Showing papers in "Physical Review C in 1998"

Equation of state of nucleon matter and neutron star structure

Abstract: Properties of dense nucleon matter and the structure of neutron stars are studied using variational chain summation methods and the new Argonne ${v}_{18}$ two-nucleon interaction, which provides an excellent fit to all of the nucleon-nucleon scattering data in the Nijmegen database. The neutron star gravitational mass limit obtained with this interaction is 1.67${M}_{\ensuremath{\bigodot}}.$ Boost corrections to the two-nucleon interaction, which give the leading relativistic effect of order ${(v/c)}^{2},$ as well as three-nucleon interactions, are also included in the nuclear Hamiltonian. Their successive addition increases the mass limit to 1.80 and 2.20 ${M}_{\ensuremath{\bigodot}}.$ Hamiltonians including a three-nucleon interaction predict a transition in neutron star matter to a phase with neutral pion condensation at a baryon number density of $\ensuremath{\sim}0.2 {\mathrm{fm}}^{\ensuremath{-}3}.$ Neutron stars predicted by these Hamiltonians have a layer with a thickness on the order of tens of meters, over which the density changes rapidly from that of the normal to the condensed phase. The material in this thin layer is a mixture of the two phases. We also investigate the possibility of dense nucleon matter having an admixture of quark matter, described using the bag model equation of state. Neutron stars of 1.4${M}_{\ensuremath{\bigodot}}$ do not appear to have quark matter admixtures in their cores. However, the heaviest stars are predicted to have cores consisting of a quark and nucleon matter mixture. These admixtures reduce the maximum mass of neutron stars from 2.20 to 2.02 (1.91) ${M}_{\ensuremath{\bigodot}}$ for bag constant $B=200 (122) {\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}}^{3}.$ Stars with pure quark matter in their cores are found to be unstable. We also consider the possibility that matter is maximally incompressible above an assumed density, and show that realistic models of nuclear forces limit the maximum mass of neutron stars to be below 2.5${M}_{\ensuremath{\bigodot}}.$ The effects of the phase transitions on the composition of neutron star matter and its adiabatic index $\ensuremath{\Gamma}$ are discussed.

Methods for analyzing anisotropic flow in relativistic nuclear collisions

Abstract: The strategy and techniques for analyzing anisotropic flow (directed, elliptic, etc.) in relativistic nuclear collisions are presented. The emphasis is on the use of the Fourier expansion of azimuthal distributions. We present formulas relevant for this approach, and in particular, show how the event multiplicity enters into the event plane resolution. We also discuss the role of nonflow correlations and a method for introducing flow into a simulation.

New Skyrme interaction for normal and exotic nuclei

Abstract: A new set of Skyrme parameters is obtained from a fit to the binding energies, rms charge radii, and single-particle energies of both normal and exotic spherical nuclei. Nuclear matter and neutron matter properties are used to put constraints on the parameters which are not well determined from the nuclear data. Special problems with the Nolen-Schiffer anomaly and the spin-orbit interaction are discussed. @S0556-2813 ~98!02107-4#

Pseudospin symmetry in relativistic mean field theory

Abstract: Relating the pseudospin symmetry back to the Dirac equation through the framework of relativistic Hartree-Bogoliubov (RHB) theory, the pseudospin approximation in real nuclei is discussed. From the Dirac equation, the mechanism behind the pseudospin symmetry was studied and the pseudospin symmetry is shown to be connected with the competition between the centrifugal barrier (CB) and the pseudospin orbital potential (PSOP), which is mainly decided by the derivative of the difference between the scalar and vector potentials. With the scalar and vector potentials derived from a self-consistent relativistic Hartree-Bogoliubov calculation, the pseudospin symmetry and its energy dependence in real nuclei is discussed.

Massive gluons and quarks and the equation of state obtained from SU(3) lattice QCD

Abstract: We analyze recent results of SU(3) lattice QCD calculations with a phenomenological parametrization for the quark-gluon plasma equation of state based on a quasiparticle picture with massive quarks and gluons. At high temperature we obtain a good fit to the lattice data using perturbative thermal quark and gluon masses from an improved HTL scheme. At temperatures close to the confinement phase transition the fitted masses increase above the perturbative value, and a nonzero (but small) bag constant is required to fit the lattice data.

