Showing papers in "Physical Review C in 1996"

Mean-field description of ground-state properties of drip-line nuclei: Pairing and continuum effects

Abstract: Ground-state properties of exotic even-even nuclei with extreme neutron-to-proton ratios are described in the framework of self-consistent mean-field theory with pairing formulated in coordinate space. This theory properly accounts for the influence of the particle continuum, which is particularly important for weakly bound systems. The pairing properties of nuclei far from stability are studied with several interactions emphasizing different aspects, such as the range and density dependence of the effective interaction. Measurable consequences of spatially extended pairing fields are presented, and the sensitivity of the theoretical predictions to model details is discussed. {copyright} {ital 1996 The American Physical Society.}

Two-nucleon potential from chiral Lagrangians

Abstract: Chiral symmetry is consistently implemented in the two-nucleon problem at low-energy through the general effective chiral Lagrangian. The potential is obtained up to a certain order in chiral perturbation theory both in momentum and coordinate space. Results of a fit to scattering phase shifts and bound state data are presented, where satisfactory agreement is found for laboratory energies up to about 100 MeV. {copyright} {ital 1996 The American Physical Society.}

Nonlocal nature of the nuclear force and its impact on nuclear structure.

Abstract: We calculate the triton binding energy with a nonlocal {ital NN} potential that fits the world {ital NN} data below 350 MeV with the almost perfect {chi}{sup 2}/datum of 1.03. The nonlocality is derived from relativistic meson field theory. The result obtained in a 34-channel, charge-dependent Faddeev calculation is 8.00 MeV, which is 0.4 MeV above the predictions by local {ital NN} potentials. The increase in binding energy can be clearly attributed to the off-shell behavior of the nonlocal potential. Our result cuts in half the discrepancy between theory and experiment established from local {ital NN} potentials. Implications for other areas of miscroscopic nuclear structure, in which underbinding is a traditional problem, are discussed. {copyright} {ital 1996 The American Physical Society.}

Hyperon rich matter in neutron stars

Abstract: We study the equation of state of hyperon-rich matter in neutron stars using an extended relativistic mean-field model. We take special care of the recently proposed nonlinear behavior of the vector field providing a much better agreement with Dirac-Br\"uckner calculations. The hyperon-hyperon interaction is also implemented by introducing additional meson exchanges. With these new terms we avoid the instability found at high densities while keeping the excellent description for finite nuclear systems. We also demonstrate within the mean-field approach that the presence of hyperons inside neutron stars on one hand and the hyperon-hyperon interactions on the other hand make the onset of kaon condensation less favorable. \textcopyright{} 1996 The American Physical Society.

Hadronization in the SU(3) Nambu-Jona-Lasinio model

Abstract: The hadronization process for quarks combining into two mesons, qq\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}${\mathit{MM}}^{\ensuremath{'}}$ at temperature T is described within the SU(3) Nambu--Jona-Lasinio model with finite current quark masses. Invariant matrix elements, cross sections, and transition rates are calculated to leading order in a 1/${\mathit{N}}_{\mathit{c}}$ expansion. Four independent classes, ud\ifmmode\bar\else\textasciimacron\fi{}, us\ifmmode\bar\else\textasciimacron\fi{}, u\ifmmode \bar{u}\else \={u}\fi{}, and ss\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}hadrons are analyzed and the yield is found to be dominated by pion production. Threshold behavior is determined by the exothermic or endothermic nature of the processes constituting the hadronization class. A strong suppression of transition rates is found at the pionic Mott temperature ${\mathit{T}}_{\mathit{M}\mathrm{\ensuremath{\pi}}}$=212 MeV, at which the pion becomes a resonant state. The mean time for hadronization is calculated to be 2--4 fm/c near the Mott temperature. The calculation of strangeness changing processes indicates that hadronization accounts for a 1% increase in the absolute value of the kaon to pion ratio at T=150 MeV. \textcopyright{} 1996 The American Physical Society.

