We present a coherent theoretical study of ultrarelativistic heavy-ion data obtained at the CERN/SPS by the NA35/NA49 Collaborations using (3+1)-dimensional relativistic hydrodynamics. We find excellent agreement with the rapidity spectra of negative hadrons and protons and with the correlation measurements in two experiments: S+S at 200A GeV and Pb+Pb at 160A GeV (preliminary results). Within our model this implies that for Pb+Pb (S+S) a quark-gluon plasma of initial volume 174 ${\mathrm{fm}}^3$ (24 ${\mathrm{fm}}^3$) with a lifetime 3.4 fm/c (1.5 fm/c) was formed. It is found that the Bose-Einstein correlation measurements do not determine the maximal effective radii of the hadron sources because of the large contributions from resonance decay at small momenta. Also within this study we present an NA49 acceptance-corrected two-pion Bose-Einstein correlation function in the invariant variable ${\mathit{Q}}_{\mathrm{inv}}$. © 1996 The American Physical Society.

There are two types of scales present simultaneously in the spacelike as well as in the timelike directions in a model class describing a cylindrically symmetric, finite, three-dimensionally expanding boson source. One type of scale is related to the finite lifetime or geometrical size of the system, and the other type is governed by the rate of change of the local momentum distribution in the considered temporal or spatial direction. The parameters of the Bose-Einstein correlation function may obey an ${\mathit{M}}_{\mathit{t}}$ scaling, as observed in S+Pb and Pb+Pb reactions at CERN SPS. This ${\mathit{M}}_{\mathit{t}}$ scaling may imply that the Bose-Einstein correlation functions view only a small part of a large and expanding system. The full sizes of the expanding system at the last interaction are shown to be measurable with the help of the invariant momentum distribution of the emitted particles. A vanishing duration parameter can also be generated, with a specific ${\mathit{M}}_{\mathit{t}}$ dependence, in the considered model class. © 1996 The American Physical Society.

In a previous paper, we calculated the fully quantum-mechanical cross section for electromagnetic excitation during peripheral heavy-ion collisions. Here, we examine the sensitivity of that cross section to the detailed structure of the projectile and target nuclei. At the transition energies relevant to nuclear physics, we find the cross section to be weakly dependent on the projectile charge radius and to be sensitive to only the leading momentum-transfer dependence of the target transition form factors. We exploit these facts to derive a quantum-mechanical ‘‘equivalent-photon spectrum’’ valid in the long-wavelength limit. This improved spectrum includes the effects of projectile size, the finite longitudinal momentum transfer required by kinematics, and the response of the target nucleus to the off-shell photon. © 1996 The American Physical Society.

We discuss the quark-antiquark loop diagrams of an extended version of the Nambu–Jona-Lasinio (NJL) model that includes a description of confinement. We show how the constraints of gauge invariance affect the calculations of such diagrams, if the qq¯ system has the quantum numbers of the rho or omega mesons. In the NJL model without confinement, gauge invariance may be maintained by using a regularization scheme that respects that symmetry. Alternatively, gauge-invariance constraints may be imposed by introducing a substraction procedure when evaluating the qq¯ loop integrals (vacuum polarization diagrams). We discuss the merits of such subtraction procedures in the presence of a confining interaction. We also provide a calculation of the vacuum polarization diagrams of the NJL model that includes the effects of confinement and also satisfies the constraints imposed by gauge invariance. Our analysis provides some justification for the subtraction procedure used in several studies of the NJL model. We also show, in a rather clear fashion, how confinement serves to change the analytic properties of the polarization tensors, so that the tensor for the theory with confinement has only a physical cut structure in the region of interest. © 1996 The American Physical Society.

The resonant electric quadrupole amplitude in the transition γN⇆Δ(1232) is of great interest for the understanding of baryon structure. Various dynamical models have been developed to extract it from the corresponding photoproduction multipole of pions on nucleons. It is shown that once such a model is specified, a whole class of unitarily equivalent models can be constructed, all of them providing exactly the same fit to the experimental data. However, they may predict quite different resonant amplitudes. Therefore, the extraction of the E2/M1(γN⇆Δ) ratio (bare or dressed), which is based on a dynamical model using a largely phenomenological πN interaction, is not unique. © 1996 The American Physical Society.