Radiative energy loss of high energy partons traversing an expanding QCD plasma

Abstract: We study analytically the medium-induced energy loss of a high energy parton passing through a finite size QCD plasma, which is expanding longitudinally according to Bjorken{close_quote}s model. We extend the Baier-Dokshitzer-Mueller-Peign{acute e}-Schiff formalism already applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and we show that the resulting radiative energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature T=T(L), which is reached after the quark propagates a distance L. {copyright} {ital 1998} {ital The American Physical Society}

Systematic study of high $p_{T}$ hadron spectra in $p p$, $p$ A and A A collisions from SPS to RHIC energies

Abstract: High-p{sub T} particle spectra in p+p (p(bar sign)+p), p+A, and A+B collisions are calculated in a parton model with QCD-inspired hard scattering and evolution in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic k{sub T} and its broadening in p+A and A+B collisions due to initial multiple parton scattering are found to be very important at low energies ({radical}(s) 1 GeV/c) in A+B collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This suggests that hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at p{sub T}{approx}2-10 GeV/c. However, there is no evidence of jet quenching or parton energy loss. Assuming this model of parton scattering, nuclear broadening and parton energy loss, one can exclude an effective parton energy loss dE{sub q}/dx>0.01 GeV/fm and a mean free path {lambda}{sub q}<7 fm from the experimental data of A+B collisions at the SPS energies.more » Predictions for high p{sub T} particle spectra in p+A and A+A collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed. (c) 2000 The American Physical Society.« less

Results on ν μ →ν e oscillations from pion decay in flight neutrinos

Abstract: A search for ${\ensuremath{ u}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{ u}}_{e}$ oscillations has been conducted at the Los Alamos Meson Physics Facility using ${\ensuremath{ u}}_{\ensuremath{\mu}}$ from ${\ensuremath{\pi}}^{+}$ decay in flight. An excess in the number of beam-related events from the ${\ensuremath{ u}}_{e}\stackrel{\ensuremath{\rightarrow}}{C}{e}^{\ensuremath{-}}X$ inclusive reaction is observed. The excess is too large to be explained by normal ${\ensuremath{ u}}_{e}$ contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of $(2.6\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ is consistent with the previously reported ${\overline{\ensuremath{ u}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{ u}}}_{e}$ oscillation evidence from LSND.

Effects of jet quenching on high p T hadron spectra in high-energy nuclear collisions

Abstract: Since large-p{sub T} particles in high-energy hadronic or nuclear collisions come from jet fragmentation, jet quenching due to parton energy loss in dense matter will cause the suppression of large-p{sub T} hadron spectra in high-energy heavy-ion collisions. Assuming an effective energy loss dE/dx for the high-E{sub T} partons, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed. Using such effective fragmentation functions, high-p{sub T} hadron spectra and particle suppression factors relative to pp collisions are estimated in central high-energy nuclear collisions with a given range of the assumed dE/dx. It is found that the suppression factors are very sensitive to the value of the effective energy loss. Systematic nuclear and flavor dependence of the hadron spectra are also studied. {copyright} {ital 1998} {ital The American Physical Society}

Pion nucleon scattering in a new approach to chiral perturbation theory

Abstract: We study pion-nucleon scattering with a chiral Lagrangian of pions, nucleons, and $\ensuremath{\Delta}$ isobars. The scattering amplitude is evaluated to one-loop ${Q}^{3}$ order, where $Q$ is a generic small momentum, using an approach which is equivalent to heavy baryon chiral perturbation theory. We obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to 100 MeV. A $\ensuremath{\sigma}$ term greater than 45 MeV is favored, but the value is not well determined.

Effects of correlations on neutrino opacities in nuclear matter

Abstract: Including nucleon-nucleon correlations due to both Fermi statistics and nuclear forces, we have developed a general formalism for calculating the neutral-current neutrino-nucleon scattering rates in nuclear matter. We derive corrections to the dynamic structure factors due to both density and spin correlations and find that neutrino-nucleon scattering rates are suppressed by large factors around and above nuclear density. Hence, in particular for the ${\ensuremath{ u}}_{\ensuremath{\mu}}$ and ${\ensuremath{ u}}_{\ensuremath{\tau}}$ neutrinos, but also for the ${\ensuremath{ u}}_{e}$ neutrinos, supernova cores are more ``transparent'' than previously thought. The many-body corrections increase with density, decrease with temperature, and are roughly independent of incident neutrino energy. In addition, we find that the spectrum of energy transfers in neutrino scattering is considerably broadened by the interactions in the medium. An identifiable component of this broadening comes from the absorption and emission of quanta of collective modes akin to the Gamow-Teller and giant dipole resonances in nuclei (zero sound; spin sound), with \ifmmode \check{C}\else \v{C}\fi{}erenkov kinematics. Under the assumption that both the charged-current and the neutral-current cross sections are decreased by many-body effects, we calculate a set of ad hoc protoneutron star cooling models to gauge the potential importance of the new opacities to the supernova itself. While the early luminosities are not altered, the luminosities after many hundreds of milliseconds to seconds can be increased by factors that range from 10 to 100 %. Such enhancements may have a bearing on the efficacy of the neutrino-driven supernova mechanism, the delay to explosion, the energy of the explosion, and the strength and relative role of convective overturn at late times. However, the actual consequences, if any, of these new neutrino opacities remain to be determined.

