13,865 research outputs found
Sequential sampling of junction trees for decomposable graphs
The junction-tree representation provides an attractive structural property
for organizing a decomposable graph. In this study, we present a novel
stochastic algorithm, which we call the junction-tree expander, for sequential
sampling of junction trees for decomposable graphs. We show that recursive
application of the junction-tree expander, expanding incrementally the
underlying graph with one vertex at a time, has full support on the space of
junction trees with any given number of underlying vertices. A direct
application of our suggested algorithm is demonstrated in a sequential Monte
Carlo setting designed for sampling from distributions on spaces of
decomposable graphs, where the junction-tree expander can be effectively
employed as proposal kernel; see the companion paper Olsson et al. 2019 [16]. A
numerical study illustrates the utility of our approach by two examples: in the
first one, how the junction-tree expander can be incorporated successfully into
a particle Gibbs sampler for Bayesian structure learning in decomposable
graphical models; in the second one, we provide an unbiased estimator of the
number of decomposable graphs for a given number of vertices. All the methods
proposed in the paper are implemented in the Python library trilearn.Comment: 31 pages, 7 figure
Consequences of Approximate Symmetry of the Neutrino Mass Matrix
Assuming that the neutrino mass matrix is dominated by a term with the
permutation symmetry it is possible to explain
neutrino data only if the masses are quasi-degenerate. A sub-dominant term with
an approximate symmetry leads to an approximate tri-bimaximal form.
Experimental consequences are discussed.Comment: 7 pages, 2 figures, 1 table, RevTe
Indian Bt cotton varieties do not affect the performance of cotton aphids.
Cotton varieties expressing Cry proteins derived from the soil bacterium Bacillus thuringiensis (Bt) are grown worldwide for the management of pest Lepidoptera. To prevent non-target pest outbreaks and to retain the biological control function provided by predators and parasitoids, the potential risk that Bt crops may pose to non-target arthropods is addressed prior to their commercialization. Aphids play an important role in agricultural systems since they serve as prey or host to a number of predators and parasitoids and their honeydew is an important energy source for several arthropods. To explore possible indirect effects of Bt crops we here examined the impact of Bt cotton on aphids and their honeydew. In climate chambers we assessed the performance of cotton aphids, Aphis gossypii Glover (Hemiptera: Aphididae) when grown on three Indian Bt (Cry1Ac) cotton varieties (MECH 12, MECH 162, MECH 184) and their non-transformed near isolines. Furthermore, we examined whether aphids pick up the Bt protein and analyzed the sugar composition of aphid honeydew to evaluate its suitability for honeydew-feeders. Plant transformation did not have any influence on aphid performance. However, some variation was observed among the three cotton varieties which might partly be explained by the variation in trichome density. None of the aphid samples contained Bt protein. As a consequence, natural enemies that feed on aphids are not exposed to the Cry protein. A significant difference in the sugar composition of aphid honeydew was detected among cotton varieties as well as between transformed and non-transformed plants. However, it is questionable if this variation is of ecological relevance, especially as honeydew is not the only sugar source parasitoids feed on in cotton fields. Our study allows the conclusion that Bt cotton poses a negligible risk for aphid antagonists and that aphids should remain under natural control in Bt cotton fields
Effects of charge-dependent vibrational frequencies and anharmonicities in transport through molecules
As a step towards a more realistic modeling of vibrations in single-molecule
devices, we investigate the effects of charge-dependent vibrational frequencies
and anharmonic potentials on electronic transport. For weak phonon relaxation,
we find that in both cases vibrational steps split into a multitude of
substeps. This effectively leads to a bias-dependent broadening of vibrational
features in current-voltage and conductance characteristics, which provides a
fingerprint of nonequilibrium vibrations whenever other broadening mechanisms
are secondary. In the case of an asymmetric molecule-lead coupling, we observe
that frequency differences can also cause negative differential conductance.Comment: 4+ pages, 3 figures; accepted for publication in Phys. Rev. B. (Brief
Reports
FASER: ForwArd Search ExpeRiment at the LHC
New physics has traditionally been expected in the high- region at
high-energy collider experiments. If new particles are light and
weakly-coupled, however, this focus may be completely misguided: light
particles are typically highly concentrated within a few mrad of the beam line,
allowing sensitive searches with small detectors, and even extremely
weakly-coupled particles may be produced in large numbers there. We propose a
new experiment, ForwArd Search ExpeRiment, or FASER, which would be placed
downstream of the ATLAS or CMS interaction point (IP) in the very forward
region and operated concurrently there. Two representative on-axis locations
are studied: a far location, from the IP and just off the beam
tunnel, and a near location, just from the IP and right behind
the TAN neutral particle absorber. For each location, we examine leading
neutrino- and beam-induced backgrounds. As a concrete example of light,
weakly-coupled particles, we consider dark photons produced through light meson
decay and proton bremsstrahlung. We find that even a relatively small and
inexpensive cylindrical detector, with a radius of and
length of , depending on the location, can discover dark photons
in a large and unprobed region of parameter space with dark photon mass and kinetic mixing parameter . FASER will clearly also be sensitive to many other forms of
new physics. We conclude with a discussion of topics for further study that
will be essential for understanding FASER's feasibility, optimizing its design,
and realizing its discovery potential.Comment: 35 Pages, 12 figures. Version 2, references added, minor change
The Demographics of Georgia IV: Hispanic Immigration Economic Policy Issues - Brief
This report analyzes the economic policy issues in education, health care, the labor market, financial services and the fiscal impact arising from the large increase in Hispanic immigration in Georgia. FRC Brief 12
Enumerating Colorings, Tensions and Flows in Cell Complexes
We study quasipolynomials enumerating proper colorings, nowhere-zero
tensions, and nowhere-zero flows in an arbitrary CW-complex , generalizing
the chromatic, tension and flow polynomials of a graph. Our colorings, tensions
and flows may be either modular (with values in for
some ) or integral (with values in ). We obtain
deletion-contraction recurrences and closed formulas for the chromatic, tension
and flow quasipolynomials, assuming certain unimodularity conditions. We use
geometric methods, specifically Ehrhart theory and inside-out polytopes, to
obtain reciprocity theorems for all of the aforementioned quasipolynomials,
giving combinatorial interpretations of their values at negative integers as
well as formulas for the numbers of acyclic and totally cyclic orientations of
.Comment: 28 pages, 3 figures. Final version, to appear in J. Combin. Theory
Series
The Demographics of Georgia IV: Hispanic Immigration Economic Policy Issues
This report analyzes the economic policy issues in education, health care, the labor market, financial services and the fiscal impact arising from the large increase in Hispanic immigration in Georgia. FRC Report 12
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