Realistic Model of the Nucleon Spectral Function in Few- and Many- Nucleon Systems

Abstract: By analyzing the high-momentum features of the nucleon momentum distribution in light and complex nuclei, it is argued that the basic two-nucleon configurations generating the structure of the nucleon spectral function at high values of the nucleon momentum and removal energy can be properly described by a factorized ansatz for the nuclear wave function, which leads to a nucleon spectral function in the form of a convolution integral involving the momentum distributions describing the relative and center-of-mass motion of a correlated nucleon-nucleon pair embedded in the medium. The spectral functions of $^{3}\mathrm{He}$ and infinite nuclear matter resulting from the convolution formula and from many-body calculations are compared, and a very good agreement in a wide range of values of nucleon momentum and removal energy is found. Applications of the model to the analysis of inclusive and exclusive processes are presented, illustrating those features of the cross section which are sensitive to that part of the spectral function which is governed by short-range and tensor nucleon-nucleon correlations. \textcopyright{} 1996 The American Physical Society.

Realistic collective nuclear Hamiltonian

Abstract: The residual part of the realistic forces{emdash}obtained after extracting the monopole terms responsible for bulk properties{emdash}is strongly dominated by pairing and quadrupole interactions, with important {sigma}{tau}{center_dot}{sigma}{tau}, octupole, and hexadecapole contributions Their forms retain the simplicity of the traditional pairing plus multipole models, while eliminating their flaws through a normalization mechanism dictated by a universal {ital A}{sup {minus}1/3} scaling Coupling strengths and effective charges are calculated and shown to agree with empirical values Comparisons between different realistic interactions confirm the claim that they are very similar {copyright} {ital 1996 The American Physical Society}

Evidence for neutrino oscillations from muon decay at rest.

Abstract: A search for \ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$\ensuremath{\rightarrow}\ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ oscillations has been conducted at the Los Alamos Meson Physics Facility using \ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$ from ${\mathrm{\ensuremath{\mu}}}^{+}$ decay at rest. The \ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ are detected via the reaction \ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{ep}}$\ensuremath{\rightarrow}${\mathit{e}}^{+}$n, correlated with the 2.2 MeV \ensuremath{\gamma} from np\ensuremath{\rightarrow}d\ensuremath{\gamma}. The use of tight cuts to identify ${\mathit{e}}^{+}$ events with correlated \ensuremath{\gamma} rays yields 22 events with ${\mathit{e}}^{+}$ energy between 36 and 60 MeV and only 4.6\ifmmode\pm\else\textpm\fi{}0.6 background events. The probability that this excess is due entirely to a statistical fluctuation is 4.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$. A ${\mathrm{\ensuremath{\chi}}}^{2}$ fit to the entire ${\mathit{e}}^{+}$ sample results in a total excess of 51.${0}_{\mathrm{\ensuremath{-}}19.5}^{+20.2}$\ifmmode\pm\else\textpm\fi{}8.0 events with ${\mathit{e}}^{+}$ energy between 20 and 60 MeV. If attributed to \ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$\ensuremath{\rightarrow}\ensuremath{ u}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ oscillations, this corresponds to an oscillation probability (averaged over the experimental energy and spatial acceptance) of (0.31\ifmmode\pm\else\textpm\fi{}0.12\ifmmode\pm\else\textpm\fi{}0.05)%. \textcopyright{} 1996 The American Physical Society.

Meson-exchange model for pi N scattering and gamma N--> pi N reaction.