Particle production at high baryon density in central Au+Au reactions at 11.6A GeV/c

Abstract: Semi-inclusive proton and pion distributions from central Au+Au reactions at 11.6A GeV/c have been measured. The proton rapidity distribution shows significantly increased stopping compared to lighter systems, providing strong evidence for the formation of a state of matter with baryon density substantially greater than normal nuclear matter. Unlike reactions at this energy induced by lighter heavy ions, at low m{sub t}-m{sub 0} the proton invariant spectra deviate from a single exponential shape and become flatter, while {pi}{sup {minus}} spectra are found to rise faster than the {pi}{sup +} spectra. {copyright} {ital 1998} {ital The American Physical Society}

Chiral Lagrangian for strange hadronic matter

Abstract: A generalized Lagrangian for the description of hadronic matter based on the linear $\mathrm{SU}{(3)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(3)}_{R}$ $\ensuremath{\sigma}$ model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. We discuss the difficulties and possibilities to construct a chiral invariant baryon-meson interaction that leads to a realistic equation of state. It is found that a coupling of the strange condensate to nucleons is needed to describe the hyperon potentials correctly. The effective baryon masses and the appearance of an abnormal phase of nearly massless nucleons at high densities are examined. A nonlinear realization of chiral symmetry is considered, to retain a Yukawa-type baryon-meson interaction and to establish a connection to the Walecka model.

Equation of state, radial flow, and freeze-out in high energy heavy ion collisions

Abstract: We have shown that recent experimental data on radial flow, both from AGS and SPS energies, are in agreement with the equation of state (EOS) including the QCD phase transition. A new hydrokinetic model (HKM) is developed, which incorporates a hydrodynamical treatment of the expansion and proper kinetics of the freeze-out. We show that the freeze-out surfaces for different secondaries and different collisions are very different, and they are not at all isotherms $T$=const (as was assumed in most previous hydrodynamics works). Comparison of HKM results with the cascade-based event generator RQMD is also made in some detail: we found that both the EOS and flow are in rather good agreement, while the space-time picture is still somewhat different.

Onset of hyperon formation in neutron star matter from Brueckner theory

Abstract: We determine fully self-consistent single-particle potentials and chemical potentials of nucleons and hyperons in asymmetric nuclear matter, using an extended Brueckner-Hartree-Fock formalism. We carefully analyze the onset of ${\ensuremath{\Sigma}}^{\ensuremath{-}}$ and $\ensuremath{\Lambda}$ formation in $\ensuremath{\beta}$ stable and charge neutral matter. The role played by the three-body nucleon interaction is widely discussed. The results indicate that formation of hyperons sets in at about (2--3) times normal nuclear matter density, for all the different nucleonic equations of state that are considered.

Large-basis shell-model calculations for p-shell nuclei

Abstract: Results of large-basis shell-model calculations for nuclei with $A=7\ensuremath{-}11$ are presented. The effective interactions used in the study were derived microscopically from the Reid93 potential and take into account the Coulomb potential as well as the charge dependence of $T=1$ partial waves. For $A=7$, a $6\ensuremath{\Elzxh}\ensuremath{\Omega}$ model space was used, while for the rest of the studied nuclides, the calculations were performed in a $4\ensuremath{\Elzxh}\ensuremath{\Omega}$ model space. It is demonstrated that the shell model combined with microscopic effective interactions derived from modern nucleon-nucleon potentials is capable of providing good agreement with the experimental properties of the ground state as well as with those of the low-lying excited states.

Cascade hypernuclei in the (K-,K+) reaction on 12C

Abstract: Cascade hypernuclei have been studied in the ${(K}^{\ensuremath{-}}{,K}^{+})$ reaction on a scintillating fiber target. The experimental result is compared with a theoretical calculation in order to extract information concerning the ${\ensuremath{\Xi}}^{\ensuremath{-}}$ nucleus potential.

