We study the relations between moments of fusion cross sections and averages of angular momentum. The role of the centrifugal barrier and the target deformation in determining the effective barrier radius are clarified. A simple method for extracting average angular momentum from fusion cross sections is demonstrated using numerical examples as well as actual data. © 1996 The American Physical Society.

The ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$(γ,${\mathrm{\pi }}^{+}$${)}^{14}$${\mathrm{C}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}$ cross section at ${\mathit{E}}_{\gamma }$=173 MeV has been calculated using shell model wave functions determined in complete (0+2)ħω and (0+2+4)ħω shell model spaces. Using the (0+2+4)ħω shell model wave functions yields an improvement over older results obtained with Cohen-Kurath wave functions by a factor of 2, but still overpredicts the back-angle data. When correlated with analyses of inelastic electron and proton scattering from ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, the present results indicate the need for an improved shell model calculation and possibly an improved description of the elementary pion production process in nuclei. © 1996 The American Physical Society.

The optical potential and observables for proton${\mathrm{-}}^{9,11}$Li elastic scattering are calculated microscopically using the single scattering approximation to the Kerman-McManus-Thaler multiple scattering expansion. The importance of the central and spin-orbit terms of the optical potential, and of the core and halo nucleon contributions, is clarified in terms of the momentum space behaviors of the nucleon-nucleon amplitudes and relevant density distributions. Calculations for the ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ systems at 60 and 62 MeV/nucleon, respectively, are compared with the available experimental data and with calculations and data for the proton${\mathrm{-}}^8$He system at similar energy. Three-body models of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ are used. The effects of the halo distribution in ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ are clearly manifest in the elastic cross section but there are small differences between observables for structure models with realistic two-neutron asymptotic behaviors. Calculations suggest that the ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ core structure is not realistically described by simple models. © 1996 The American Physical Society.

Associated strangeness production has been studied in the p¯p→Λ¯Λ→p¯${\mathrm{\pi }}^{+}$p${\mathrm{\pi }}^{\mathrm{-}}$ reaction at the CERN antiproton facility LEAR using the experimental setup of PS185. Results from two high-statistics measurements at incident antiproton momenta of 1.642 and 1.918 GeV/c are reported. Approximately 40 000 reconstructed events at each momentum have allowed us to measure the total and differential cross sections, the spin polarizations, the spin correlations, and the singlet fractions of the ΛΛ¯ pair. Since the decays of both the Λ and the Λ¯ were simultaneously observed in the same detector, we are able to provide upper limits on CP and CPT violation phenomena in the weak interaction. © 1996 The American Physical Society.

Two-particle rapidity correlations for pp, p${\mathrm{\pi }}^{\mathrm{-}}$, and ${\mathrm{\pi }}^{\mathrm{-}}$${\mathrm{\pi }}^{\mathrm{-}}$ pairs in C+Ta interactions at 4.2 GeV/c per nucleon are presented. The experimental data are obtained with a 2 m propane bubble chamber exposed on the JINR, Dubna, synchrophasotron. It is found that the strength of the correlations does not depend on particle combination, but strongly depends on the rapidity and collision centrality. The quark-gluon-string model describes these features of the correlations well, but needs to be tuned in the target fragmentation region. Since the quark-gluon-string model simulates the nucleus-nucleus collisions by the superposition of hadron-hadron collisions, we conclude that there is no evidence of a collective behavior in our data. This conclusion is additionally supported by the intermittency analysis. © 1996 The American Physical Society.

Multiplicity and pseudorapidity distributions of shower particles produced in interactions of the ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at 10.6A GeV, ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at 14.5A GeV, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ at 200A GeV beams were studied in nuclear emulsion. The pseudorapidity (η) distributions are approximately Gaussian in shape. Their widths increase monotonically from central to peripheral collisions with energy and with the size of the target. The widths of the pseudorapidity distributions, the values of their centroid, pseudorapidity density and ${\mathrm{\eta }}_{\mathrm{peak}}$ values are compared among the different groups of their multiplicity at different energies as a function of centrality for all four beams. © 1996 The American Physical Society.

Lambda flow in Ni+Ni collisions at SIS energies is studied in the relativistic transport model (RVUU 1.0). It is found that for primordial lambdas the flow is considerably weaker than proton flow. The inclusion of final-state interactions, especially the propagation of lambdas in the mean-field potential, brings the lambda flow close to that of protons. An accurate determination of the lambda flow in heavy-ion experiments is shown to be very useful for studying lambda properties in dense matter. © 1996 The American Physical Society.

The production of dileptons is studied within a hadronic transport model. We investigate the sensitivity of the dilepton spectra to the initial configuration of the hadronic phase in a ultrarelativistic heavy-ion collision. Possible in-medium corrections due to the modifications of pions and the pion form factor in a hadronic gas are discussed. © 1996 The American Physical Society.

