Collisions between ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$ + ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ at 19.1 MeV/nucleon were studied using two 4π detection systems. A reconstruction procedure was developed to determine the mass, kinetic, and excitation energies of the primary projectile and targetlike fragments. The results show a broad range of mechanisms. These results were compared with predictions of the quantum-molecular dynamics model. © 1996 The American Physical Society.

The energy distributions of the complex fragments (3≤Z≤11) emitted in the reaction ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(96 MeV) + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ have been measured in the angular range 10°≤${\mathrm{\theta }}_{\mathrm{lab}}$≤60°. The lighter fragments (3≤Z≤6) have been found to be emitted predominantly due to the asymmetric fissionlike decay of the compound nucleus, whereas the heavier fragments (Z≥10) have been identified as evaporation residues. The shapes of the fragment energy distributions, as well as the total elemental yields for the lighter fragments (3≤Z≤6) have been explained fairly well by the asymmetric binary fission model. The binary fragment yields from the reaction ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(96 MeV) + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ have been compared with those obtained in α(60 MeV) + ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$(47 MeV) + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ reactions, all producing the same composite ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ at same excitation energy. No significant entrance channel asymmetry dependence has been observed. © 1996 The American Physical Society.

High precision fusion data for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{144}$Sm are analyzed in a coupled channels treatment which includes the excitation of the lowest $2^{+}$ and $3^{\mathrm{-}}$ states in both projectile and target. We find that certain higher-order couplings, which are often neglected in coupled channels analyses, play a significant role and improve the fit to the data. The nuclear potential which gives the best fit has a diffuseness of 0.64 fm. This is consistent with the prediction of folding models and also with the empirical expectation, based on analyses of elastic scattering data. © 1996 The American Physical Society.

The advent of new electron accelerators with few-GeV beam energies makes the (e,${\mathit{e}}^{\prime }$p) reaction a promising tool for investigating new aspects of the electromagnetic interaction. To this purpose it is crucial to set the scale of final-state interactions (FSI) at high ejectile energies. Usually, the problem is faced by mutuating well-established results of the Glauber method in the framework of elastic (p,p) scattering. Since the generalization of this eikonal approximation to the (e,${\mathit{e}}^{\prime }$p) case is not straightforward, we have analyzed the constraints which make the comparison a meaningful one, using the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(e,${\mathit{e}}^{\prime }$p${)}^{11}$${\mathrm{B}}_{\mathit{s}1/2}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}_{\mathit{s}1/2}^{}{}_{}{}^{}$(p,p) reactions with outgoing-proton momenta of 4 GeV/c as a test case. The FSI dominance at large deflection angles produces in the distributions a universal behavior resembling the coherent diffractive scattering between the ejected proton and the (residual) nucleus. Because of the selected sensitivity of the (e,${\mathit{e}}^{\prime }$p) distribution to different theoretical ingredients depending on different values of the deflection angle (or transverse missing momentum), it is argued that the previous comparison with elastic proton scattering may represent a convenient tool to disentangle effects due to the (hard) electromagnetic vertex from (exotic) effects related to the propagation of the struck hadron through the nuclear medium. © 1996 The American Physical Society.

We analyze the properties of the inclusive d+p reactions, with particular interest in the domain of nucleonic resonances excitation. The calculated cross section and polarization observables show that it is possible to disentangle the different reaction mechanisms (ω,σ,η exchange) and bring new information about the electromagnetic form factors of the deuteron as well as of the nucleonic resonances excitation. Existing data on the tensor analyzing power are in agreement with the predictions based on the ω-exchange model. © 1996 The American Physical Society.

The iterative perturbative inversion method is extended to determine explicitly energy and parity dependent potentials from S matrices specified at a series of discrete energies. The energy dependence is inserted only in the potential magnitude, although enhanced ‘‘energy bite’’ techniques allow this approximation to be tested. With a linear energy dependence in both real and imaginary components, S-matrix data for p+α scattering is fitted with a single potential for a wide range of energies above the inelastic threshold. The imaginary potential is shown to be parity dependent. The real potential is consistent, in both the potential shape and the parity and energy dependence, with potentials established for subthreshold energies. The method is also applied to resonating group model (RGM) phase shifts for n${+}^{16}$O and p+α scattering to give energy and parity dependent potentials in both cases. For p+α, a close correspondence is obtained between the RGM and the empirical potentials in both the energy and parity dependence up to about 65 MeV. For n${+}^{16}$O, potentials are determined for positive and negative energies. © 1996 The American Physical Society.

At a pion kinetic energy of 292 MeV, comparison of averages of ground-state (g.s.) and double analog (DIAS) cross sections on isotopes of Ni and Se suggests that the g.s. cross section is comparable to that per nucleon pair for the DIAS. © 1996 The American Physical Society.