3 P 2 − 3 F 2 pairing in neutron matter with modern nucleon-nucleon potentials

Abstract: We present results for the ${}^{3}{P}_{2}{\ensuremath{-}}^{3}{F}_{2}$ pairing gap in neutron matter with several realistic nucleon-nucleon potentials, in particular with recent phase-shift-equivalent potentials. We find that their predictions for the gap cannot be trusted at densities above $\ensuremath{\rho}\ensuremath{\approx}1.7{\ensuremath{\rho}}_{0},$ where ${\ensuremath{\rho}}_{0}$ is the saturation density for symmetric nuclear matter. In order to make predictions above that density, potential models which fit the nucleon-nucleon phase shifts up to about 1 GeV are required.

Lowest order constrained variational calculation for asymmetrical nuclear matter with the new Argonne potential

Abstract: The lowest order constrained variational method is used to calculate the properties of asymmetrical nuclear matter with the new charge-independent breaking, Argonne ${V}_{18}$ ${(AV}_{18}),$ as well as Argonne ${V}_{14}$ ${(AV}_{14})$ potentials, for a wide range of density and proton to neutron ratio. The new ${\mathrm{AV}}_{18}$ potential, unlike ${\mathrm{AV}}_{14}$ and \ensuremath{\Delta}-Reid interactions, overbinds nuclear matter at a saturation density of about $0.31{\mathrm{fm}}^{\mathrm{\ensuremath{-}}3}$ which agrees well with the variational method based on hypernetted chain summation techniques. It is shown that it is not a good approximation to use the result of nuclear and neutron matter to get the equation of state of asymmetrical nuclear matter with an empirical parabolic approximation. Finally, various properties of asymmetrical nuclear matter such as incompressibility, symmetry energy, etc., are given and a comparison is made with the other many-body techniques.

Observation of kaonic hydrogen atom x rays

Abstract: We successfully observed clear K-series kaonic hydrogen X-rays for the first time taking advantage of two charged pions tagging technique and a gaseous hydrogen target. The strong-interaction energy shift and width of 1s state of K−p atom were determined to be ΔE1s = -323 ± 63 (statistical) ± 11 (systematic) eV and Г1s = 407 ± 208 ± 100 eV. Our result means that the K−p strong interaction near threshold is repulsive and the long-standing kaonic hydrogen puzzle is solved.

Model of charmonium absorption by light mesons

Abstract: We calculate the cross sections for dissociation of $J/\ensuremath{\psi}$ by pions and \ensuremath{\rho} mesons within the framework of a meson exchange model. We find that these cross sections are small at center-of-mass energies less than 1 GeV above threshold, and that dissociation rates are less than $0.01\mathrm{fm}/c$ in a thermal meson gas at temperatures where such a description makes sense.

Unitary model for meson-nucleon scattering

Abstract: We extract nucleon resonance parameters from an effective Lagrangian model employing the K-matrix approximation. To this end we analyze simultaneously all available data for reactions involving the final states {pi}N, {pi}{pi}N, {eta}N, and K{Lambda} in the energy range m{sub N}+m{sub {pi}}{le}{radical} (s) {le}1.9 GeV. The background contributions are generated consistently from the relevant Feynman amplitudes, which significantly reduces the number of free parameters. The sensitivity of the parameters upon the {pi}N{endash}partial-wave analysis and the details of the Lagrangians and form factors used are discussed. {copyright} {ital 1998} {ital The American Physical Society}

Hyperonic Nuclear Matter in Brueckner Theory

Abstract: We determine in an extended Brueckner-Hartree-Fock formalism self-consistent single-particle potentials of nucleons, lambda, and sigma hyperons for a system consisting of symmetric nuclear matter and lambda hyperons of uniform densities rN and rL , respectively. The binding energy per baryon of this system is discussed and its maximum strangeness content preserving binding is evaluated. The results are used to introduce a hyperonic symmetry energy term in a generalized mass formula for multistrange hypernuclei. @S0556-2813 ~98!01101-7#

Shell model study of the neutron rich isotopes from oxygen to silicon

Abstract: In this paper we extend the shell model calculations previously made for the very neutron-rich nuclei with $Zg~14$ to Al, Mg, Na, Ne, F, and O, using the same valence space and effective interaction. Predictions are made for the neutron separation energies and for the location of the neutron drip line. We find that the isotopes of Ne, Na, and Mg are deformed for $Ng~22.$ In ${}^{40}\mathrm{Mg},$ which is at the edge of the drip line, the $N=28$ shell closure does not stand. By enlarging the valence space to include intruder states we are able to account for the vanishing of the $N=20$ neutron shell closure in a small region around ${}^{31}\mathrm{Na}$ that we delineate. The dominance of the intruders explains the collective features experimentally found in this region.