Inclusive single photon spectra have been measured in the transverse momentum (${\mathit{p}}_{\mathit{T}}$) range from a few MeV/c up to approximately 1 GeV/c at various nucleon-nucleon center of momentum rapidities (${\mathit{y}}_{\mathit{NN}}$) in the interval from -2.4<${\mathit{y}}_{\mathit{NN}}$<+0.5 in 18 GeV/c p-Be and p-W collisions. These distributions were used to estimate the inclusive ${\mathit{p}}_{\mathit{T}}$ and rapidity distributions for ${\mathrm{\pi }}^0$ production over approximately the same kinematic range. The measured photon spectra are compared with the expected distribution of photons from ${\mathrm{\pi }}^0$ decays and hadronic bremsstrahlung in order to search for new sources of ‘‘soft’’ photons at low transverse momentum. The results indicate no anomalous sources of soft photons in either p-Be or p-W interactions at these energies. The p-W data at forward angles set a limit on any excess of low ${\mathit{p}}_{\mathit{T}}$ photons at <2.65 times hadronic bremsstrahlung at the 99% C.L. © 1996 The American Physical Society.

The analyzing power ${\mathit{A}}_{\mathit{y}}$ for ${\mathrm{\pi }}^{+}$p→ scattering at 68.3 MeV has been measured at the Paul Scherrer Institut with the magnetic spectrometer LEPS. The measurements cover the angular range 40°≤${\mathrm{\theta }}_{\mathrm{lab}}$≤70°. The protons have been polarized in a butanol target, operated in frozen spin mode. The S31 phase shift comes out by about 1° smaller than the Koch-Pietarinen [Nucl. Phys. A 336, 331 (1980)] phase shift analysis, supporting the necessity of an alternative dispersion analysis of πN scattering to determine the σ term and the πN coupling constant. © 1996 The American Physical Society.

We study the electromagnetic form factors of the nucleon in a collective model of baryons. Using the algebraic approach to hadron structure, we derive closed expressions for both elastic and transition form factors, and consequently for the helicity amplitudes that can be measured in electro- and photoproduction. Effects of spin-flavor symmetry breaking and of swelling of hadrons with increasing excitation energy are considered. © 1996 The American Physical Society.

The extraction of the nucleon’s strangeness axial charge Δs from inclusive, quasielastic neutral current neutrino cross sections is studied within the framework of the plane-wave impulse approximation. We find that the value of Δs can depend significantly on the choice of nuclear model used in analyzing the quasielastic cross section. This model dependence may be reduced by one order of magnitude when Δs is extracted from the ratio of total proton to neutron yields. We apply this analysis to the interpretation of low-energy neutrino cross sections and arrive at a nuclear theory uncertainty of ±0.03 on the value of Δs expected to be determined from the ratio of proton and neutron yields measured by the LSND Collaboration. This error compares favorably with estimates of the SU(3)-breaking uncertainty in the value of Δs extracted from inclusive, polarized deep-inelastic structure function measurements. We also point out several general features of the quasielastic neutral current neutrino cross section and compare them with the analogous features in inclusive, quasielastic electron scattering. © 1996 The American Physical Society.

η-deuteron scattering lengths are calculated. A summation of the multiple scattering series is carried out and the result is checked against more involved calculations. The necessity to go beyond the fixed nucleon approximation is emphasized. It is shown that a quasibound or virtual state in the η-deuteron system may occur within the range of η-nucleon scattering lengths suggested by other experiments. © 1996 The American Physical Society

The proton-neutron monopole Lipkin model, which exhibits some properties that are relevant for those double beta decay (ββ) transitions mediated by the Fermi matrix elements, is solved exactly in the proton-neutron two-quasiparticle space. The exact results are compared with the ones obtained by using the quasiparticle random phase approximation (QRPA) and renormalized QRPA (RQRPA) approaches. It is shown that the RQRPA violates the Ikeda sum rule and that this violation may be common to any extension of the QRPA where scattering terms are neglected in the participant one-body operators as well as in the Hamiltonian. This finding underlines the need of additional developments before the RQRPA could be adopted as a reliable tool to compute ββ processes. © 1996 The American Physical Society.

The excitation function of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O reaction and the angular distribution of its γ rays were measured at nine center-of-mass energies ranging from E=1.37 to 2.98 MeV. These measurements allowed the separation of the E1 and E2 contributions and their extrapolation to the region of astrophysical interest. The analysis of all the available E1 cross sections with the K-matrix method and with a three-level R-matrix method yields a consistent prediction of 79±16 keV b for the E1 S factor at 300 keV. The E2 contribution at the same energy is 36±6 keV b from a cluster model analysis of the present data. The best estimate of the total S factor at 300 keV is 120 keV b, and it is unlikely that it will fall outside the range of 80–160 keV b. © 1996 The American Physical Society.