Abstract: An effective Hamiltonian consisting of bare \ensuremath{\Delta}\ensuremath{\leftrightarrows}\ensuremath{\pi}N, \ensuremath{\gamma}N vertex interactions and energy-independent meson-exchange \ensuremath{\pi}N\ensuremath{\leftrightarrows}\ensuremath{\pi}N,\ensuremath{\gamma}N transition operators is derived by applying a unitary transformation to a model Lagrangian with N,\ensuremath{\Delta},\ensuremath{\pi}, \ensuremath{\rho}, \ensuremath{\omega}, and \ensuremath{\gamma} fields. With appropriate phenomenological form factors and coupling constants for \ensuremath{\rho} and \ensuremath{\Delta}, the model can give a good description of \ensuremath{\pi}N scattering phase shifts up to the \ensuremath{\Delta} excitation energy region. It is shown that the best reproduction of the recent LEGS data of the photon-asymmetry ratios in \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$p reactions provides rather restricted constraints on the coupling strengths ${\mathit{G}}_{\mathit{E}}$ of the electric E2 and ${\mathit{G}}_{\mathit{M}}$ of the magnetic M1 transitions of the bare \ensuremath{\Delta}\ensuremath{\leftrightarrows}\ensuremath{\gamma}N vertex and the less well-determined coupling constant ${\mathit{g}}_{\mathrm{\ensuremath{\omega}}\mathit{NN}}$ of \ensuremath{\omega} meson. Within the ranges that ${\mathit{G}}_{\mathit{M}}$=1.9\ifmmode\pm\else\textpm\fi{}0.05, ${\mathit{G}}_{\mathit{E}}$=0.0\ifmmode\pm\else\textpm\fi{}0.025, and 7\ensuremath{\le}${\mathit{g}}_{\mathrm{\ensuremath{\omega}}\mathit{NN}}$\ensuremath{\le}10.5, the predicted differential cross sections and photon-asymmetry ratios are in an overall good agreement with the data of \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$p, \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$n, and \ensuremath{\gamma}n\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p reactions from 180 MeV to the \ensuremath{\Delta} excitation region. The predicted ${\mathit{M}}_{{1}^{+}}$ and ${\mathit{E}}_{{1}^{+}}$ multipole amplitudes are also in good agreement with the empirical values determined by the amplitude analyses. The constructed effective Hamiltonian is free of the nucleon renormalization problem and hence is suitable for nuclear many-body calculations. We have also shown that the assumptions made in the K-matrix method, commonly used in extracting empirically the \ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\Delta} transition amplitudes from the data, are consistent with our meson-exchange dynamical model. It is found that the helicity amplitudes calculated from our bare \ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\Delta} vertex are in good agreement with the predictions of the constituent quark model. The differences between these bare amplitudes and the dressed amplitudes, which are closer to the empirical values listed by the Particle Data Group, are shown to be due to the nonresonant meson exchange mechanisms. Within the range 7\ensuremath{\le}${\mathit{g}}_{\mathrm{\ensuremath{\omega}}\mathit{NN}}$\ensuremath{\le}10.5 of the \ensuremath{\omega} meson coupling favored by the data of the photon-asymmetry ratios in \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$p reactions, our values of the E2/M1 ratio for the \ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\Delta} transition are (0.0\ifmmode\pm\else\textpm\fi{}1.3)% for the bare vertex and (-1.8\ifmmode\pm\else\textpm\fi{}0.9)% for the dressed vertex. \textcopyright{} 1996 The American Physical Society.

Bose-Einstein Correlations for Three-Dimensionally Expanding, Cylindrically Symmetric, Finite Systems

Abstract: 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. \textcopyright{} 1996 The American Physical Society.

Conclusive evidence for the influence of nuclear orientation on quasifission.

Abstract: Fission fragment anisotropies and mass distributions have been measured to high accuracy, over a wide range of angles, for the $^{16}\mathrm{O}$ + $^{238}\mathrm{U}$ reaction. The bombarding energies spanned the fusion barrier distribution, in steps of 1 MeV. Fission following transfer reactions was rejected by making use of the deduced velocity vectors of the fissioning nuclei. The resulting mass distributions for full momentum transfer fission show a small but significant skewness, which increases as the beam energy falls through the fusion barrier region, displaying a similar energy dependence as the fission fragment angular anisotropies. This is conclusive evidence for the interpretation that collisions with the tips of the deformed $^{238}\mathrm{U}$ target nuclei lead to quasifission, while collisions with the sides result in fusion-fission. \textcopyright{} 1996 The American Physical Society.