Probing the halo structure of 1 9 , 1 7 , 1 5 C and 14 B

Abstract: We have measured the parallel momentum distributions of outcoming fragments in the one-neutron breakup of the odd-mass carbon isotopes ${}^{19,17,15}\mathrm{C}$ and ${}^{14}\mathrm{B}$. Owing to their low neutron separation energies, the study of these nuclei is of particular interest regarding the appearance of the halo phenomenon. Discrepancies between experiment and theory observed for ${}^{15}\mathrm{C}$ and ${}^{14}\mathrm{B}$ indicate that their halos are not as prominent as in ${}^{11}$Be or ${}^{11}$Li, and that core excitations start playing a role in the breakup mechanism. In ${}^{17}\mathrm{C}$ the halo appears hindered by a $d$-wave neutron ground state configuration. Finally, the data on ${}^{19}\mathrm{C}$ suggests an $s$-wave neutron around the 2${}^{+}$ excited state in ${}^{18}\mathrm{C}$.

Effective theory for neutron-deuteron scattering: Energy dependence

Abstract: We report on results of the effective theory method applied to neutron-deuteron scattering. We extend previous results in the $J=3/2$ channel to nonzero energies and find very good agreement with the experiment without any parameter fitting.

Octet and decuplet baryons in a covariant and confining diquark - quark model

Abstract: The baryon octet and decuplet masses and Bethe-Salpeter vertex and wave functions are calculated in the ladder approximation to the quark exchange between a scalar or axialvector diquark and a constituent quark. These functions reflecting full Lorentz covariance are given in terms of an expansion in Gegenbauer polynomials. In the rest frame of the baryon, a complete partial wave decomposition of the Bethe-Salpeter wave function is performed. The confinement of quarks and diquarks is implemented via a parametrisation of the corresponding propagators. We also discuss some aspects of the momentum routing in the ladder approximation to the Bethe-Salpeter equation. Numerical results for the octet and decuplet masses with broken flavour SU(3) in the conserved isospin limit are presented.

Neutrino capture and r process nucleosynthesis

Abstract: We explore neutrino capture during $r$-process nucleosynthesis in neutrino-driven ejecta from nascent neutron stars. We focus on the interplay between charged-current weak interactions and element synthesis, and we delineate the important role of equilibrium nuclear dynamics. During the period of coexistence of free nucleons and light and/or heavy nuclei, electron neutrino capture inhibits the $r$-process. At all stages, capture on free neutrons has a larger impact than capture on nuclei. However, neutrino capture on heavy nuclei by itself, if it is very strong, is also detrimental to the $r$-process until large nuclear equilibrium clusters break down and the classical neutron-capture phase of the $r$-process begins. The sensitivity of the $r$-process to neutrino irradiation means that neutrino-capture effects can strongly constrain the $r$-process site, neutrino physics, or both. These results apply also to $r$-process scenarios other than the neutrino-heated winds.

Dipole excitations to bound states in Sn-116 and Sn-124

Abstract: Dipole transitions to bound states in the Sn nuclei 116Sn and 124Sn have been investigated by means of nuclear resonance fluorescence (NRF) using 12 MeV linearly polarized bremsstrahlung and unpolarized bremsstrahlung with different end point energies (4.1, 7.5, and 10 MeV). The measurements enable the determination in a completely model-independent way of reduced transition probabilities, multipolarities, and parities of the observed transitions. More than 150 new dipole ground state transitions have been identified. The observed dipole strength distribution displays for both isotopes a clear concentration around 6.5 MeV. For about half of the observed dipole excitations parities could be extracted. They all turned out to be E1 excitations except for three tentative M1 assignments. The NRF results are compared with data from tagged photon scattering experiments and quasiparticle phonon model calculations.

Weak capture of protons by protons

Abstract: The cross section for the proton weak capture reaction {sup 1}H(p,e{sup +}{nu}{sub e}){sup 2}H is calculated with wave functions obtained from a number of modern, realistic high-precision interactions. To minimize the uncertainty in the axial two-body current operator, its matrix element has been adjusted to reproduce the measured Gamow-Teller matrix element of tritium {beta} decay in model calculations using trinucleon wave functions from these interactions. A thorough analysis of the ambiguities that this procedure introduces in evaluating the two-body current contribution to the pp capture is given. Its inherent model dependence is in fact found to be very weak. The overlap integral {Lambda}{sup 2}(E=0) for the pp capture is predicted to be in the range 7.05--7.06, including the axial two-body current contribution, for all interactions considered.