9,464 research outputs found
Particle Correlations in Z and WW Events
Important information about the dynamics of hadron production can be obtained
by the study of particle correlations. More than 16 million hadronic Z0 decays
and several thousand W+W- events have been recorded from the four LEP
collaborations between 1989 and 2000. Recently, in Z0 decays, new results of
Bose-Einstein correlations in pairs of pions and Fermi-Dirac correlations for
antiproton pairs were reported. In fully-hadronic W+W- decays particle
correlations were used to study whether the two W bosons decay independently.Comment: To appear in the proceedings of XXXVIIth Rencontres de Moriond: QCD
and High Energy Hadronic Interactions, March 16-23, 2002, Les Arcs, Franc
GOES observations of solar protons during ground level enhancements
Since 1974, the U.S. National Oceanic and Atmospheric Administration (NOAA) has observed solar proton fluxes from the Geostationary Operational Environmental Satellites (GOES). These observations frequently have served as measurements of the primary component of ground level enhancements (GLEs). Until March 2020, when GOES-14 and -15 were turned off, solar proton measurements were made by the Energetic Particle Sensor (EPS) and the High-Energy Proton and Alpha Detector (HEPAD). EPS had poor energy resolution above 100 MeV, and NOAA derived a >100 MeV integral flux from the EPS channels to support alerts issued by the Space Weather Forecast Office. HEPAD provided some energy resolution in the 330-700 MeV range and a >700 MeV integral channel. Starting with GOES-16, a new instrument called the Solar and Galactic Proton Sensor (SGPS) has replaced EPS and HEPAD. SGPS uses three solid-state telescopes to observe solar proton fluxes between 1 and 500 MeV with a >500 MeV integral channel. The >100 MeV integral flux is now derived from SGPS observations and includes the >500 MeV flux in its derivation. In this paper, we describe the older EPS and HEPAD observations and the new SGPS solar proton observations. We also compare methods for detecting solar proton event onsets currently used with GOES and neutron monitor observations and recommend some innovations
Ab initio study of reflectance anisotropy spectra of a sub-monolayer oxidized Si(100) surface
The effects of oxygen adsorption on the reflectance anisotropy spectrum (RAS)
of reconstructed Si(100):O surfaces at sub-monolayer coverage (first stages of
oxidation) have been studied by an ab initio DFT-LDA scheme within a
plane-wave, norm-conserving pseudopotential approach. Dangling bonds and the
main features of the characteristic RAS of the clean Si(100) surface are mostly
preserved after oxidation of 50% of the surface dimers, with some visible
changes: a small red shift of the first peak, and the appearance of a distinct
spectral structure at about 1.5 eV. The electronic transitions involved in the
latter have been analyzed through state-by-state and layer-by-layer
decompositions of the RAS. We suggest that new interplay between present
theoretical results and reflectance anisotropy spectroscopy experiments could
lead to further clarification of structural and kinetic details of the Si(100)
oxidation process in the sub-monolayer range.Comment: 21 pages, 8 figures. To be published in Physical Rev.
Orthogonal, solenoidal, three-dimensional vector fields for no-slip boundary conditions
Viscous fluid dynamical calculations require no-slip boundary conditions.
Numerical calculations of turbulence, as well as theoretical turbulence closure
techniques, often depend upon a spectral decomposition of the flow fields.
However, such calculations have been limited to two-dimensional situations.
Here we present a method that yields orthogonal decompositions of
incompressible, three-dimensional flow fields and apply it to periodic
cylindrical and spherical no-slip boundaries.Comment: 16 pages, 2 three-part figure
Assessing access of galactic cosmic rays at Moon\u27s orbit
[1] Characterizing the lunar radiation environment is essential for preparing future robotic and human explorations on lunar bases. Galactic cosmic rays (GCR) represent one source of ionizing radiation at the Moon that poses a biological risk. Because GCR are charged particles, their paths are affected by the magnetic fields along their trajectories. Unlike the Earth, the Moon has no strong, shielding magnetic field of its own. However, as it orbits Earth, the Moon traverses not only the weak interplanetary magnetic field but also the distant magnetic tail of Earth\u27s magnetosphere. We combine an empirical magnetic field model of Earth\u27s magnetosphere with a fully-relativistic charged particle trajectory code to model and assess the access of GCR at the Moon\u27s orbit. We follow protons with energies of 1, 10 and 100 MeV starting from an isotropic distribution at large distances outside a volume of space including Earth\u27s magnetosphere and the lunar orbit. The simulation result shows that Earth\u27s magnetosphere does not measurably modify protons of energy greater than 1 MeV at distances outside the geomagnetic cutoff imposed by Earth\u27s strong dipole field very near to the planet. Therefore, in contrast to Winglee and Harnett (2007), we conclude that Earth\u27s magnetosphere does not provide any substantial magnetic shielding at the Moon\u27s orbit. These simulation results will be compared to LRO/CRaTER data after its planned launch in June 2009
Local estimates for entropy densities in coupled map lattices
We present a method to derive an upper bound for the entropy density of
coupled map lattices with local interactions from local observations. To do
this, we use an embedding technique being a combination of time delay and
spatial embedding. This embedding allows us to identify the local character of
the equations of motion. Based on this method we present an approximate
estimate of the entropy density by the correlation integral.Comment: 4 pages, 5 figures include
Magnetohydrodynamic activity inside a sphere
We present a computational method to solve the magnetohydrodynamic equations
in spherical geometry. The technique is fully nonlinear and wholly spectral,
and uses an expansion basis that is adapted to the geometry:
Chandrasekhar-Kendall vector eigenfunctions of the curl. The resulting lower
spatial resolution is somewhat offset by being able to build all the boundary
conditions into each of the orthogonal expansion functions and by the
disappearance of any difficulties caused by singularities at the center of the
sphere. The results reported here are for mechanically and magnetically
isolated spheres, although different boundary conditions could be studied by
adapting the same method. The intent is to be able to study the nonlinear
dynamical evolution of those aspects that are peculiar to the spherical
geometry at only moderate Reynolds numbers. The code is parallelized, and will
preserve to high accuracy the ideal magnetohydrodynamic (MHD) invariants of the
system (global energy, magnetic helicity, cross helicity). Examples of results
for selective decay and mechanically-driven dynamo simulations are discussed.
In the dynamo cases, spontaneous flips of the dipole orientation are observed.Comment: 15 pages, 19 figures. Improved figures, in press in Physics of Fluid
Constraining the Drag Coefficients of Meteors in Dark Flight
Based on data in the aeronautics literature, we have derived functions for the drag coefficients of spheres and cubes as a function of Mach number. Experiments have shown that spheres and cubes exhibit an abrupt factor-of-two decrease in the drag coefficient as the object slows through the transonic regime. Irregularly shaped objects such as meteorites likely exhibit a similar trend. These functions are implemented in an otherwise simple projectile motion model, which is applicable to the non-ablative dark flight of meteors (speeds less than .+3 km/s). We demonstrate how these functions may be used as upper and lower limits on the drag coefficient of meteors whose shape is unknown. A Mach-dependent drag coefficient is potentially important in other planetary and astrophysical situations, for instance, in the core accretion scenario for giant planet formation
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