Neutrinoless double beta decay within the quasiparticle random-phase approximation with proton-neutron pairing.

Abstract: We have investigated the role of proton-neutron pairing in the context of the quasiparticle random phase approximation formalism. This way the neutrinoless double beta decay matrix elements of the experimentally interesting A = 48, 76, 82, 96, 100, 116, 128, 130, and 136 systems have been calculated. We have found that the inclusion of proton-neutron pairing influences the neutrinoless double beta decay rates significantly, in all cases allowing for larger values of the expectation value of light neutrino masses. Using the best presently available experimental limits on the half lifetime of neutrinoless double beta decay we have extracted the limits on lepton number violating parameters. \textcopyright{} 1996 The American Physical Society.

Effective gA in the pf shell.

Abstract: We have calculated the Gamow-Teller matrix elements of 64 decays of nuclei in the mass range {ital A}=41{endash}50. In all the cases the valence space of the full {ital pf} shell is used. Agreement with the experimental results demands the introduction of an average quenching factor {ital q}=0.744{plus_minus}0.015 slightly smaller but statistically compatible with the {ital sd}-shell value, thus indicating that the present number is close to the limit for large {ital A}. {copyright} {ital 1996 The American Physical Society.}

Radii of halo nuclei from cross section measurements

Abstract: The root mean square matter radii of halo nuclei provide a basic measure in constructing, constraining, and assessing theoretical models of halo structures. We consider corrections to static density (optical limit) Glauber model calculations of reaction cross sections of such nuclei at high energy giving careful consideration to their intrinsic few-body structure and the adiabatic nature of the halo nucleus-target interaction. We take as important examples the loosely bound two- and three-body systems $^{11}\mathrm{Be}$, $^{6}\mathrm{He}$, $^{11}\mathrm{Li}$, and $^{14}\mathrm{Be}$. The contribution of the valence particles to the calculated reaction cross sections are shown to be significantly reduced, requiring increased halo radii to reproduce experimental data. The implications of these changes for structure models of extended two- and three-body systems are discussed. \textcopyright{} 1996 The American Physical Society.

Spectroscopic study of (Lambda) B-10, (Lambda) C-12, (Lambda) Si-28, (Lambda) Y-89, (Lambda) La-139, and (Lambda) Pb-208 by the (pi+, K+) reaction

Abstract: Hypernuclear mass spectra of ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{10}$B, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{12}$C, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{28}$Si, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{89}$Y, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{139}$La, and ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{208}$Pb were measured by the (${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathit{K}}^{+}$) reaction with an energy resolution of 2 MeV (FWHM). The experiment was carried out with a 1.06 GeV/c ${\mathrm{\ensuremath{\pi}}}^{+}$ beam from the KEK 12-GeV PS and with the superconducting kaon spectrometer (SKS). For light hypernuclei, core-excited states were resolved for the first time. For heavy hypernuclei, peak structure reflecting \ensuremath{\Lambda} shell structure was observed. \textcopyright{} 1996 The American Physical Society.

Updated resonance photodecay amplitudes to 2 GeV.

Abstract: We present the results of an energy-dependent and set of single-energy partial-wave analyses of single-pion photoproduction data. These analyses extend from threshold to 2 GeV in the laboratory photon energy, and update our previous analyses to 1.8 GeV. Photo-decay amplitudes are extracted for the baryon resonances within this energy range. We consider two photoproduction sum rules and the contributions of two additional resonance