The quark-meson coupling model for nuclear matter, which describes nuclear matter as nonoverlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons, is modified by introducing medium modification of the bag constant. We model the density dependence of the bag constant in two different ways: One invokes a direct coupling of the bag constant to the scalar meson field, and the other relates the bag constant to the in-medium nucleon mass. Both models feature a decreasing bag constant with increasing density. We find that when the bag constant is significantly reduced in nuclear medium with respect to its free-space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology and finite-density QCD sum rules. This suggests that the reduction of bag constant in nuclear medium may play an important role in low- and medium-energy nuclear physics. © 1996 The American Physical Society.

Ambiguities in pseudoscalar-meson photoproduction, arising from incomplete experimental data, have analogs in pion-nucleon scattering. Amplitude ambiguities have important implications for the problems of amplitude extraction and resonance identification in partial-wave analysis. The effect of these ambiguities on observables is described. We compare our results with those found in earlier studies. © 1996 The American Physical Society.

The N¯N→ππ annihilation reaction is investigated in a model with both baryon-exchange diagrams and meson-pole diagrams. The main features of the observed differential cross sections for p¯p→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$ from 360 to 1190 MeV/c are well understood in this model. The backward enhancement of the differential cross section is mainly due to the NΔπ tensor coupling while the process p¯p→${\mathit{f}}_2$→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$ contributes to the bump structure around 100° developing for ${\mathit{P}}_{\mathrm{lab}}$≳680 MeV/c. This hints at nontrivial quark spin contributions beyond the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_0$ model (which favored S and P waves) and hints at a large gluonic component in tensor mesons with masses between 1.27 and 2.2 GeV. © 1996 The American Physical Society.

The neutron capture cross sections of ${}_{}{}^{114}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{115}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{117}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{118}{}_{}{}^{}\mathrm{Sn}$, and ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$ were measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$(p,n${)}^7$Be reaction using a pulsed proton beam. Capture events were registered with the Karlsruhe 4π barium fluoride detector. The experiment was complicated by the small (n,γ) cross sections of the proton magic tin isotopes and by the comparably low enrichment of the rare isotopes ${}_{}{}^{114}{}_{}{}^{}\mathrm{Sn}$ and ${}_{}{}^{115}{}_{}{}^{}\mathrm{Sn}$. Despite significant corrections for capture of scattered neutrons and for isotopic impurities, the high efficiency and the spectroscopic quality of the ${\mathrm{BaF}}_2$ detector allowed the determination of the cross-section ratios with overall uncertainties of 1–2 %, five times smaller compared to existing data. Based on these results, Maxwellian averaged (n,γ) cross sections were calculated for thermal energies between kT=10 and 100 keV. These data are used for a discussion of the solar tin abundance and for an improved determination of the isotopic s- and r-process components. © 1996 The American Physical Society.

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of ${}_{}{}^{134}{}_{}{}^{}\mathrm{Ba}$ and ${}_{}{}^{136}{}_{}{}^{}\mathrm{Ba}$ at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3–5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT≊8-12 keV) favored by the most recent stellar models of the s process. By fitting the data with a multilevel R-matrix code, we determined parameters for 86 resonances in ${}_{}{}^{134}{}_{}{}^{}\mathrm{Ba}$ below 11 keV and 92 resonances in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Ba}$ below 35 keV. Astrophysical reaction rates were calculated using these parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature kT=30 keV, but show significant differences at lower temperatures. We determined that these differences were due to the effect of resonances below the energy range of previous experiments and to the use of incorrect neutron widths in a previous resonance analysis. Our data show that the ratio of reaction rates for these two isotopes depends more strongly on temperature than previous measurements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at ${}_{}{}^{134}{}_{}{}^{}\mathrm{Cs}$ is too low to be consistent with the temperature derived from other branching points. This inconsistency is evidence for the need for more sophisticated models of the s process beyond the classical model. We used a reaction network code to explore the changes in the calculated isotopic abundances resulting from our new reaction rates for an s-process scenario based on a stellar model. These calculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates. © 1996 The American Physical Society.

We have determined the absolute intensity of weak γ transitions in the level scheme of ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}$, observed following the electron capture decay of ${}_{}{}^{144}{}_{}{}^{}\mathrm{Pm}$. The absolute intensity of the 1397-keV E3 branch from the 2093-keV ($5_1^{\mathrm{-}}$) level was determined to be (4.9 ± 0.7) × ${10}^{\mathrm{-}4}$%. This leads to a revised absolute transition rate of B(E3;$5_1^{\mathrm{-}}$→$2_1^{+}$)=${26}_{\mathrm{-}12}^{+15\mathrm{}}$ Weisskopf units, which is still consistent with an interpretation of the $5_1^{\mathrm{-}}$ level based on quadrupole-octupole coupling. © 1996 The American Physical Society.

