3,078 research outputs found
Short-range nuclear effects on axion emissivities by nucleon-nucleon bremsstrahlung
The rates of axion emission by nucleon-nucleon (NN) bremsstrahlung are
reconsidered by taking into account the NN short range correlations. The
analytical formulas for the neutron-neutron (nn), proton-proton (pp) and
neutron-proton (np) processes with the inclusion of the full momentum
dependence of an one- and two- pion exchange nuclear potentials, in the
non-degenerate limit, are explicitly given. We find that the two-pion exchange
(short range) effects can give a significant contribution to the emission
rates, and are temperature dependent. Other short range nuclear effects like
effective nucleon mass, polarization effects and use of correlated wave
functions, are discused as well. The trend of all these nuclear effects is to
diminish the corresponding axion emission rates. Further, we estimate that the
values of the emission rates calculated with the inclusion of all these effects
can differ from the corresponding ones derived with constant nuclear matrix
elements by a factor of . This leads to an uncertainty factor of when extracting bounds of the axion parametersComment: 11 pages, 4 figure
No-Core shell model for A = 47 and A = 49
We apply an {\it ab-initio} approach to the nuclear structure of odd-mass
nuclei straddling . Starting with the NN interaction, that fits
two-body scattering and bound state data we evaluate the nuclear properties of
and nuclei in a no-core approach. Due to model space
limitations and the absence of 3-body interactions, we incorporate
phenomenological terms determined by fits to nuclei in a previous
effort. Our modified Hamiltonian produces reasonable spectra for these odd mass
nuclei. In addition to the differences in single-particle basis states, the
absence of a single-particle Hamiltonian in our no-core approach obscures
direct comparisons with valence effective NN interactions. Nevertheless, we
compare the fp-shell matrix elements of our initial and modified Hamiltonians
in the harmonic oscillator basis with a recent model fp-shell interaction, the
GXPF1 interaction of Honma, Otsuka, Brown and Mizusaki. Notable differences
emerge from these comparisons. In particular, our diagonal two-body
matrix elements are, on average, about 800-900keV more attractive. Furthermore,
while our initial and modified NN Hamiltonian fp-shell matrix elements are
strongly correlated, there is much less correlation with the GXPF1 matrix
elements.Comment: 17 pages including 14 figure
Avatar: A Time- and Space-Efficient Self-Stabilizing Overlay Network
Overlay networks present an interesting challenge for fault-tolerant
computing. Many overlay networks operate in dynamic environments (e.g. the
Internet), where faults are frequent and widespread, and the number of
processes in a system may be quite large. Recently, self-stabilizing overlay
networks have been presented as a method for managing this complexity.
\emph{Self-stabilizing overlay networks} promise that, starting from any
weakly-connected configuration, a correct overlay network will eventually be
built. To date, this guarantee has come at a cost: nodes may either have high
degree during the algorithm's execution, or the algorithm may take a long time
to reach a legal configuration. In this paper, we present the first
self-stabilizing overlay network algorithm that does not incur this penalty.
Specifically, we (i) present a new locally-checkable overlay network based upon
a binary search tree, and (ii) provide a randomized algorithm for
self-stabilization that terminates in an expected polylogarithmic number of
rounds \emph{and} increases a node's degree by only a polylogarithmic factor in
expectation
The Friedmann-Lemaitre-Robertson-Walker Big Bang singularities are well behaved
We show that the Big Bang singularity of the
Friedmann-Lemaitre-Robertson-Walker model does not raise major problems to
General Relativity. We prove a theorem showing that the Einstein equation can
be written in a non-singular form, which allows the extension of the spacetime
before the Big Bang. The physical interpretation of the fields used is
discussed. These results follow from our research on singular semi-Riemannian
geometry and singular General Relativity.Comment: 10 pages, 5 figure
- …