78,664 research outputs found
The diffuse neutrino flux from the inner Galaxy: constraints from very high energy gamma-ray observations
Recently, the MILAGRO collaboration reported on the detection of a diffuse
multi-TeV emission from a region of the Galactic disk close to the inner
Galaxy. The emission is in excess of what is predicted by conventional models
for cosmic ray propagation, which are tuned to reproduce the spectrum of cosmic
rays observed locally. By assuming that the excess detected by MILAGRO is of
hadronic origin and that it is representative for the whole inner Galactic
region, we estimate the expected diffuse flux of neutrinos from a region of the
Galactic disk with coordinates . Our estimate has
to be considered as the maximal expected neutrino flux compatible with all the
available gamma ray data, since any leptonic contribution to the observed
gamma-ray emission would lower the neutrino flux. The diffuse flux of
neutrinos, if close to the maximum allowed level, may be detected by a
km--scale detector located in the northern hemisphere. A detection would
unambiguously reveal the hadronic origin of the diffuse gamma-ray emission.Comment: submitted to Astroparticle Physic
Characterization of Power-to-Phase Conversion in High-Speed P-I-N Photodiodes
Fluctuations of the optical power incident on a photodiode can be converted
into phase fluctuations of the resulting electronic signal due to nonlinear
saturation in the semiconductor. This impacts overall timing stability (phase
noise) of microwave signals generated from a photodetected optical pulse train.
In this paper, we describe and utilize techniques to characterize this
conversion of amplitude noise to phase noise for several high-speed (>10 GHz)
InGaAs P-I-N photodiodes operated at 900 nm. We focus on the impact of this
effect on the photonic generation of low phase noise 10 GHz microwave signals
and show that a combination of low laser amplitude noise, appropriate
photodiode design, and optimum average photocurrent is required to achieve
phase noise at or below -100 dBc/Hz at 1 Hz offset a 10 GHz carrier. In some
photodiodes we find specific photocurrents where the power-to-phase conversion
factor is observed to go to zero
From Small-Scale Dynamo to Isotropic MHD Turbulence
We consider the problem of incompressible, forced, nonhelical, homogeneous,
isotropic MHD turbulence with no mean magnetic field. This problem is
essentially different from the case with externally imposed uniform mean field.
There is no scale-by-scale equipartition between magnetic and kinetic energies
as would be the case for the Alfven-wave turbulence. The isotropic MHD
turbulence is the end state of the turbulent dynamo which generates folded
fields with small-scale direction reversals. We propose that the statistics
seen in numerical simulations of isotropic MHD turbulence could be explained as
a superposition of these folded fields and Alfven-like waves that propagate
along the folds.Comment: kluwer latex, 7 pages, 7 figures; Proceedings of the International
Workshop "Magnetic Fields and Star Formation: Theory vs. Observations",
Madrid, 21-25 April 2003 -- published version (but the e-print is free of
numerous typos introduced by the publisher
A nanometer-scale optical electrometer
Self-assembled semiconductor quantum dots show remarkable optical and spin
coherence properties, which have lead to a concerted research effort examining
their potential as a quantum bit for quantum information science1-6. Here, we
present an alternative application for such devices, exploiting recent
achievements of charge occupation control and the spectral tunability of the
optical emission of quantum dots by electric fields7 to demonstrate
high-sensitivity electric field measurement. In contrast to existing
nanometer-scale electric field sensors, such as single electron transistors8-11
and mechanical resonators12,13, our approach relies on homodyning light
resonantly Rayleigh scattered from a quantum dot transition with the excitation
laser and phase sensitive lock-in detection. This offers both static and
transient field detection ability with high bandwidth operation and near unity
quantum efficiency. Our theoretical estimation of the static field sensitivity
for typical parameters, 0.5 V/m/ \surd Hz, compares favorably to the
theoretical limit for single electron transistor-based electrometers. The
sensitivity level of 5 V/m/ \surd Hz we report in this work, which corresponds
to 6.4 * 10-6 e/ \surd Hz at a distance of 12 nm, is worse than this
theoretical estimate, yet higher than any other result attained at 4.2 K or
higher operation temperature
Fluctuations in network dynamics
Most complex networks serve as conduits for various dynamical processes,
ranging from mass transfer by chemical reactions in the cell to packet transfer
on the Internet. We collected data on the time dependent activity of five
natural and technological networks, finding that for each the coupling of the
flux fluctuations with the total flux on individual nodes obeys a unique
scaling law. We show that the observed scaling can explain the competition
between the system's internal collective dynamics and changes in the external
environment, allowing us to predict the relevant scaling exponents.Comment: 4 pages, 4 figures. Published versio
Ectomycorrhizas in association with Pinus patula in Sabie, South Africa
Forestry is an economically important industry in South Africa,involving extensive exotic plantations of Eucalyptus, Pinus and Acacia species. These tree species have fungal associations, such as ectomycorrhizas, that have become locally naturalized. The forestry industry is increasingly faced with problems of long-term sustainability, increasing soil acidity and depletion of soil nutrients. It is, therefore, essential that the fundamental importance of the ectomycorrhizal (ECM) symbioses in the nutrient cycling, growth, health and survival of these tree species should not be ignored. Research on the species diversity of ECM fungi associated with forestry plant species has been hampered by the difficulty of identifying the fungi involved in the symbiosis. This investigation focused on the ECM fungi associated with Pinus patula (Schlecht. et Cham.) grown in managed plantations in the Sabie region, Mpumalanga province, South Africa. ECM roots were morphotyped and DNA was extracted. The internal transcribed spacer (ITS) region was amplified using the ITS 1F and ITS 4 primers. The sequences were BLASTed using the GenBank and UNITE databases. Twenty-seven extractions were successfully amplified representing 17 different morphotypes. Of the 27 sequences, 21 were identified as ECM fungi and, from the BLAST results, eleven different ECM species could be identified. Selected ECM root types were morphologically and anatomically described according to root morphology, mantle structure, specialized hyphae and rhizomorph arrangement. Seven dominant field types were described and identified as two Amanita species, Scleroderma citrinum, a suilloid species, Thelephora terrestris, a tometelloid species and one resembled an Albatrellus species
Possibility of Coherent Phenomena like Bloch Oscillations with Single Photons via W-States
We examine the behavior of single photons at multiport devices and inquire if
coherent effects are possible. In particular we study how single photons need
to be manipulated in order to study coherent phenomena. We show that single
photons need to be produced in W states which lead to vanishing mean amplitude
but nonzero correlations between the inputs at different ports. Such
correlations restore coherent effects with single photons. As a specific
example we demonstrate Bloch oscillations with single photons and thus provide
strict analog of Bloch oscillation of electrons.Comment: 5 pages, 7 figure
Mixed Dark Matter from Axino Distribution
We study the possibility of mixed dark matter obtained through the phase
space distribution of a single particle. An example is offered in the context
of SUSY models with a Peccei-Quinn symmetry. Axinos in the 100 keV range can
naturally have both thermal and non-thermal components. The latter one arises
from the lightest neutralino decays and derelativizes at z ~ 10^4.Comment: Figures added, references fixed. Version accepted for publication on
Phys. Rev. D. LaTeX. 9 pages, 3 figures, uses epsfig.st
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