17,737 research outputs found
Color transparency and short-range correlations in exclusive pion photo- and electroproduction from nuclei
A relativistic and quantum mechanical framework to compute nuclear
transparencies for pion photo- and electroproduction reactions is presented.
Final-state interactions for the ejected pions and nucleons are implemented in
a relativistic eikonal approach. At sufficiently large ejectile energies, a
relativistic Glauber model can be adopted. At lower energies, the framework
possesses the flexibility to use relativistic optical potentials. The proposed
model can account for the color-transparency (CT) phenomenon and short-range
correlations (SRC) in the nucleus. Results are presented for kinematics
corresponding to completed and planned experiments at Jefferson Lab. The
influence of CT and SRC on the nuclear transparency is studied. Both the SRC
and CT mechanisms increase the nuclear transparency. The two mechanisms can be
clearly separated, though, as they exhibit a completely different dependence on
the hard scale parameter. The nucleon and pion transparencies as computed in
the relativistic Glauber approach are compared with optical-potential and
semi-classical calculations. The similarities in the trends and magnitudes of
the nuclear transparencies indicate that they are not subject to strong model
dependencies.Comment: 33 pages, 14 figure
On the Critical Behaviour of Heat Conducting Sphere out of Hydrostatic Equilibrium
We comment further on the behaviour of a heat conducting fluid when a
characteristic parameter of the system approaches a critical value.Comment: 4 pages, emTex (LaTex 2.09), submitted to Classical and Quantum
Gravity (Comments and Addenda
Superscaling Predictions for Neutral Current Quasielastic Neutrino-Nucleus Scattering
The application of superscaling ideas to predict neutral-current (NC)
quasielastic (QE) neutrino cross sections is investigated. Results obtained
within the relativistic impulse approximation (RIA) using the same relativistic
mean field potential (RMF) for both initial and final nucleons -- a model that
reproduces the experimental (e,e') scaling function -- are used to illustrate
the ideas involved. While NC reactions are not so well suited for scaling
analyses, to a large extent the RIA-RMF predictions do exhibit superscaling.
Independence of the scaled response on the nuclear species is very well
fulfilled. The RIA-RMF NC superscaling function is in good agreement with the
experimental (e,e') one. The idea that electroweak processes can be described
with a universal scaling function, provided that mild restrictions on the
kinematics are assumed, is shown to be valid.Comment: 4 pages, 4 figures, published in PR
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
Thermal Conduction in Systems out of Hydrostatic Equilibrium
We analyse the effects of thermal conduction in a relativistic fluid, just
after its departure from hydrostatic equilibrium, on a time scale of the order
of thermal relaxation time. It is obtained that the resulting evolution will
critically depend on a parameter defined in terms of thermodynamic variables,
which is constrained by causality requirements.Comment: 16 pages, emTex (LaTex 2.09). To appear in Classical and Quantum
Gravit
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