Standard neutrino spectrum from B 8 decay

Abstract: We present a systematic evaluation of the shape of the neutrino energy spectrum produced by beta decay of $^{8}\mathrm{B}$. We place special emphasis on determining the range of uncertainties permitted by existing laboratory data and theoretical ingredients (such as forbidden and radiative corrections). We review and compare the available experimental data on the $^{8}\mathrm{B}$(${\mathrm{\ensuremath{\beta}}}^{+}$${)}^{8}$Be(2\ensuremath{\alpha}) decay chain. We analyze the theoretical and experimental uncertainties quantitatively. We give a numerical representation of the best-fit (standard-model) neutrino spectrum, as well as two extreme deviations from the standard spectrum that represent the total (experimental and theoretical) effective \ifmmode\pm\else\textpm\fi{}3\ensuremath{\sigma} deviations. Solar neutrino experiments that are currently being developed will be able to measure the shape of the $^{8}\mathrm{B}$ neutrino spectrum above about 5 MeV. An observed distortion of the $^{8}\mathrm{B}$ solar neutrino spectrum outside the range given in the present work could be considered as evidence, at an effective significance level greater than three standard deviations, for physics beyond the standard electroweak model. We use the most recent available experimental data on the Gamow-Teller strengths in the A=37 system to calculate the $^{8}\mathrm{B}$ neutrino absorption cross section on chlorine: ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{Cl}}$=(1.14\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}42}$ ${\mathrm{cm}}^{2}$ (\ifmmode\pm\else\textpm\fi{}3\ensuremath{\sigma} errors). The chlorine cross section is also given as a function of the neutrino energy. The $^{8}\mathrm{B}$ neutrino absorption cross section in gallium is ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{Ga}}$=(2.${46}_{\mathrm{\ensuremath{-}}1.1}^{+2.1}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}42}$ ${\mathrm{cm}}^{2}$ (\ifmmode\pm\else\textpm\fi{}3\ensuremath{\sigma} errors). \textcopyright{} 1996 The American Physical Society.

Breakup reactions of the halo nuclei 11Be and 8B.

Abstract: We calculate the nuclear induced breakup of {sup 11}Be and {sup 8}B using a more realistic treatment of the diffraction and stripping processes than in previous work. The breakup is treated in the eikonal approximation with a profile function calculated from a realistic optical potential at low energies and from free nucleon-nucleon cross sections at high energies. This treatment gives a good description of measured breakup cross sections, as well as the longitudinal momentum distribution of the corelike fragments, which is narrower than predicted in the transparent limit. The real part of the potential is found to be significant and enhances the diffractive breakup at low energies. {copyright} {ital 1996 The American Physical Society.}

alpha decay of 273110: Shell closure at N=162.

Abstract: In bombardments of $^{244}\mathrm{Pu}$ with $^{34}\mathrm{S}$ we discovered the \ensuremath{\alpha}-decaying nuclide $^{273}110$. We conducted an extensive off-line search of the raw data for event sequences which fit the expected pattern of implantation in a position-sensitive detector and subsequent decay of $^{273}110$ and its descendants. We observed one three-member sequence of genetically linked \ensuremath{\alpha} decays, resulting in ${\mathit{E}}_{\mathrm{\ensuremath{\alpha}}}$=11.35 MeV, a half-life of 0.${3}_{\mathrm{\ensuremath{-}}0.2}^{+1.3}$ ms, and a production cross section of about 0.4 pb for $^{273}110$. Other possible $^{273}110$ event chains were also observed. The measured \ensuremath{\alpha}-particle energy for the N=163 nuclide $^{273}110$ provides direct evidence for a neutron shell closure at N=162. \textcopyright{} 1996 The American Physical Society.

Electromagnetic production of associated strangeness.

