An energy density derived from the relativistic mean-field theory has been used to calculate the real optical potential of ${}_{}{}^{40}{}_{}{}^{}\mathrm{-}_{}^{40}{}_{}{}^{}$Ca nuclei at different energies. We used two sets of mean-field parameters which correctly produce the binding energy and saturation density of nuclear matter. We studied the effect of the relativistic corrections on the real ion-ion potential. It is found that the heavy ion potential calculated using field theory approach agrees well with that derived from nonrelativistic energy density. © 1996 The American Physical Society.

An accurate impact parameter determination in a heavy ion collision is crucial for almost all further analysis. The capabilities of an artificial neural network are investigated in that respect. A novel input generation for the network is proposed, namely, the transverse and longitudinal momentum distributions of all outgoing (or actually detectable) particles. The neural network approach yields an improvement in performance of a factor of 2 as compared to classical techniques. To achieve this improvement simple network architectures and a 5×5 input grid in (${\mathit{p}}_{\mathit{t}}$,${\mathit{p}}_{\mathit{z}}$) space are sufficient. © 1996 The American Physical Society.

Production of soft photons in relativistic heavy ion collisions due to bremsstrahlung processes in quark matter and hadronic matter is studied. The contribution of pion-driven processes is found to dominate the yield. © 1996 The American Physical Society.

Dilepton production from resonance scattering in hot hadronic matter is studied. Including the widths of these resonances, which enhance the phase space for dilepton production, we find that the production rate is significantly increased if a resonance appears in the extended phase space. For the reaction π+ρ→${\mathit{l}}^{+}$+${\mathit{l}}^{\mathrm{-}}$, the finite ρ meson width extends the invariant mass below the ω-meson mass, so a peak at the ω-meson mass is seen in the dilepton spectrum. Similarly, a ρ-meson peak appears in the reaction ρ+ρ→${\mathit{l}}^{+}$+${\mathit{l}}^{\mathrm{-}}$. On the other hand, the effect of particle widths in the reaction π+${\mathit{a}}_1$→${\mathit{l}}^{+}$+${\mathit{l}}^{\mathrm{-}}$ is small since the extended phase space does not include any resonance. © 1996 The American Physical Society.

The relativistic deuteron structure described by the Bethe-Salpeter formalism is used to obtain the deuteron-nucleus interaction from the covariant convolution of Dirac-phenomenology nucleon-nucleus potentials. Multiple scattering contributions to deuteron-nucleus scattering are taken into account by the positive energy subtracted impulse approximation which results from an analysis of the role played by the relativistic degrees of freedom. A detailed comparison of calculations with deuteron-nucleus elastic scattering data at intermediate energies, including polarization observables, is presented. It is shown that good agreement with the data is obtained when the full spin structure of the interaction that results from the relativistic model is included in the calculations. © 1996 The American Physical Society.

We examine the implications of the constraints arising due to strangeness conservation on the strangeness production in various existing thermal hadron-gas models. The dependence of strangeness chemical potential ${\mathrm{\mu }}_{\mathit{S}}$ on the baryon chemical potential ${\mathrm{\mu }}_{\mathit{B}}$ and temperature T is investigated. The incorporation of finite-size, hard-core, repulsive interactions in the thermodynamically consistent description of hot and dense hadron gas alters the results obtained for pointlike particles. We compare results in two extreme alternative cases: (1) K and ${\mathit{K}}^{\mathrm{*}}$ mesons are treated as point particles and they can penetrate all volumes occupied by baryons and antibaryons and (2) the volume occupied by the baryons and antibaryons is not accessible to them. We find that the results indeed depend on the assumptions made. Moreover, the anomalous results obtained for the ratios Ξ¯/Ξ and Λ¯/Λ rule out the second possibility. © 1996 The American Physical Society.

Previously the quark and meson properties in a many quark system at finite temperature have been studied within effective QCD approaches in the Hartree approximation. In the present paper we consider the influence of the mesonic correlations on the quark self-energy and on the quark propagator within a systematic 1/${\mathit{N}}_{\mathit{c}}$ expansion. Using a general separable ansatz for the nonlocal interaction, we derive a self-consistent equation for the 1/${\mathit{N}}_{\mathit{c}}$ correction to the quark propagator. For a separable model with cutoff form factor, we obtain a decrease of the condensate of the order of 20% at zero temperature. A lowering of the critical temperature for the onset of the chiral restoration transition due to the inclusion of mesonic correlations is obtained with results that seem to be closer to those from lattice calculations. © 1996 The American Physical Society.

We demonstrate that the main part of the observed nuclear suppression of dilepton and charmonium production, especially the x dependence of the suppression, can be explained by the absorption of fast initial partons. We describe several reactions at different energies using one empirical form of the parton absorption cross section. The factorization theorem constraints and some alternative models are discussed. The obtained description of data suggests that the parton recombination and the excess pion contribution are unimportant in nuclei. A gluon excess in nuclei at x∼0.1 is compatible with the present model. © 1996 The American Physical Society.

