4,552 research outputs found
The SERENDIP 2 SETI project: Current status
Over the past 30 years, interest in extraterrestrial intelligence has progressed from philosophical discussion to rigorous scientific endeavors attempting to make contact. Since it is impossible to assess the probability of success and the amount of telescope time needed for detection, Search for Extraterrestrial Intelligence (SETI) Projects are plagued with the problem of attaining the large amounts of time needed on the world's precious few large radio telescopes. To circumvent this problem, the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP) instrument operates autonomously in a piggyback mode utilizing whatever observing plan is chosen by the primary observer. In this way, large quantities of high-quality data can be collected in a cost-effective and unobtrusive manner. During normal operations, SERENDIP logs statistically significant events for further offline analysis. Due to the large number of terrestrial and near-space transmitters on earth, a major element of the SERENDIP project involves identifying and rejecting spurious signals from these sources. Another major element of the SERENDIP Project (as well as most other SETI efforts) is detecting extraterrestrial intelligence (ETI) signals. Events selected as candidate ETI signals are studied further in a targeted search program which utilizes between 24 to 48 hours of dedicated telescope time each year
Final-State Interactions in the Superscaling Analysis of Neutral-Current Quasielastic Neutrino Scattering
Effects of strong final-state interactions in the superscaling properties of
neutral-current quasielastic neutrino cross sections are investigated using the
Relativistic Impulse Approximation as guidance. First- and second-kind scaling
are analyzed for neutrino beam energies ranging from 1 to 2 GeV for the cases
of 12C, 16O and 40Ca. Different detection angles of the outgoing nucleon are
considered in order to sample various nucleon energy regimes. Scaling of the
second kind is shown to be very robust. Validity of first-kind scaling is found
to be linked to the kinematics of the process. Superscaling still prevails even
in the presence of very strong final-state interactions, provided that some
kinematical restrains are kept, and the conditions under which superscaling can
be applied to predict neutral-current quasielastic neutrino scattering are
determined.Comment: 39 pages, 16 figures, accepted for publication in Phys. Rev.
Pionic correlations and meson-exchange currents in two-particle emission induced by electron scattering
Two-particle two-hole contributions to electromagnetic response functions are
computed in a fully relativistic Fermi gas model. All one-pion exchange
diagrams that contribute to the scattering amplitude in perturbation theory are
considered, including terms for pionic correlations and meson-exchange currents
(MEC). The pionic correlation terms diverge in an infinite system and thus are
regularized by modification of the nucleon propagator in the medium to take
into account the finite size of the nucleus. The pionic correlation
contributions are found to be of the same order of magnitude as the MEC.Comment: 14 pages, 15 figure
Inelastic electron-nucleus scattering and scaling at high inelasticity
Highly inelastic electron scattering is analyzed within the context of the
unified relativistic approach previously considered in the case of quasielastic
kinematics. Inelastic relativistic Fermi gas modeling that includes the
complete inelastic spectrum - resonant, non-resonant and Deep Inelastic
Scattering - is elaborated and compared with experimental data. A
phenomenological extension of the model based on direct fits to data is also
introduced. Within both models, cross sections and response functions are
evaluated and binding energy effects are analyzed. Finally, an investigation of
the second-kind scaling behavior is also presented.Comment: 39 pages, 13 figures; formalism extended and slightly reorganized,
conclusions extended; to appear in Phys. Rev.
Superscaling of non-quasielastic electron-nucleus scattering
The present study is focused on the superscaling behavior of electron-nucleus
cross sections in the region lying above the quasielastic peak, especially the
region dominated by electroexcitation of the Delta. Non-quasielastic cross
sections are obtained from all available high-quality data for Carbon 12 by
subtracting effective quasielastic cross sections based on the superscaling
hypothesis. These residuals are then compared with results obtained within a
scaling-based extension of the relativistic Fermi gas model, including an
investigation of violations of scaling of the first kind in the region above
the quasielastic peak. A way potentially to isolate effects related to
meson-exchange currents by subtracting both impulsive quasielastic and
impulsive inelastic contributions from the experimental cross sections is also
presented.Comment: RevTeX, 34 pages including 11 figure
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
Hysteresis effects in rotating Bose-Einstein condensates
We study the formation of vortices in a dilute Bose-Einstein condensate
confined in a rotating anisotropic trap. We find that the number of vortices
and angular momentum attained by the condensate depends upon the rotation
history of the trap and on the number of vortices present in the condensate
initially. A simplified model based on hydrodynamic equations is developed, and
used to explain this effect in terms of a shift in the resonance frequency of
the quadrupole mode of the condensate in the presence of a vortex lattice.
Differences between the spin-up and spin-down response of the condensate are
found, demonstrating hysteresis phenomena in this system.Comment: 16 pages, 7 figures; revised after referees' report
Vortices in attractive Bose-Einstein condensates in two dimensions
The form and stability of quantum vortices in Bose-Einstein condensates with
attractive atomic interactions is elucidated. They appear as ring bright
solitons, and are a generalization of the Townes soliton to nonzero winding
number . An infinite sequence of radially excited stationary states appear
for each value of , which are characterized by concentric matter-wave rings
separated by nodes, in contrast to repulsive condensates, where no such set of
states exists. It is shown that robustly stable as well as unstable regimes may
be achieved in confined geometries, thereby suggesting that vortices and their
radial excited states can be observed in experiments on attractive condensates
in two dimensions.Comment: 4 pages, 3 figure
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