We perform a classical dynamics study of the proton transfer reactions measured for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${+}^{197}$Au and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{197}$Au systems. The nuclei are assumed to move along classical trajectories, and the proton transfer probability is considered as a tunneling process around the point of closest approach. The absorption due to the imaginary part of the optical potential also is included. At the highest energies considered, several trajectories contribute to each scattering angle. The contributions associated to the different trajectories are added to obtain the proton transfer probability. We find that, for a properly selected value of the strength parameter in the imaginary part of the optical potential, the theoretical results fit adequately the experimentally measured values. © 1996 The American Physical Society.

We generate meson exchange currents mechanisms for the (γ,${\mathrm{\pi }}^{+}$) reaction in nuclei starting from the γN→ππN amplitude on one nucleon and allowing one of the pions to be produced off shell and be absorbed by a second nucleon. Detailed calculations are presented for the γ ${}_{}{}^3{}_{}{}^{}\mathrm{\pi }_{}^{+3\mathrm{}}{}_{}{}^{}$H reaction, where we show that the cross section at large momentum transfers is dominated by these mechanisms, helping improve the agreement with experimental data. It is also shown that the meson exchange currents produce important effects in the photon asymmetry in the Δ-resonance region. © 1996 The American Physical Society.

Energy-dependent in-medium nucleon-nucleon (NN) cross sections are obtained from the experimental data of proton-induced reaction cross sections. Total reaction cross sections of the proton-nucleus reaction reflect only the magnitude of NN cross sections around the incident beam energy. Therefore energy-dependent in-medium NN cross sections can be determined by reproducing proton-induced reaction cross sections for various target nuclei. Calculations of the reaction cross section are made by using antisymmetrized molecular dynamics. © 1996 The American Physical Society.

The effect of projectile shape on cross sections and momentum distributions of fragments from heavy ion reactions is studied. We propose a new approach that implements the underlying symmetries of each isotope with a few parameters directly in the density. Various densities and their nuclear structure are analyzed in the reactions of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$, and ${}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ on a carbon target. © 1996 The American Physical Society.

We present the results of a study performed on the ${}_{}{}^{208}{}_{}{}^{}\mathrm{emulsion}$ interactions at 160A GeV. In a minimum-bias sample of 1034 nuclear collisions, charges are assigned to all of the emitted 3431 helium and 1820 heavier projectile fragments during the breakup of the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ beam in emulsion. Multiplicity distributions of the target-associated slow-moving particles and of the fast-moving projectile fragments are investigated. The polar angles of a subsample of 337 interactions having at least four projectile fragments of charge Z≥2 are accurately measured and their pseudorapidity distributions are obtained. Multiplicity distributions, two- and three-body asymmetries, and conditional moments of the fast-moving projectile particles are investigated in terms of the total charge remaining in bound in the multiply charged projectile fragments. The results are compared with our existing 10.6A GeV ${}_{}{}^{197}{}_{}{}^{}\mathrm{emulsion}$ data obtained in experiment No. 875 conducted at the Brookhaven National Laboratory. Multiplicity distributions of slow particles emitted from the ${}_{}{}^{208}{}_{}{}^{}\mathrm{-}$ and ${}_{}{}^{197}{}_{}{}^{}\mathrm{induced}$ emulsion reactions are found to be independent of the beam masses and their energies. An insignificant number of fission events is observed in this work. Some differences in the average yields of helium nuclei and heavier fragments are observed, which may be attributed to an energy effect or to the limited statistics. However, two- and three-body asymmetries and conditional moments indicate that the breakup mechanism of the projectiles over a wide span of energies seems to be energy independent. © 1996 The American Physical Society.

We present a measurement of the W-to-Be per-nucleon cross-section ratio, ${\mathit{R}}_{\mathrm{W}/\mathrm{B}\mathrm{e}}$, for single hadrons and for opposite-sign hadron pairs produced in p-A collisions at √s=38.8 GeV. The data fill in regions of intermediate transverse momentum and dihadron mass that previously have revealed cross-section enhancements for scattering from nuclear targets. When combined with previous measurements, the new results help delineate the dependence of nuclear effects on the kinematic variables ${\mathit{p}}_{\mathit{t}}$, m, and √s. Both the single-hadron and hadron-pair enhancements in ${\mathit{R}}_{\mathrm{W}/\mathrm{B}\mathrm{e}}$ seen at intermediate values of ${\mathit{p}}_{\mathit{t}}$ and mass decrease with increasing √s, consistent with a multiple-scattering model of parton scattering. © 1996 The American Physical Society.

We investigate entropy production for an expanding system of particles and resonances with isospin symmetry—in our case pions and ρ mesons—within the framework of relativistic kinetic theory. A cascade code to simulate the kinetic equations is developed and results for entropy production and particle spectra are presented. © 1996 The American Physical Society.

