41,702 research outputs found
Transit Lightcurve Signatures of Artificial Objects
The forthcoming space missions, able to detect Earth-like planets by the
transit method, will a fortiori also be able to detect the transit of
artificial planet-size objects. Multiple artificial objects would produce
lightcurves easily distinguishable from natural transits. If only one
artificial object transits, detecting its artificial nature becomes more
difficult. We discuss the case of three different objects (triangle, 2-screen,
louver-like 6-screen) and show that they have a transit lightcurve
distinguishable from the transit of natural planets, either spherical or
oblate, although an ambiguity with the transit of a ringed planet exists in
some cases. We show that transits, especially in the case of multiple
artificial objects, could be used for the emission of attention-getting
signals, with a sky coverage comparable to the laser pulse method. The large
number of expected planets (several hundreds) to be discovered by the transit
method by next space missions will allow to test these ideas.Comment: Accepted for publication in ApJ. Manuscript: 17 pages, 8 figure
Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space
The electromagnetic bias is an error present in radar altimetry of the ocean due to the non-uniform reflection from wave troughs and crests. A study of the electromagnetic bias became necessary to permit error reduction in mean sea level measurements of satellite radar altimeters. Satellite radar altimeters have been used to find the upper and lower bounds for the electromagnetic bias. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to predict the electromagnetic bias. The predicted electromagnetic bias will be compared to measurements at C and Ku bands
Geos 1 observations at Malvern, England
Satellite observation techniques and data processing methods at optical tracking station in Malvern, Englan
Electrical conductivity and thermal dilepton rate from quenched lattice QCD
We report on a continuum extrapolation of the vector current correlation
function for light valence quarks in the deconfined phase of quenched QCD. This
is achieved by performing a systematic analysis of the influence of cut-off
effects on light quark meson correlators at using clover
improved Wilson fermions. We discuss resulting constraints on the electrical
conductivity and the thermal dilepton rate in a quark gluon plasma. In addition
new results at 1.2 and 3.0 will be presented.Comment: 4 pages, 6 eps figures, to appear in the proceedings of Quark Matter
2011, 23-28 May 2011, Annecy, Franc
Perturbative and Nonperturbative Kolmogorov Turbulence in a Gluon Plasma
In numerical simulations of nonabelian plasma instabilities in the hard-loop
approximation, a turbulent spectrum has been observed that is characterized by
a phase-space density of particles with exponent , which is larger than expected from relativistic
scatterings. Using the approach of Zakharov, L'vov and Falkovich, we analyse
possible Kolmogorov coefficients for relativistic -particle
processes, which give at most perturbatively for an energy cascade.
We discuss nonperturbative scenarios which lead to larger values. As an extreme
limit we find the result generically in an inherently nonperturbative
effective field theory situation, which coincides with results obtained by
Berges et al.\ in large- scalar field theory. If we instead assume that
scaling behavior is determined by Schwinger-Dyson resummations such that the
different scaling of bare and dressed vertices matters, we find that
intermediate values are possible. We present one simple scenario which would
single out .Comment: published versio
Spaces of finite element differential forms
We discuss the construction of finite element spaces of differential forms
which satisfy the crucial assumptions of the finite element exterior calculus,
namely that they can be assembled into subcomplexes of the de Rham complex
which admit commuting projections. We present two families of spaces in the
case of simplicial meshes, and two other families in the case of cubical
meshes. We make use of the exterior calculus and the Koszul complex to define
and understand the spaces. These tools allow us to treat a wide variety of
situations, which are often treated separately, in a unified fashion.Comment: To appear in: Analysis and Numerics of Partial Differential
Equations, U. Gianazza, F. Brezzi, P. Colli Franzone, and G. Gilardi, eds.,
Springer 2013. v2: a few minor typos corrected. v3: a few more typo
correction
Weak-triplet, color-octet scalars and the CDF dijet excess
We extend the standard model to include a weak-triplet and color-octet
scalar. This `octo-triplet' field consists of three particles, two charged and
one neutral, whose masses and renormalizable interactions depend only on two
new parameters. The charged octo-triplet decay into a W boson and a gluon is
suppressed by a loop factor and an accidental cancellation. Thus, the main
decays of the charged octo-triplet may occur through higher-dimensional
operators, mediated by a heavy vectorlike fermion, into quark pairs. For an
octo-triplet mass below the t\bar{b} threshold, the decay into Wb\bar{b} or
Wb\bar{s} through an off-shell top quark has a width comparable to that into
c\bar{s} or c\bar{b}. Pair production with one octo-triplet decaying into two
jets and the other decaying into a W and two soft b jets may explain the
dijet-plus-W excess reported by the CDF Collaboration. Using a few kinematic
distributions, we compare two mechanisms of octo-triplet pair production:
through an s-channel coloron and through the coupling to gluons. The
higher-dimensional operators that allow dijet decays also lead to CP violation
in B_s - \bar B_s mixing.Comment: 18 pages. New CDF kinematic distributions using 7.3 fb^{-1} compared
to both resonant and gluon-induced pair production of octets. Corrections in
Section 3.1. Comment on the D0 Wjj result included in Section 3.3.
Implications for LHC expanded in Section 3.
A simple extended-cavity diode laser
Operating a laser diode in an extended cavity which provides frequency-selective feedback is a very effective method of reducing the laser's linewidth and improving its tunability. We have developed an extremely simple laser of this type, built from inexpensive commercial components with only a few minor modifications, A 780 nm laser built to this design has an output power of 80 mW, a Linewidth of 350 kHz, and it has been continuously locked to a Doppler-free rubidium transition for several days
Regional and seasonal patterns of epipelagic fish assemblages from the central California Current
The coastal Pacific Ocean off northern and central California encompasses the strongest seasonal upwelling zone in the California Current ecosystem. Headlands and bays
here generate complex circulation features and confer unusual oceanographic complexity. We sampled the coastal epipelagic fish community of this region with a surface trawl in the summer and fall of 2000–05 to assess patterns of spatial and temporal community structure. Fifty-three species of fish were captured in 218 hauls at 34 fixed stations, with clupeiform species dominating. To examine spatial patterns, samples were grouped by location relative to a prominent headland at Point Reyes and the resulting
two regions, north coast and Gulf of the Farallones, were plotted by using nonmetric multidimensional scaling.
Seasonal and interannual patterns were also examined, and representative species were identified for each distinct community. Seven oceanographic variables measured concurrently with trawling were plotted by principal components analysis and tested for correlation with biotic
patterns. We found significant differences in community structure by region, year, and season, but no interaction among main effects. Significant differences in oceanographic
conditions mirrored the biotic patterns, and a match between biotic and hydrographic structure was detected.
Dissimilarity between assemblages was mostly the result of differences in abundance and frequency of occurrence of about twelve common species. Community patterns were best
described by a subset of hydrographic variables, including water depth, distance from shore, and any one of several correlated variables associated with upwelling intensity. Rather than discrete communities with clear borders and distinct member species, we found gradients in community
structure and identified stations with similar fish communities by region and by proximity to features such as
the San Francisco Bay
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