341 research outputs found
On Reconciling Gottfried Sum Rule Violation with Cabibbo Theory
We discuss the seemingly contradictory constraints of simultaneously
preserving the SU(3)-symmetric Cabibbo description of the weak vector baryon
matrix elements, accounting for SU(3) flavor symmetry breaking and describing
the observed violation of the Gottfried Sum Rule. We try to construct a simple
model that will satisfy these constraints and use it to explain the generic
difficulties and tradeoffs.Comment: 10 pages, late
Ground state correlations and mean-field in O
We use the coupled cluster expansion ( method) to generate the
complete ground state correlations due to the NN interaction. Part of this
procedure is the calculation of the two-body G matrix inside the nucleus in
which it is being used. This formalism is being applied to in a
configuration space of 50 . The resulting ground state wave
function is used to calculate the binding energy and one- and two-body
densities for the ground state of .Comment: 9 pages, 9 figures, LaTe
Short-range correlations in low-lying nuclear excited states
The electromagnetic transitions to various low-lying excited states of 16O,
48Ca and 208Pb are calculated within a model which considers the short-range
correlations. In general the effects of the correlations are small and do not
explain the required quenching to describe the data.Comment: 6 pages, 2 postscript figures, 1 tabl
The Magnetorotational Instability in Core Collapse Supernova Explosions
We investigate the action of the magnetorotational instability (MRI) in the
context of iron-core collapse. Exponential growth of the field on the rotation
time scale by the MRI will dominate the linear growth process of field line
"wrapping" with the same characteristic time. We examine a variety of initial
rotation states, with solid body rotation or a gradient in rotational velocity,
that correspond to models in the literature. A relatively modest value of the
initial rotation, a period of ~ 10 s, will give a very rapidly rotating PNS and
hence strong differential rotation with respect to the infalling matter. We
assume conservation of angular momentum on spherical shells. Results are
discussed for two examples of saturation fields, a fiducial field that
corresponds to Alfven velocity = rotational velocity and a field that
corresponds to the maximum growing mode of the MRI. Modest initial rotation
velocities of the iron core result in sub-Keplerian rotation and a
sub-equipartition magnetic field that nevertheless produce substantial MHD
luminosity and hoop stresses: saturation fields of order 10^{15} - 10^{16} G
develop within 300 msec after bounce with an associated MHD luminosity of about
10^{52} erg/s. Bi-polar flows driven by this MHD power can affect or even cause
the explosions associated with core-collapse supernovae.Comment: 42 pages, including 15 figures. Accepted for publication in ApJ. We
have revised to include an improved treatment of the convection, and some
figures have been update
Spin Contents of Nucleons with SU(3) Breaking
We apply a model for SU(3) breaking to the analysis of the spin contents of
the nucleon from the the latest data on first moments of the spin dependent
structure functions and include higher order QCD corrections. The results show
that the value of the total quark spin contribution to the nucleon spin remains about 0.3 and is very
insensitive to SU(3) breaking, while the result for the strange quark
contribution varies considerably with SU(3) breaking.Comment: Six pages of LaTeX plus three figures (tarred, compressed, and
uuencoded
The Quark/Antiquark Asymmetry of the Nucleon Sea
Although the distributions of sea quarks and antiquarks generated by
leading-twist QCD evolution through gluon splitting
are necessarily CP symmetric, the distributions of nonvalence quarks and
antiquarks which are intrinsic to the nucleon's bound state wavefunction need
not be identical. In this paper we investigate the sea quark/antiquark
asymmetries in the nucleon wavefunction which are generated by a light-cone
model of energetically-favored meson-baryon fluctuations. The model predicts
striking quark/antiquark asymmetries in the momentum and helicity distributions
for the down and strange contributions to the proton structure function: the
intrinsic and quarks in the proton sea are predicted to be negatively
polarized, whereas the intrinsic and antiquarks give zero
contributions to the proton spin. Such a picture is supported by experimental
phenomena related to the proton spin problem and the violation of the
Ellis-Jaffe sum rule. The light-cone meson-baryon fluctuation model also
suggests a structured momentum distribution asymmetry for strange quarks and
antiquarks which could be relevant to an outstanding conflict between two
different determinations of the strange quark sea in the nucleon. The model
predicts an excess of intrinsic pairs over pairs, as
supported by the Gottfried sum rule violation. We also predict that the
intrinsic charm and anticharm helicity and momentum distributions are not
identical.Comment: LaTex 18 pages, 4 figures. To obtain a copy, send e-mail to
[email protected]
Mass and width of the resonance in nuclei
We calculated the mass and width of the resonance inside nuclei within a
nucleon- model by including the self-energy of the in the
propagator. We found that in the nuclear medium the width of the
is increased by one order of magnitude while its mass changes only by a few
MeV. This broadening of the width of the resonance embedded in nuclei is
consistent with the experimental observations so that the can be
understood as a resonance. Thus, given the freedom between either
isospin 0 or isospin 2 for the , our results give weigth to the isospin-2
assignment.Comment: 14 pages, RevteX type, 2 eps figures. To be published in Phys. Rev. C
(September
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