724 research outputs found
Gluing construction of initial data with Kerr-de Sitter ends
We construct initial data sets which satisfy the vacuum constraint equa-
tions of General Relativity with positive cosmologigal constant. More pre-
silely, we deform initial data with ends asymptotic to Schwarzschild-de Sitter
to obtain non-trivial initial data with exactly Kerr-de Sitter ends. The method
is inspired from Corvino's gluing method. We obtain here a extension of a
previous result for the time-symmetric case by Chru\'sciel and Pollack.Comment: 27 pages, 3 figure
Metrics with Prescribed Ricci Curvature near the Boundary of a Manifold
Suppose is a manifold with boundary. Choose a point . We
investigate the prescribed Ricci curvature equation \Ric(G)=T in a
neighborhood of under natural boundary conditions. The unknown here is
a Riemannian metric. The letter in the right-hand side denotes a
(0,2)-tensor. Our main theorems address the questions of the existence and the
uniqueness of solutions. We explain, among other things, how these theorems may
be used to study rotationally symmetric metrics near the boundary of a solid
torus . The paper concludes with a brief discussion of the Einstein
equation on .Comment: 13 page
The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses
In spectroscopy, it is conventional to treat pulses much stronger than the
linewidth as delta-functions. In NMR, this assumption leads to the prediction
that pi pulses do not refocus the dipolar coupling. However, NMR spin echo
measurements in dipolar solids defy these conventional expectations when more
than one pi pulse is used. Observed effects include a long tail in the CPMG
echo train for short delays between pi pulses, an even-odd asymmetry in the
echo amplitudes for long delays, an unusual fingerprint pattern for
intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments
that set limits on possible extrinsic causes for the phenomena are reported. We
find that the action of the system's internal Hamiltonian during any real pulse
is sufficient to cause the effects. Exact numerical calculations, combined with
average Hamiltonian theory, identify novel terms that are sensitive to
parameters such as pulse phase, dipolar coupling, and system size.
Visualization of the entire density matrix shows a unique flow of quantum
coherence from non-observable to observable channels when applying repeated pi
pulses.Comment: 24 pages, 27 figures. Revised from helpful referee comments. Added
new Table IV, new paragraphs on pages 3 and 1
Complementary subicular pathways to the anterior thalamic nuclei and mammillary bodies in the rat and macaque monkey brain
The origins of the hippocampal (subicular) projections to the anterior thalamic nuclei and mammillary bodies were compared in rats and macaque monkeys using retrograde tracers. These projections form core components of the Papez circuit, which is vital for normal memory. The study revealed a complex pattern of subicular efferents, consistent with the presence of different, parallel information streams, whose segregation appears more marked in the rat brain. In both species, the cells projecting to the mammillary bodies and anterior thalamic nuclei showed laminar separation but also differed along other hippocampal axes. In the rat, these diencephalic inputs showed complementary topographies in the proximalâdistal (columnar) plane, consistent with differential involvement in object-based (proximal subiculum) and context-based (distal subiculum) information. The medial mammillary inputs, which arose along the anteriorâposterior extent of the rat subiculum, favoured the central subiculum (septal hippocampus) and the more proximal subiculum (temporal hippocampus). In contrast, anterior thalamic inputs were largely confined to the dorsal (i.e. septal and intermediate) subiculum, where projections to the anteromedial nucleus favoured the proximal subiculum while those to the anteroventral nucleus predominantly arose in the distal subiculum. In the macaque, the corresponding diencephalic inputs were again distinguished by anteriorâposterior topographies, as subicular inputs to the medial mammillary bodies predominantly arose from the posterior hippocampus while subicular inputs to the anteromedial thalamic nucleus predominantly arose from the anterior hippocampus. Unlike the rat, there was no clear evidence of proximalâdistal separation as all of these medial diencephalic projections preferentially arose from the more distal subiculum
Calibration of the Milagro Cosmic Ray Telescope
The Milagro detector is an air shower array which uses the water Cherenkov
technique and is capable of continuously monitoring the sky at energies near 1
TeV. The detector consists of 20000 metric tons of pure water instrumented with
723 photo-multiplier tubes (PMTs). The PMTs are arranged in a two-layer
structure on a lattice of 3 m spacing covering 5000 area. The direction
of the shower is determined from the relative timing of the PMT signals,
necessitating a common time reference and amplitude slewing corrections to
improve the time resolution. The calibration system to provide these consists
of a pulsed laser driving 30 diffusing light sources deployed in the pond to
allow cross-calibration of the PMTs. The system is capable of calibrating times
and the pulse-heights from the PMTs using the time-over-threshold technique.
The absolute energy scale is provided using single muons passing through the
detector. The description of the calibration system of the Milagro detector and
its prototype Milagrito will be presented.Comment: 4 pages, submitted to the XXVI International Cosmic Ray Conferenc
Milagrito Detection of TeV Emission from Mrk 501
The Milagro water Cherenkov detector near Los Alamos, New Mexico, has been
operated as a sky monitor at energies of a few TeV between February 1997 and
April 1998. Serving as a test run for the full Milagro detector, Milagrito has
taken data during the strong and long-lasting 1997 flare of Mrk 501. We present
results from the analysis of Mrk 501 and compare the excess and background
rates with expectations from the detector simulations.Comment: 4 pages, submitted to XXVI International Cosmic Ray Conference, Salt
Lake Cit
Study of the Shadows of the Moon and the Sun with VHE Cosmic Rays
Milagrito, a prototype for the Milagro detector, operated for 15 months in
1997-8 and collected 8.9 billion events. It was the first extensive air shower
(EAS) array sensitive to showers intiated by primaries with energy below 1 TeV.
The shadows of the sun and moon observed with cosmic rays can be used to study
systematic pointing shifts and measure the angular resolution of EAS arrays.
Below a few TeV, the paths of cosmic rays coming toward the earth are bent by
the helio- and geo-magnetic fields. This is expected to distort and displace
the shadows of the sun and the moon. The moon shadow, offset from the nominal
(undeflected) position, has been observed with high statistical significance in
Milagrito. This can be used to establish energy calibrations, as well as to
search for the anti-matter content of the VHE cosmic ray flux. The shadow of
the sun has also been observed with high significance.Comment: 4 pages, submitted to XXVI International Cosmic Ray Conference, Salt
Lake Cit
Search for Short Duration Bursts of TeV Gamma Rays with the Milagrito Telescope
The Milagrito water Cherenkov telescope operated for over a year. The most
probable gamma-ray energy was ~1 TeV and the trigger rate was as high as 400
Hz. We have developed an efficient technique for searching the entire sky for
short duration bursts of TeV photons. Such bursts may result from "traditional"
gamma-ray bursts that were not in the field-of-view of any other instruments,
the evaporation of primordial black holes, or some as yet undiscovered
phenomenon. We have begun to search the Milagrito data set for bursts of
duration 10 seconds. Here we will present the technique and the expected
results. Final results will be presented at the conference.Comment: submitted to XXVI International Cosmic Ray Conference, Salt Lake Cit
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