Abstract: A formalism, based on an isobaric approach using Feynman diagrammatic techniques, which includes the nucleonic (spin \ensuremath{\le} 5/2), hyperonic (spin 1/2), and kaonic resonances, is developed Using this formalism, a thorough investigation of the following electromagnetic strangeness processes, for which experimental results are available, is performed: \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathit{K}}^{+}$\ensuremath{\Lambda}, ${\mathit{K}}^{+}$${\mathrm{\ensuremath{\Sigma}}}^{0}$, ${\mathit{K}}^{0}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, for ${\mathit{E}}_{\ensuremath{\gamma}}^{\mathrm{lab}}$\ensuremath{\le} 21 GeV, ep\ensuremath{\rightarrow}${\mathit{e}}^{\ensuremath{'}}$${\mathit{K}}^{+}$\ensuremath{\Lambda}, ${\mathit{e}}^{\ensuremath{'}}$${\mathit{K}}^{+}$${\mathrm{\ensuremath{\Sigma}}}^{0}$, and ${\mathit{K}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Lambda},\ensuremath{\gamma}${\mathrm{\ensuremath{\Sigma}}}^{0}$ A reaction mechanism, describing well enough the data, is found to include a reasonable number of baryonic resonances among a very large number of potential candidates The extracted main kaon-hyperon-nucleon coupling constants are in good agreement with values predicted using SU(3) symmetry The main findings of this model are compared with the results of other recent phenomenological studies Predictions for the upcoming photoproduction polarization and electroproduction observables are presented, and their sensitivity to the phenomenological models ingredients are emphasized \textcopyright{} 1996 The American Physical Society

New precompound decay model

Abstract: A new precompound Monte Carlo simulation model is presented. It permits unlimited multiple precompound emission for each interaction, may be used to give exclusive spectra and yields, and does not rely upon those exciton state densities shown to be inconsistent with the assumed two body transitions in the hybrid and exciton model formulations. Results of the new model are compared with $^{90}\mathrm{Zr}$ (p,xn) single differential spectra for incident energies from 25 to 160 MeV, and for $^{59}\mathrm{Co}$ (p,x) and $^{51}\mathrm{V}$ (p,x) excitation functions for incident proton energies up to 200 MeV. The new model gives significant improvement over the hybrid model for calculation of excitation functions. Multiple precompound decay predictions are compared for the two models. It is shown that the restriction to two precompound nucleons per decay in the hybrid model becomes a poor approximation for incident proton energies above \ensuremath{\approxeq}80 MeV. \textcopyright{} 1996 The American Physical Society.

Role of color neutrality in nuclear physics: Modifications of nucleonic wave functions

Abstract: The influence of the nuclear medium upon the internal structure of a composite nucleon is examined. The interaction with the medium is assumed to depend on the relative distances between the quarks in the nucleon consistent with the notion of color neutrality, and to be proportional to the nucleon density. In the resulting description the nucleon in matter is a superposition of the ground state (free nucleon) and radial excitations. The effects of the nuclear medium on the electromagnetic and weak nucleon form factors and the nucleon structure function are computed using a light-front constituent quark model. Further experimental consequences are examined by considering the electromagnetic nuclear response functions. The effects of color neutrality supply small but significant corrections to predictions of observables. {copyright} {ital 1996 The American Physical Society.}

Femtometer toroidal structures in nuclei

Abstract: The two-nucleon density distributions in states with isospin T=0, spin S=1 and projection M{sub S}=0 and {+-}1 are studied in {sup 2}H, {sup 3,4}He, {sup 6,7}Li and {sup 16}O. The equidensity surfaces for M{sub S}=0 distributions are found to be toroidal in shape, while those of M{sub S}={+-}1 have dumbbell shapes at large density. The dumbbell shapes are generated by rotating tori. The toroidal shapes indicate that the tensor correlations have near maximal strength at r < 2 fm in all these nuclei. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron, the quasi-deuteron model, and the dp, dd and {alpha}d L=2 (D-wave) components in {sup 3}He, {sup 4}He and {sup 6}Li. The toroidal distribution has a maximum-density diameter of {approximately}1 fm and a half-maximum density thickness of {approximately}0.9 fm. Many realistic models of nuclear forces predict these values, which are supported by the observed electromagnetic form factors of the deuteron, and also predicted by classical Skyrme effective Lagrangians, related to QCD in the limit of infinite colors. Due to the rather small size of this structure, it could have a revealing relation to certain aspects of QCD.