A detailed investigation of the low-energy chiral expansion is presented within a model truncation of QCD. The truncation allows for a phenomenological description of the quark-quark interaction in a framework which maintains the global symmetries of QCD and permits a 1/${\mathit{N}}_{\mathit{c}}$ expansion. The model dependence of the chiral coefficients is tested for several forms of the quark-quark interaction by varying the form of the running coupling, α(${\mathit{q}}^2$), in the infrared region. The pattern in the coefficients that arises at tree level is consistent with large ${\mathit{N}}_{\mathit{c}}$ QCD, and is related to the model truncation. © 1996 The American Physical Society.

Pion photoproduction up to 770 MeV photon laboratory energy is described by a manifestly covariant wave equation, which includes a treatment of the final state πN interactions consistent with the covariant, unitary, resonance model of πN scattering previously developed. The kernel of the equation includes nucleon (N), Roper (${\mathit{N}}^{\mathrm{*}}$), delta (Δ), and ${\mathit{D}}_{13}$ poles and their crossed poles, as well as π, ρ, and ω exchange terms. The Kroll-Rudermann term and other interaction currents ensure that the model is exactly gauge invariant to all orders in the strong coupling ${\mathit{g}}_{\mathrm{\pi }\mathit{NN}}$. The threshold value of the ${\mathit{E}}_{0+}$ amplitude is in good agreement with recent estimates obtained from chiral perturbation theory. Elastic unitarity to first order in the charge e (Watson theorem) is maintained up to the two-pion production threshold. The complete development of this model, which gives a good fit to all L≤2 multiples up to 770 MeV, is presented. © 1996 The American Physical Society.

We investigate the light and heavy meson spectra in the context of the instantaneous approximation to the Bethe-Salpeter equation (Salpeter’s equation). We use a static kernel consisting of a one-gluon-exchange component and a confining contribution. Salpeter’s equation is known to be formally equivalent to a random-phase-approximation equation; as such, it can develop imaginary eigenvalues. Thus our study cannot be complete without first discussing the stability of Salpeter’s equation. The stability analysis limits the form of the kernel and reveals that a Lorentz scalar confining interaction in the Salpeter equation leads to instabilities (imaginary eigenvalues), whereas one transforming as the time component of a vector does not. Moreover, the stability analysis sets an upper limit on the size of the one-gluon-exchange component; the value for the critical coupling is determined through a solution of the ‘‘semirelativistic’’ Coulomb problem. These limits place important constraints on the interaction and suggest that a more sophisticated model is needed to describe the light and heavy quarkonia. © 1996 The American Physical Society.

We derive the contribution of Δ-h excitations to quasielastic charged-current neutrino-nucleus scattering in the framework of relativistic mean-field theory. We discuss the effect of Δ production on the determination of the axial mass ${\mathit{M}}_{\mathit{A}}$ in neutrino scattering experiments. © 1996 The American Physical Society.

A setup with Moxon-Rae detectors was optimized for measurements of (n,γ) cross sections in the keV region. The experimental technique makes use of the quasi-Maxwellian spectrum that can be obtained via the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$(p,n${)}^7$Be reaction and is particularly suited for the determination of Maxwellian-averaged cross sections for nucleosynthesis studies related to s-process scenarios. It allows measurements at extremely short flight paths with the time-of-flight method as an option for background reduction. The experimental determination of the efficiency with calibrated γ sources and with two-step cascades from selected resonances of the ${}_{}{}^{34}{}_{}{}^{}\mathrm{S}$(p,γ) reaction revealed new properties of the response function. The reliability of the method was first demonstrated at the example of the well-known ratio of the (n,γ) cross sections of Ta and Au, and then used for a measurement of the stellar ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ cross section. With this new value 〈σv〉/${\mathit{v}}_{\mathit{T}}$=15.5±1.5 mb, a discrepancy in the previously existing data could be resolved. © 1996 The American Physical Society.

In a neutron-rich, α-rich environment that could arise during a supernova explosion the reaction chain ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(αn,γ${)}^9$Be(α,n${)}^{12}$C dominates the creation of carbon, which leads to the creation of heavier elements in the subsequent r process [S. E. Woosley and R. D. Hoffman, Astrophys. J. 395, 202 (1992); B. S. Meyer, G. J. Matthews, W. M. Howard, S. G. Woosley, and R. D. Hoffman, ibid. 399, 656 (1992); W. M. Howard, S. Goriely, M. Rayet, and M. Arnould, ibid. 417, 713 (1993)]. For this reason the excitation function of the reaction ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$(α,n${)}^{12}$C has been measured in the energy range from ${\mathit{E}}_{\mathrm{\alpha }}$=366 keV up to 3552 keV (corresponding ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=254–2450 keV in the center-of-mass system) to obtain consistent data in the whole range, which were lacking in the literature. The measurements have been performed with a 4π detector and the yield contributions of the ${\mathit{n}}_0$ and ${\mathit{n}}_1$ neutron group could be resolved. Solid metallic targets of thickness 430 Å (7.75 μg/${\mathrm{cm}}^2$) and 650 Å (11.72 μg/${\mathrm{cm}}^2$) were used. Reaction rates and an analytic expression for them have been evaluated for 0.001≤${\mathit{T}}_9$≤10. © 1996 The American Physical Society.