We used the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${(}^3$He,n${)}^{18}$Ne, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{12}$C${,}^6$He${)}^{18}$Ne, and ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$(p,t${)}^{18}$Ne reactions to study ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ states up to an excitation energy of 10 MeV, with emphasis on levels corresponding to ${}_{}{}^{14}{}_{}{}^{}\mathrm{O}$(α,p${)}^{17}$F and ${}_{}{}^{17}{}_{}{}^{}\mathrm{F}$(p,γ${)}^{18}$Ne resonances that could strongly affect these reaction rates in hot stellar environments. Excitation energies, widths, absolute cross sections, and angular distributions were measured. We found previously unidentified states at ${\mathit{E}}_{\mathit{x}}$=6.15±0.01 MeV, 7.12±0.02 MeV, 7.35±0.02 MeV, 7.62±0.02 MeV, 8.30±0.02 MeV, (8.45±0.03 MeV), 8.55±0.03 MeV, 8.94±0.02 MeV, and 9.58±0.02 MeV. We combined level width, cross section, and angular distribution data to infer ${\mathit{J}}^{\mathrm{\pi }}$ values for a number of the new levels as well as for the previously known 5.1-MeV doublet. Using information from our experiments, we recalculated the ${}_{}{}^{14}{}_{}{}^{}\mathrm{O}$(α,p${)}^{17}$F reaction rate, which constitutes a possible path out of the hot CNO cycle into the rp process and could play an important role in transforming nuclei involved in the hot CNO cycle into heavier nuclei with Z≥10. © 1996 The American Physical Society.

At the Karlsruhe pulsed 3.75 MV Van de Graaff accelerator the thermonuclear ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$(n,γ${)}^{49}$Ca(8.72 min) cross section was measured by the fast cyclic activation technique via the 3084.4 keV γ-ray line of the ${}_{}{}^{49}{}_{}{}^{}\mathrm{Ca}$ decay. Samples of ${\mathrm{CaCO}}_3$ enriched in ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ by 77.87% were irradiated between two gold foils which served as capture standards. The capture cross section was measured at the neutron energies 25, 151, 176, and 218 keV, respectively. Additionally, the thermal capture cross section was measured at the reactor BR1 in Mol, Belgium, via the prompt and decay γ-ray lines using the same target material. The ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$(n,γ${)}^{49}$Ca cross section in the thermonuclear and thermal energy range has been calculated using the direct-capture model combined with folding potentials. The potential strengths are adjusted to the scattering length and the binding energies of the final states in ${}_{}{}^{49}{}_{}{}^{}\mathrm{Ca}$. The small coherent elastic cross section of ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$+n is explained through the nuclear Ramsauer effect. Spectroscopic factors of ${}_{}{}^{49}{}_{}{}^{}\mathrm{Ca}$ have been extracted from the thermal capture cross section with better accuracy than from a recent (d,p) experiment. Within the uncertainties both results are in agreement. The nonresonant thermal and thermonuclear experimental data for this reaction can be reproduced using the direct-capture model. A possible interference with a resonant contribution is discussed. The neutron spectroscopic factors of ${}_{}{}^{49}{}_{}{}^{}\mathrm{Ca}$ determined from shell-model calculations are compared with the values extracted from the experimental cross sections for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$(d,p${)}^{49}$Ca and ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$(n,γ${)}^{49}$Ca. © 1996 The American Physical Society.

The possibility of noncausal signal propagation is examined for various theories of dense matter. This investigation requires a discussion of definitions of causality, together with interpretations of spacetime position. Specific examples are used to illustrate the satisfaction or violation of causality in realistic calculations. © 1996 The American Physical Society.

We calculate the neutrino mean free path for neutrino-nucleon scattering in highly asymmetric nuclear matter (N≫Z) at densities typical for the core of a neutron star. Nuclear matter is described by means of a relativistic field theory of nucleons coupled to neutral, scalar (σ), and vector (ω) mesons and to charged vector mesons (d). The correlation effects induced by the strong interaction of nucleons are taken into account within a self-consistent mean field approximation. The contribution of the exchange terms to the nucleon-nucleon interaction is also included. We present results for physical situations which are expected to occur in the cooling of neutron stars. Exchange terms have the effect of increasing the neutrino mean free path by about 50%. © 1996 The American Physical Society.

Measurements were carried out on mass, energy correlations of fission fragments for the systems of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$${+}^{237}$Np at ${\mathit{E}}_{\mathrm{lab}}$=76 MeV and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{209}$Bi at ${\mathit{E}}_{\mathrm{lab}}$=100 MeV at forward and perpendicular angles of emission with respect to the beam direction. The fragment anisotropy averaged over all fragment masses matches well for both the systems with the standard saddle point statistical model calculations. Variation of the angular anisotropy and total kinetic energy of fragments were studied as a function of the mass of the fragment pair. For the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$${+}^{237}$Np system, the fragment anisotropy is seen to be nearly independent of the fragment mass, whereas for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{209}$Bi system, the anisotropy is seen to decrease with increasing fragment mass asymmetry. The results are discussed on the basis of the statistical model of fragment angular distributions and mass division in the fission process. © 1996 The American Physical Society.