Utilizing the Lorentz invariance of the correlation function we study the effects of anisotropic transverse flow on the Hanbury-Brown–Twiss (HBT) correlation function. In particular we show that directed flow would evidence itself by nonzero ‘‘side-long’’ and ‘‘side-out’’ terms in the correlation function. We also show that the study of ratios of the correlation functions evaluated on different event subsamples and/or different Lorentz systems provides information on the source dimensions and velocity which could be less biased by corrections of different kinds. We present a fitting technique appropriate for HBT analysis which does not require binning and is especially useful for multidimensional fitting. © 1996 The American Physical Society.

Temperature effects on the electromagnetic couplings of pions in hot hadronic matter are studied with an effective chiral Lagrangian. We show that the Ward-Takahashi identity is satisfied at nonzero temperature in the soft pion limit. The in-medium electromagnetic form factor of the pion is obtained in the timelike region and shown to be reduced in magnitude, especially near the vector-meson resonance region. Finally, we discuss the consequences of this medium effect on dilepton production from hot hadronic matter. © 1996 The American Physical Society.

We develop a formally covariant quark-diquark model of the nucleon. The nucleon is treated as a bound state of a constituent quark and a diquark interacting via a quark exchange. We include both scalar and axial-vector diquarks. The underlying Bethe-Salpeter equation is transformed into a pair of coupled Salpeter equations. The electromagnetic form factors of the nucleon are calculated in the Mandelstam formalism. We obtain a very good description of the proton electric and magnetic form factors and of the neutron electric form factor for momentum transfers up to -3(GeV/c${)}^2$. © 1996 The American Physical Society.

We evaluate the πNN, πΣΣ, and πΣΛ coupling constants using QCD sum rules based on pion-to-vacuum matrix elements of correlators of two interpolating baryon fields. The parts of the correlators with Dirac structure k/${\gamma }_5$ are used, keeping all terms up to dimension 5 in the OPE and including continuum contributions on the phenomenological side. The ratios of these sum rules to baryon mass sum rules yield stable results with values for the couplings of ${\mathit{g}}_{\mathrm{\pi }\mathit{NN}}$=12±5, ${\mathit{g}}_{\mathrm{\pi }\mathrm{\Sigma }\mathrm{\Sigma }}$=7±4, and ${\mathit{g}}_{\mathrm{\pi }\mathrm{\Sigma }\mathrm{\Lambda }}$=6±3. The sources of uncertainty are discussed. © 1996 The American Physical Society.

We investigate the structure of the one- and two-nucleon axial-current operators necessary for the partial conservation of the nuclear axial-current elastic matrix element in the one-boson-exchange approximation to the two-nucleon Bethe-Salpeter equation. We use three models for this purpose: (a) the linear sigma model, (b) the nonlinear sigma model, (c) a hybrid model, which is, roughly speaking, a linear combination of (a) and (b). We construct a partially conserved nuclear axial-current elastic matrix element in models (a) and (c) provided that the associated nuclear wave functions are solutions to the Bethe-Salpeter equation with a potential made of one-boson-exchange diagrams. In the nonlinear sigma model the nuclear one-body axial current is partially conserved by itself, without reference to the nuclear wave function, whereas the two-body axial current partial conservation is violated by terms of order 1/${\mathit{f}}_{\mathrm{\pi }}^2$. The complete axial current in models (a) and (c) and the one-body axial current in model (b) are applicable to the construction of the deuteron electroweak process amplitudes, for example. The divergence of the axial-current matrix element is proportional to the pion absorption nuclear matrix element, which leads to another potential application in the foundation of chiral perturbation theory for pion-two-nucleon processes. Consistency between the nuclear axial-currents and the underlying nuclear dynamics in models (a) and (c) is a new condition imposed by the partial conservation of the nuclear axial-current matrix element. We also examine conditions imposed on the form of the nucleon self-energy by the nucleon and meson axial Ward-Takahashi identities, as well as the approximations that satisfy the said conditions. We show that besides the first Born approximation, the so-called Hartree+random-phase approximation satisfies chiral Ward-Takahashi idenitities in models (a) and (c). © 1996 The American Physical Society.

We have measured cross sections for (d${,}^2$He)-induced reactions on ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$ at an energy of ${\mathit{E}}_{\mathit{d}}$=125.2 MeV. The measured Gamow-Teller strength is significantly smaller than that inferred from (p,n) reactions. We demonstrate that ${\mathrm{\beta }}^{+}$ Gamow-Teller strengths for ${\mathit{T}}_0$+1 states deduced from (p,n) reactions may have significant systematic uncertainties due to ambiguities in the large backgrounds that must be subtracted. © 1996 The American Physical Society.