The reactions ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$(p,α${)}^7$Be and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$(p,α${)}^8$Be are studied at thermonuclear energies using DWBA calculations. For both reactions, transitions to the ground states and first excited states are investigated. In the case of ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$(p,α${)}^7$Be, a resonance at ${\mathit{E}}_{\mathrm{res}}$=10 keV can be consistently described in the potential model, thereby allowing the extension of the astrophysical S-factor data to very low energies. Strong interference with a resonance at about ${\mathit{E}}_{\mathrm{res}}$=550 keV require a Breit-Wigner description of that resonance and the introduction of an interference term for the reaction ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$(p,${\mathrm{\alpha }}_1$${)}^7$${\mathrm{Be}}^{\mathrm{*}}$. Two isospin T=1 resonances (at ${\mathit{E}}_{\mathrm{res}\mathrm{}1}$=149 keV and ${\mathit{E}}_{\mathrm{res}\mathrm{}2}$=619 keV) observed in the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$+p reactions necessitate Breit-Wigner resonance and interference terms to fit the data of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$(p,α${)}^8$Be reaction. S-factors and thermonuclear reaction rates are given for each reaction. The present calculation is the first consistent parametrization for the transition to the ground states and first excited states at low energies. © 1996 The American Physical Society.

We examine the effects on the nuclear neutral current Gamow-Teller (GT) strength of a finite contribution from a polarized strange quark sea. We perform nuclear shell model calculations of the neutral current GT strength for a number of nuclei likely to be present during stellar core collapse. We compare the GT strength when a finite strange quark contribution is included to the strength without such a contribution. As an example, the process of neutral current nuclear deexcitation via νν¯ pair production is examined for the two cases. © 1996 The American Physical Society.

The isotope ${}_{}{}^{181}{}_{}{}^{}\mathrm{Pb}$ was produced in ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$ bombardments of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and, together with other reaction products, was passed through a recoil mass separator and implanted in a double-sided silicon strip detector for α-particle assay. The half-life and energy of the main ${}_{}{}^{181}{}_{}{}^{}\mathrm{Pb}$ α transition were determined to be 45 (20) ms and 7065 (20) keV, respectively. This ${}_{}{}^{181}{}_{}{}^{}\mathrm{Pb}$ ${\mathit{E}}_{\mathrm{\alpha }}$ agrees with one previously measured value [7044 (15) keV], but not with the one [7211 (10) keV] used as input to the 1993 Atomic Mass Evaluation. The 6180-keV α transition assigned to ${}_{}{}^{181}{}_{}{}^{}\mathrm{Tl}$ by Bolshakov et al. was observed, but the 6566-keV α particles ascribed to a 2.7-ms level in ${}_{}{}^{181}{}_{}{}^{}\mathrm{Tl}$ by Schneider were not. © 1996 The American Physical Society.

Excited states in ${}_{}{}^{145}{}_{}{}^{}\mathrm{Nd}$ and ${}_{}{}^{147}{}_{}{}^{}\mathrm{Sm}$ nuclei have been studied using heavy-ion induced reactions. Level schemes of both nuclei are very similar to the level scheme of ${}_{}{}^{149}{}_{}{}^{}\mathrm{Gd}$. In analogy to the ${}_{}{}^{149}{}_{}{}^{}\mathrm{Gd}$ isotone most of the observed excitations are interpreted as single-particle neutron excitations with octupole vibrations coupled to them. Candidates for double octupole-phonon states were identified in both nuclei. © 1996 The American Physical Society.

High-spin states have been studied in neutron-deficient ${}_{}{}^{115}{}_{}{}^{}\mathrm{Xe}$ produced in the ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$${(}^{58}$Ni,2pnγ) reaction. Transitions have been assigned to ${}_{}{}^{115}{}_{}{}^{}\mathrm{Xe}$, for the first time, through their characteristic transition yields with respect to the total γ-ray sum energy and by systematic comparison to neighboring ${}_{}{}^{113}{}_{}{}^{}\mathrm{Xe}$ and ${}_{}{}^{117}{}_{}{}^{}\mathrm{Xe}$. © 1996 The American Physical Society.

In this paper we have studied the low energy structure of ${}_{}{}^{115–119}{}_{}{}^{}\mathrm{Sn}$ isotopes within the vibrational limit for bosons under ${\mathrm{U}}^{\mathit{BF}}$(5)⊗${\mathrm{SU}}^{\mathit{F}}$(4) dynamical symmetry of the graded Lie group U(6/20). Calculation of excitation energy spectra of positive parity levels and B(E2) values of these isotopes has been performed using this dynamical symmetry. Comparison of experimental and calculated energy spectra and B(E2) values of these nuclei suggests the existence of an approximate U(6/20) supersymmetry in these isotopes. © 1996 The American Physical Society.