96 research outputs found
Flavor and Charge Symmetry in the Parton Distributions of the Nucleon
Recent calculations of charge symmetry violation(CSV) in the valence quark
distributions of the nucleon have revealed that the dominant symmetry breaking
contribution comes from the mass associated with the spectator quark
system.Assuming that the change in the spectator mass can be treated
perturbatively, we derive a model independent expression for the shift in the
parton distributions of the nucleon. This result is used to derive a relation
between the charge and flavor asymmetric contributions to the valence quark
distributions in the proton, and to calculate CSV contributions to the nucleon
sea. The CSV contribution to the Gottfried sum rule is also estimated, and
found to be small
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
suppression in Pb+Pb collisions and broadening
We have analysed the NA50 data, on the centrality dependence of
broadening of 's, in Pb+Pb collisions, at the CERN-SPS. The data were
analysed in a QCD based model, where 's are suppressed in 'nuclear'
medium. Without any free parameter, the model could explain the NA50
broadening data. The data were also analysed in a QGP based threshold model,
where suppression is 100% above a critical density. The QGP based
model could not explain the NA50 broadening data. We have also predicted
the centrality dependence of suppression and broadening at RHIC
energy. Both the models, the QGP based threshold model and the QCD based
nuclear absorption model, predict broadening very close to each other.Comment: The paper was completely revised. The conclusion is also changed. 5
pages, 4 figure
Valence Quark Distribution in A=3 Nuclei
We calculate the quark distribution function for 3He/3H in a relativistic
quark model of nuclear structure which adequately reproduces the nucleon
approximation, nuclear binding energies, and nuclear sizes for small nuclei.
The results show a clear distortion from the quark distribution function for
individual nucleons (EMC effect) arising dominantly from a combination of
recoil and quark tunneling effects. Antisymmetrization (Pauli) effects are
found to be small due to limited spatial overlaps. We compare our predictions
with a published parameterization of the nuclear valence quark distributions
and find significant agreement.Comment: 18pp., revtex4, 4 fig
Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces
The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4 metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5 metals than in 4
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5 series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23
May 199
Theoretical analysis of the electronic structure of the stable and metastable c(2x2) phases of Na on Al(001): Comparison with angle-resolved ultra-violet photoemission spectra
Using Kohn-Sham wave functions and their energy levels obtained by
density-functional-theory total-energy calculations, the electronic structure
of the two c(2x2) phases of Na on Al(001) are analysed; namely, the metastable
hollow-site structure formed when adsorption takes place at low temperature,
and the stable substitutional structure appearing when the substrate is heated
thereafter above ca. 180K or when adsorption takes place at room temperature
from the beginning. The experimentally obtained two-dimensional band structures
of the surface states or resonances are well reproduced by the calculations.
With the help of charge density maps it is found that in both phases, two
pronounced bands appear as the result of a characteristic coupling between the
valence-state band of a free c(2x2)-Na monolayer and the
surface-state/resonance band of the Al surfaces; that is, the clean (001)
surface for the metastable phase and the unstable, reconstructed "vacancy"
structure for the stable phase. The higher-lying band, being Na-derived,
remains metallic for the unstable phase, whereas it lies completely above the
Fermi level for the stable phase, leading to the formation of a
surface-state/resonance band-structure resembling the bulk band-structure of an
ionic crystal.Comment: 11 pages, 11 postscript figures, published in Phys. Rev. B 57, 15251
(1998). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Prompt photons at RHIC
We calculate the inclusive cross section for prompt photon production in
heavy-ion collisions at RHIC energies ( GeV and
GeV) in the central rapidity region including next-to-leading order,
, radiative corrections, initial state nuclear
shadowing and parton energy loss effects. We show that there is a significant
suppression of the nuclear cross section, up to at
GeV, due to shadowing and medium induced parton energy loss effects. We find
that the next-to-leading order contributions are large and have a strong
dependence.Comment: 9 pages, 5 figures, expanded discussion of the K facto
Valence Quark Spin Distribution Functions
The hyperfine interactions of the constituent quark model provide a natural
explanation for many nucleon properties, including the Delta-N splitting, the
charge radius of the neutron, and the observation that the proton's quark
distribution function ratio d(x)/u(x)->0 as x->1. The hyperfine-perturbed quark
model also makes predictions for the nucleon spin-dependent distribution
functions. Precision measurements of the resulting asymmetries A_1^p(x) and
A_1^n(x) in the valence region can test this model and thereby the hypothesis
that the valence quark spin distributions are "normal".Comment: 16 pages, 2 Postscript figure
An embedding scheme for the Dirac equation
An embedding scheme is developed for the Dirac Hamiltonian H. Dividing space
into regions I and II separated by surface S, an expression is derived for the
expectation value of H which makes explicit reference to a trial function
defined in I alone, with all details of region II replaced by an effective
potential acting on S and which is related to the Green function of region II.
Stationary solutions provide approximations to the eigenstates of H within I.
The Green function for the embedded Hamiltonian is equal to the Green function
for the entire system in region I. Application of the method is illustrated for
the problem of a hydrogen atom in a spherical cavity and an Au(001)/Ag/Au(001)
sandwich structure using basis sets that satisfy kinetic balance.Comment: 16 pages, 5 figure
Mutual heavy ion dissociation in peripheral collisions at ultrarelativistic energies
We study mutual dissociation of heavy nuclei in peripheral collisions at
ultrarelativistic energies. Earlier this process was proposed for beam
luminosity monitoring via simultaneous registration of forward and backward
neutrons in zero degree calorimeters at Relativistic Heavy Ion Collider.
Electromagnetic dissociation of heavy ions is considered in the framework of
the Weizsacker-Williams method and simulated by the RELDIS code. Photoneutron
cross sections measured in different experiments and calculated by the GNASH
code are used as input for the calculations of dissociation cross sections. The
difference in results obtained with different inputs provides a realistic
estimation for the systematic uncertainty of the luminosity monitoring method.
Contribution to simultaneous neutron emission due to grazing nuclear
interactions is calculated within the abrasion model. Good description of CERN
SPS experimental data on Au and Pb dissociation gives confidence in predictive
power of the model for AuAu and PbPb collisions at RHIC and LHC.Comment: 46 pages with 7 tables and 13 figures, numerical integration accuracy
improved, next-to-leading-order corrections include
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