A detailed discussion and supplemental experimental information on the electron capture and ${\mathrm{\beta }}^{+}$ decay schemes of ${}_{}{}^{153}{}_{}{}^{}\mathrm{Er}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Yb}$ proposed in our previous publications are included. The three-quasiparticle state and single-proton states assigned to ${}_{}{}^{153}{}_{}{}^{}\mathrm{Ho}$ have been reproduced by a shell model calculation. The extracted quenching factor for the three-quasiparticle state was determined to be 10. The existence of the two bands in ${}_{}{}^{157}{}_{}{}^{}\mathrm{Tm}$ has been explained by a triaxial deformation of the ground state of this nucleus. The experimental evidence from systematic behavior of the compiled low-energy levels indicates that the ground-state shapes change from spherical to deformed between neutron numbers N=86 and 88 in both the odd Ho and odd Tm isotopic chains. © 1996 The American Physical Society.

We extract energies and widths of the ground states of ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^5{}_{}{}^{}\mathrm{Li}$ from recent single-level R-matrix fits to the spectra of the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(d,γ${)}^5$He and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(d,γ${)}^5$Li reactions. The widths obtained differ significantly from the formal R-matrix values but they are close to those measured as full widths at half maxima of the spectra in various experiments. The energies are somewhat lower than those given by usual estimates of the peak positions. The extracted values are close to the S-matrix poles calculated previously from the multiterm analyses of the N${\mathrm{-}}^4$He elastic scattering data. © 1996 The American Physical Society.

We calculate nuclear matter observables for the Bonn-C potential in a relativistic conserving approach. This amounts to extending the Dirac-Brueckner-Hartree-Fock model by including hole-hole propagating diagrams to all orders, which also allows us to calculate the momentum distribution using a spectral function. The trends we find upon including hole-hole propagation are very similar to what we found performing the same calculation for the Groningen potential. One particular finding is that the high momentum tail of the momentum distribution is the same for both potentials. © 1996 The American Physical Society.

Vector mesons containing light quarks are thought to have their masses reduced in dense nuclear matter, sacrificing some of their energy to the scalar field which becomes appreciable at finite baryon density. Model calculations find masses which fall by a couple tens of percents in normal nuclear matter, and by several hundred MeV in dense matter. We estimate the collision rate for ρ mesons in such a scenario and at finite temperature. Compared to its free-mass value, the collision rate changes by nearly a factor of 2 both above and below, depending on the density. This collision broadening effect could be important for estimates of low-mass dilepton production in heavy-ion collisions. © 1996 The American Physical Society.

We critically discuss the recent analysis of strangeness production in nuclear collisions at 200A GeV/c performed by Topor Pop et al. We point out various problems in connection with the interpretation of the data and the numerical model results. We also argue that the term ‘‘strangeness enhancement’’ is used in a misleading way. © 1996 The American Physical Society.

In this Reply we argue that the factor of 4 enhancement of the local midrapidity densities of Λ and the strong suppression of Λ in the projectile fragmentation region in p+S relative to p+p reactions provide strong evidence of nonequilibrium dynamics in strangeness production. Second, we show that the dramatic changes in the Λ rapidity distributions in pA cannot be attributed to nuclear baryon stopping power. © 1996 The American Physical Society.

Tests of time-reversal symmetry at low and medium energies may be analyzed in the framework of effective hadronic interactions. Here, we consider the quark structure of hadrons to make a connection to the more fundamental degrees of freedom. It turns out that for P-even T-odd interactions hadronic matrix elements evaluated in terms of quark models give rise to factors of 2 to 5. Also, it is possible to relate the strength of the anomalous part of the effective ρ-type T-odd P-even tensor coupling to quark structure effects. © 1996 The American Physical Society.

Extended calculations of the deuteron’s static properties, based on the numerical solution of the Bethe-Salpeter equation, are presented. A formalism is developed, which provides a comparative analysis of the covariant amplitudes in various representations and nonrelativistic wave functions. The magnetic and quadrupole moments of the deuteron are calculated in the Bethe-Salpeter formalism and the role of relativistic corrections is discussed. © 1996 The American Physical Society.