shell-model wave function with a ${1p}_{1/2}-{0g}_{9/2}$ configuration is used to calculate the two-particle one-hole spectroscopic amplitudes. The excitation spectra leading to both the positive- and the negative-parity states are calculated for ${}^{88}\mathrm{Sr},$ ${}^{90}\mathrm{Zr},$ and ${}^{92}\mathrm{Mo}$ nuclei. The present model calculation predicts pronounced selectivity of the excitation of several high-spin states, especially at higher incident energies.

In this paper the dependence of meson-nucleon-nucleon vertex form factors is studied as a function of temperature. The results are obtained starting from a zero temperature Bonn potential. The temperature dependence of the vertex form factors and radii is studied in the thermofield dynamics, a real-time operator formalism of finite temperature field theory. It is anticipated that these results will have an impact on the study of relativistic heavy-ion collisions as the critical temperature for the phase transition from hadronic to quark-gluon system is approached.

The analysis of parity nonconservation (PNC) measurements performed on ${}^{232}\mathrm{Th}$ by the TRIPLE Collaboration has been extended to include the neutron energy range of 250 to 1900 eV. Below 250 eV all ten statistically significant parity violations have the same sign. However, at higher energies PNC effects of both signs were observed in the transmission of longitudinally polarized neutrons through a thick thorium target. Although the limited experimental energy resolution precluded analysis in terms of the longitudinal asymmetry, parity violations were observed and the cross section differences for positive and negative neutron helicities were obtained. For comparison, a similar analysis was performed on the data below 250 eV, for which longitudinal asymmetries were obtained previously. For energies below 250 eV, the p-wave neutron strength functions for the $J=1/2\mathrm{}$ and $J=3/2\mathrm{}$ states were extracted: $S_{1/2}^1=(1.68\mathrm{}\pm 0.61)\times {10}^{-4}$ and $S_{3/2}^1=(0.75\mathrm{}\pm 0.18)\times {10}^{-4}.$ The data provide constraints on the properties of local doorway states proposed to explain the PNC sign effect in thorium.

The balance energy at which repulsive nucleon-nucleon scattering balances the attractive force due to the nuclear mean field is measured for the first time for the $\mathrm{Au}+\mathrm{Au}$ reaction. The observed balance energy of $42\pm 3_{\mathrm{stat}}\pm 1_{\mathrm{sys}}$ MeV/nucleon agrees with the previously established power law dependence of $E_{\mathrm{bal}}$ on system mass, providing evidence that the Coulomb interaction does not suppress the attractive mean field near the balance energy, in contrast to recent theoretical predictions.

The baryon phase-space density at mid-rapidity from central heavy-ion collisions is estimated from proton spectra with interferometry and deuteron coalescence measurements. It is found that the mid-rapidity phase-space density of baryons is significantly lower at the super proton synchrotron than the Alternating-Gradient Synchrotron, while those of total particles (pion + baryon) are comparable. Thermal and chemical equilibrium model calculations tend to over-estimate the phase-space densities at both energies.

Isospin singlet $\mathrm{}\left(\mathrm{pn}\right)\mathrm{}$ pairing as well as quartetting in nuclei is expected to arise near the symmetry line $N=Z.\mathrm{}$ Empirical values can be deduced from the nuclear binding energies applying special filters. Within the local density approximation, theoretical estimates for finite nuclei are obtained from results for the condensation energy of asymmetric nuclear matter. It is shown that the isospin singlet condensation energy drops down abruptly for $|N-Z|\approx 4$ for medium nuclei in the region $A=40.\mathrm{}$ Furthermore, $\alpha$-like quartetting and the influence of excitations are discussed.

We study an extended Lipkin-Meshkov-Glick model that permits a transition to a deformed phase with a broken continuous symmetry. Unlike simpler models, one sees a persistent zero-frequency Goldstone mode past the transition point into the deformed phase. We found that the RPA formula for the correlation energy provides a useful correction to the Hartree-Fock energy when the number of particle N satisfies $N>\mathrm{}3,$ and becomes accurate for large N. We conclude that the RPA correlation energy formula offers a promising way to improve the Hartree-Fock energy in a systematic theory of nuclear binding energies.

We study the cross-correlation between the spin and orbital parts of magnetic dipole transitions $M1\mathrm{}$ in both isoscalar and isovector channels. In particular, we closely examine certain cases of isovector transitions where $\sum B\left(M1\mathrm{}\right)\mathrm{}$ is very close to $\sum {B\left(M1\mathrm{}\right)\mathrm{}}_{\sigma }+\sum {B\left(M1\mathrm{}\right)\mathrm{}}_l,$ implying a cancellation of the summed interference terms. We gain some insight into this problem by considering special cases approaching the $\mathrm{SU}\left(4\right)\mathrm{}$ limit, and by examining the behavior of single-particle transitions at the beginning and towards the end of the $s-d$ shell. For isoscalar transitions there is always destructive interference between the spin and orbital amplitudes.

Using a semiclassical treatment of the linear $\sigma$ model, we simulate the dynamical evolution of an initially hot cylindrical rod endowed with a longitudinal Bjorken scaling expansion (a “Bjorken rod”). The field equation is propagated until full decoupling has occurred and the asymptotic many-body state of free pions is then obtained by a suitable Fourier decomposition of the field and a subsequent stochastic determination of the number of quanta in each elementary mode. The resulting transverse pion spectrum exhibits visible enhancements below 200 MeV due to the parametric amplification caused by the oscillatory relaxation of the chiral order parameter. Ensembles of such final states are subjected to various event-by-event analyses. The factorial moments of the multiplicity distribution suggest that the soft pions are nonstatistical. Furthermore, their emission patterns exhibit azimuthal correlations that have a bearing on the domain size in the source. Finally, the distribution of the neutral pion fraction shows a significant broadening for the soft pions which grows steadily as the number of azimuthal segments is increased. All of these features are indicative of disoriented chiral condensates and it may be interesting to apply similar analyses to actual data from high-energy nuclear collision experiments.

Chiral symmetry restoration in a dense medium is to some extent a consequence of the nuclear pion cloud. These pions induce a mixing of the axial and vector current contributions in the axial and vector correlators. We discuss their influence on hadron masses and investigate the signal produced by the remaining contribution associated with chiral symmetry restoration. Using the quark-meson coupling model we find that the latter is associated with the reduction of hadron masses.