269 research outputs found
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
Measurement of polarization-transfer to bound protons in carbon and its virtuality dependence
We measured the ratio of the transverse to longitudinal
components of polarization transferred from electrons to bound protons in
by the process at the
Mainz Microtron (MAMI). We observed consistent deviations from unity of this
ratio normalized to the free-proton ratio,
, for both -
and -shell knocked out protons, even though they are embedded in averaged
local densities that differ by about a factor of two. The dependence of the
double ratio on proton virtuality is similar to the one for knocked out protons
from and , suggesting a universal behavior.
It further implies no dependence on average local nuclear density
Anisotropies in the Neutrino Fluxes and Heating Profiles in Two-dimensional, Time-dependent, Multi-group Radiation Hydrodynamics Simulations of Rotating Core-Collapse Supernovae
Using the 2D multi-group, flux-limited diffusion version of the code
VULCAN/2D, that also incorporates rotation, we have calculated the collapse,
bounce, shock formation, and early post-bounce evolutionary phases of a
core-collapse supernova for a variety of initial rotation rates. This is the
first series of such multi-group calculations undertaken in supernova theory
with fully multi-D tools. We find that though rotation generates
pole-to-equator angular anisotropies in the neutrino radiation fields, the
magnitude of the asymmetries is not as large as previously estimated. Moreover,
we find that the radiation field is always more spherically symmetric than the
matter distribution, with its plumes and convective eddies. We present the
dependence of the angular anisotropy of the neutrino fields on neutrino
species, neutrino energy, and initial rotation rate. Only for our most rapidly
rotating model do we start to see qualitatively different hydrodynamics, but
for the lower rates consistent with the pre-collapse rotational profiles
derived in the literature the anisotropies, though interesting, are modest.
This does not mean that rotation does not play a key role in supernova
dynamics. The decrease in the effective gravity due to the centripetal effect
can be quite important. Rather, it means that when a realistic mapping between
initial and final rotational profiles and 2D multi-group
radiation-hydrodynamics are incorporated into collapse simulations the
anisotropy of the radiation fields may be only a secondary, not a pivotal
factor, in the supernova mechanism.Comment: Includes 11 low-resolution color figures, accepted to the
Astrophysical Journal (June 10, 2005; V. 626); high-resolution figures and
movies available from the authors upon reques
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
A New Algorithm for Supernova Neutrino Transport and Some Applications
We have developed an implicit, multi-group, time-dependent, spherical
neutrino transport code based on the Feautrier variables, the tangent-ray
method, and accelerated iteration. The code achieves high
angular resolution, is good to O(), is equivalent to a Boltzmann solver
(without gravitational redshifts), and solves the transport equation at all
optical depths with precision. In this paper, we present our formulation of the
relevant numerics and microphysics and explore protoneutron star atmospheres
for snapshot post-bounce models. Our major focus is on spectra, neutrino-matter
heating rates, Eddington factors, angular distributions, and phase-space
occupancies. In addition, we investigate the influence on neutrino spectra and
heating of final-state electron blocking, stimulated absorption, velocity terms
in the transport equation, neutrino-nucleon scattering asymmetry, and weak
magnetism and recoil effects. Furthermore, we compare the emergent spectra and
heating rates obtained using full transport with those obtained using
representative flux-limited transport formulations to gauge their accuracy and
viability. Finally, we derive useful formulae for the neutrino source strength
due to nucleon-nucleon bremsstrahlung and determine bremsstrahlung's influence
on the emergent and neutrino spectra.Comment: 58 pages, single-spaced LaTeX, 23 figures, revised title, also
available at http://jupiter.as.arizona.edu/~burrows/papers, accepted for
publication in the Ap.
Proton G_E/G_M from beam-target asymmetry
The ratio of the proton's electric to magnetic form factor, G_E/G_M, can be
extracted in elastic electron-proton scattering by measuring either cross
sections, beam-target asymmetry or recoil polarization. Separate determinations
of G_E/G_M by cross sections and recoil polarization observables disagree for
Q^2 > 1 (GeV/c)^2. Measurement by a third technique might uncover an unknown
systematic error in either of the previous measurements. The beam-target
asymmetry has been measured for elastic electron-proton scattering at Q^2 =
1.51 (GeV/c)^2 for target spin orientation aligned perpendicular to the beam
momentum direction. This is the largest Q^2 at which G_E/G_M has been
determined by a beam-target asymmetry experiment. The result, \muG_E/G_M =
0.884 +/- 0.027 +/- 0.029, is compared to previous world data.Comment: 8 pages, 6 figures, Updated to be version published in Physical
Review
Probing Quark-Gluon Interactions with Transverse Polarized Scattering
We have extracted QCD matrix elements from our data on double polarized
inelastic scattering of electrons on nuclei. We find the higher twist matrix
element \tilde{d_2}, which arises strictly from quark- gluon interactions, to
be unambiguously non zero. The data also reveal an isospin dependence of higher
twist effects if we assume that the Burkhardt-Cottingham Sum rule is valid. The
fundamental Bjorken sum rule obtained from the a0 matrix element is satisfied
at our low momentum transfer.Comment: formerly "Nachtmann Moments of the Proton and Deuteron Spin Structure
Functions
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