Early fragment formation in heavy ion collisions

Abstract: The fragmentation pattern of central multifragmentation events observed in the collision of heavy systems can be recognized at a time when the system is still dense and the particles are still interacting with each other. This is the result obtained by applying simulated annealing algorithms to molecular dynamics simulations. We employ this algorithm to central and peripheral reactions of heavy nuclei simulated by the quantum molecular dynamics model (QMD). We see that in central collisions the fragments can already be identified when the density is still close to normal nuclear matter density and hence the fragment nucleons never pass through a density sufficiently low to allow for a liquid gas phase transition. In peripheral reactions, however, we observe that shortly after the nuclei have passed each other a division of the spectator matter into several medium-size clusters would yield the highest binding energy. However, the spectator matter does not break into these clusters but approaches thermal equilibrium.

Reactions pd → 3 He η and pd → 3 He π + π − near the η threshold

Abstract: The reaction {ital pd}{r_arrow}{sup 3}He{eta} has been studied with the SPES2 spectrometer, for proton energies between 0.2 MeV and 11 MeV above threshold. The total cross section rises from 0.25 to 0.40 {mu}b in this region. The observed energy dependence can be attributed to the strong final state interaction. The angular distribution is nearly isotropic, consistent with {ital S} wave production. A measurement of the {ital pd}{r_arrow}{sup 3}He{pi}{sup +}{pi}{sup {minus}} cross section in the threshold region is also presented. The setup, which is capable of producing 10{sup 5} tagged {eta}/s with only a few percent background, is used to investigate rare {eta} decays. {copyright} {ital 1996 The American Physical Society.}

pp-->pp pi 0 reaction near threshold: A chiral power counting approach.

Abstract: We use power-counting arguments as an organizing principle to apply chiral perturbation theory, including an explicit {Delta}, to the {ital pp}{r_arrow}{ital pp}{pi}{sup 0} reaction near threshold. There are two lowest-order leading mechanisms expected to contribute to the amplitude with similar magnitudes: an impulse term, and a {Delta}-excitation mechanism. We examine formally subleading but potentially large mechanisms, including pion-rescattering and short-ranged contributions. We show that the pion-rescattering contribution is enhanced by off-shell effects and has a sign opposite to that of a recent estimate based on a PCAC pion interpolating field. Our result is that the impulse term interferes destructively with the pion-rescattering and {Delta}-excitation terms. In addition, we have modeled the short-ranged interaction using {sigma} and {omega} exchange mechanisms. A recoil correction to the impulse approximation is small. The total amplitude obtained including all of these processes is found to yield cross sections substantially smaller than the measured ones. {copyright} {ital 1996 The American Physical Society.}

J/Psi Suppression in an Equilibrating Parton Plasma

Abstract: Short-distance QCD is employed to calculate the J/\ensuremath{\psi} survival probability in an equilibrating parton gas whose evolution is governed by a set of master rate equations. Partons in the early stage of high-energy nuclear collisions may initially not be in equilibrium, but their average transverse momentum is sufficiently high to break up a QQ\ifmmode\bar\else\textasciimacron\fi{} bound state. Such a breakup during the evolution of the parton gas is shown to cause a substantial J/\ensuremath{\psi} suppression at both RHIC and LHC energies, using realistic estimates of the initial parton densities. The transverse momentum dependence of the suppression is also shown to be sensitive to the initial conditions and the evolution history of the parton plasma. \textcopyright{} 1996 The American Physical Society.

