274 research outputs found
Quasi-SU(3) truncation scheme for even-even sd-shell nuclei
The Quasi-SU(3) symmetry was uncovered in full pf and sdg shell-model
calculations for both even-even and odd-even nuclei. It manifests itself
through a dominance of single-particle and quadrupole-quadrupole terms in the
Hamiltonian used to describe well-deformed nuclei. A practical consequence of
the quasi-SU(3) symmetry is an efficient basis truncation scheme. In a recent
work was shown that when this type of Hamiltonian is diagonalized in an SU(3)
basis, only a few irreducible represntations (irreps) of SU(3) are needed to
describe the Yrast band, the leading S = 0 irrep augmented with the leading S =
1 irreps in the proton and neutron subspaces. In the present article the
quasi-SU(3) truncation scheme is used, in conjunction with a "realistic but
schematic" Hamiltonian that includes the most important multipole terms, to
describe the energy spectra and B(E2) transition strengths of 20-Ne, 22-Ne,
24-Mg and 28-Si. The effect of the size of the Hilbert space on both sets of
observables is discussed, as well as the structure of the Yrast band and the
importance of the various terms in the Hamiltonian.Comment: 30 pages, 8 figures. Submited to Nucl. Phys.
Pion-nucleus optical potential valid up to the DELTA-resonance region
We present in this article an optical potential for the -nucleus
interaction that can be used in various studies involving -nucleus
channels. Based on earlier treatments of the low energy -nucleus optical
potential, we have derived a potential expression applicable from threshold up
to the -resonance region. We extracted the impulse approximation form
for this potential from the scattering amplitude and then added to it
kinematical and physical corrections. The kinematic corrections arise from
transforming the impulse approximation expression from the center of
mass frame to the -nucleus center of mass frame, while the physical
corrections arise mostly from the many-body nature of the -nucleus
interaction. By taking advantage of the experimental progress in our knowledge
of the process, we have updated earlier treatments with parameters
calculated from state-of-the-art experimental measurements.Comment: 23 pages, 12 figures. Accepted for publication in Physical Review
Coherent Photoproduction of Eta-Mesons on Spin-Zero Nuclei in a Relativistic, Non-local Model
The coherent photoproduction of -mesons on spin-zero nuclei is studied
in a relativistic, non-local model, which we have previously applied to the
coherent photoproduction of pions. We find that different off-shell
extrapolations of the elementary production operator lead to large effects in
the cross section. We also show that the almost complete suppression of the
N(1535) seen in earlier studies on this reaction is a result of the local or
factorization approximation used in these works. Non-local effects can lead to
a considerable contribution from this resonance. The relative size of the
N(1535) contribution depends on the structure of the nucleus under
consideration. We give an estimate for the contribution of the N(1520)
resonance and discuss the effect of an -nucleus optical potentialComment: 29 pages, 14 figures; slight changes in presentation, extended
discussion, one new figur
Determination of the Gamow-Teller Quenching Factor from Charge Exchange Reactions on 90Zr
Double differential cross sections between 0-12 degrees were measured for the
90Zr(n,p) reaction at 293 MeV over a wide excitation energy range of 0-70 MeV.
A multipole decomposition technique was applied to the present data as well as
the previously obtained 90Zr(p,n) data to extract the Gamow-Teller (GT)
component from the continuum. The GT quenching factor Q was derived by using
the obtained total GT strengths. The result is Q=0.88+/-0.06 not including an
overall normalization uncertainty in the GT unit cross section of 16%.Comment: 11 papes, 4 figures, submitted to Physics Letters B (accepted),
gzipped tar file, changed content
Measurements of polarized photo-pion production on longitudinally polarized HD and Implications for Convergence of the GDH Integral
We report new measurements of inclusive pion production from frozen-spin HD
for polarized photon beams covering the Delta(1232) resonance. These provide
data simultaneously on both H and D with nearly complete angular distributions
of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH)
sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the
proton by 22 microbarns, suggesting as yet unmeasured high-energy components.
Our pi0 data reveal a different angular dependence than assumed in Mainz
analyses and integrate to a value that is 18 microbarns lower, suggesting a
more rapid convergence. Our results for deuterium are somewhat lower than
published data, considerably more precise and generally lower than available
calculations.Comment: 4 pages, 4 figures. Submitted for publication in Physical Review
Letter
Correlation effects in single-particle overlap functions and one-nucleon removal reactions
Single-particle overlap functions and spectroscopic factors are calculated on
the basis of the one-body density matrices (ODM) obtained for the nucleus
employing different approaches to account for the effects of
correlations. The calculations use the relationship between the overlap
functions related to bound states of the (A-1)-particle system and the ODM for
the ground state of the A-particle system. The resulting bound-state overlap
functions are compared and tested in the description of the experimental data
from (p,d) reactions for which the shape of the overlap function is important.Comment: 11 pages, 4 figures include
Observation of exclusive DVCS in polarized electron beam asymmetry measurements
We report the first results of the beam spin asymmetry measured in the
reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry
with a sin(phi) modulation is observed, as predicted for the interference term
of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The
amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and
leading-twist pQCD, the alpha is directly proportional to the imaginary part of
the DVCS amplitude.Comment: 6 pages, 5 figure
Photoproduction of phi(1020) mesons on the proton at large momentum transfer
The cross section for meson photoproduction on the proton has been
measured for the first time up to a four-momentum transfer -t = 4 GeV^2, using
the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low
four-momentum transfer, the differential cross section is well described by
Pomeron exchange. At large four-momentum transfer, above -t = 1.8 GeV^2, the
data support a model where the Pomeron is resolved into its simplest component,
two gluons, which may couple to any quark in the proton and in the .Comment: 5 pages; 7 figure
Observation of an Exotic Baryon in Exclusive Photoproduction from the Deuteron
In an exclusive measurement of the reaction , a
narrow peak that can be attributed to an exotic baryon with strangeness
is seen in the invariant mass spectrum. The peak is at
GeV/c with a measured width of 0.021 GeV/c FWHM, which is largely
determined by experimental mass resolution. The statistical significance of the
peak is . The mass and width of the observed peak are
consistent with recent reports of a narrow baryon by other experimental
groups.Comment: 5 pages, 5 figure
Measurement of Beam-Spin Asymmetries for Deep Inelastic Electroproduction
We report the first evidence for a non-zero beam-spin azimuthal asymmetry in
the electroproduction of positive pions in the deep-inelastic region. Data have
been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector
at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of
the modulation increases with the momentum of the pion relative to
the virtual photon, , with an average amplitude of for range.Comment: 5 pages, RevTEX4, 3 figures, 2 table
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