Radioactivity of neutron deficient isotopes in the region N>82>Z

Abstract: The {alpha} decay characteristics of 88 decay lines emanating from 74 isotopes are presented, including new {alpha} decay lines identified in the neutron deficient nuclides {sup 158}Ta ({ital E}{sub {alpha}}=5969{plus_minus}8 keV, {ital t}{sub 1/2}=46{plus_minus}4 ms), {sup 159}Ta ({ital E}{sub {alpha}}=5516{plus_minus}5 keV, {ital t}{sub 1/2}=1100{plus_minus}100 ms), {sup 160}Ta ({ital E}{sub {alpha}}=5313{plus_minus}5 keV, {ital t}{sub 1/2}=1700{plus_minus}200 ms), {sup 168}Ir ({ital E}{sub {alpha}}=6323{plus_minus}8 keV, {ital t}{sub 1/2}=161{plus_minus}21 ms, {ital b}{sub {alpha}}=82{plus_minus}14{percent}), and {sup 170}Ir ({ital E}{sub {alpha}}=6083{plus_minus}11 keV, {ital t}{sub 1/2}=830{plus_minus}300 ms, {ital b}{sub {alpha}}=36{plus_minus}10{percent}). Their correlations with other decay lines are discussed. The alpha decay of a high-spin isomer in {sup 157}Ta has been discovered, with an energy of 7744{plus_minus}8 keV and a half-life of 1.7{plus_minus}0.1 ms, while the half-life of the corresponding isomeric alpha decay line of {sup 158}W has been measured for the first time as 160{plus_minus}50 {mu}s. First half-life and branching ratio measurements are also reported for the 5454{plus_minus}4 keV {sup 156}Lu line ({ital t}{sub 1/2}=494{plus_minus}12 ms), {sup 162}Re ({ital b}{sub {alpha}}=85{plus_minus}9{percent}), {sup 163}Os ({ital t}{sub 1/2}=12{sub {minus}7}{sup +11} ms), {sup 166}Ir ({ital t}{sub 1/2}=12{plus_minus}1 ms), {sup 167}Ir ({ital t}{sub 1/2}=34{plus_minus}4 ms), the 6227{plus_minus}15 keV {sup 168}Ir line ({ital t}{sub 1/2}=125{plus_minus}40 ms), and {sup 171}Ir ({ital b}{sub {alpha}}=58{plus_minus}11{percent}). Newmore » decay measurements for the proton emitter {sup 156}Ta ({ital E}{sub {ital p}}=1108{plus_minus}8 keV, {ital t}{sub 1/2}=375{plus_minus}54 ms, {ital b}{sub {ital p}}=4.2{plus_minus}0.9{percent}; {ital E}{sub {ital p}}=1007{plus_minus}5 keV, {ital t}{sub 1/2}=144{plus_minus}24 ms) are presented. (Abstract Truncated)« less

Systematic study of nuclear beta decay

Abstract: \ensuremath{\beta}-decay properties of nuclei are studied in the framework of proton-neutron quasiparticle random-phase approximation with a schematic Gamow-Teller residual interaction Particle-hole and particle-particle terms of the separable Gamow-Teller force are consistently included for both ${\mathrm{\ensuremath{\beta}}}^{+}$ and ${\mathrm{\ensuremath{\beta}}}^{\mathrm{\ensuremath{-}}}$ directions, and their strengths are fixed as smooth functions of mass number A of nuclei in such a way that the calculation reproduces observed \ensuremath{\beta}-decay properties of nuclei Using the fixed interaction strengths, \ensuremath{\beta}-decay half-lives of nuclei up to A=150 are calculated, and generally good agreement with experiment is obtained A schematic force which is relevant to the unique first-forbidden decay is also included in a similar way to the Gamow-Teller force, and its effects are studied \textcopyright{} 1996 The American Physical Society

No-core shell-model calculations with starting-energy-independent multivalued effective interactions

Abstract: Large-space no-core shell-model calculations have been performed for $^{3}\mathrm{H}$, $^{4}\mathrm{He}$, $^{5}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{6}\mathrm{He}$, using a starting-energy-independent two-body effective interaction derived by application of the Lee-Suzuki similarity transformation. This transformation can be performed by direct calculation or by different iteration procedures, which are described. A possible way of reducing the auxiliary potential influence on the two-body effective interaction has also been introduced. The many-body effects have been partially taken into account by employing the recently introduced multivalued effective interaction approach. Dependence of the $^{5}\mathrm{He}$ energy levels on the harmonic-oscillator frequency as well as on the size of the model space has been studied. The Reid 93 nucleon-nucleon potential has been used in the study, but results have also been obtained using the Nijmegen II potential for comparison. \textcopyright{} 1996 The American Physical Society.