48,691 research outputs found
Landau-Pomeranchuk-Migdal resummation for dilepton production
We consider the thermal emission rate of dileptons from a QCD plasma in the
small invariant mass (Q^2 \sim \gs^2 T^2) but large energy (q^0 \gsim T)
range. We derive an integral equation which resums multiple scatterings to
include the LPM effect; it is valid at leading order in the coupling. Then we
recast it as a differential equation and show a simple algorithm for its
solution. We present results for dilepton rates at phenomenologically
interesting energies and invariant masses.Comment: 19 pages, 7 postscript figures, test program available at
http://www-spht.cea.fr/articles/T02/150/libLPM
Synthesis of Y1Ba2Cu3O(sub x) superconducting powders by intermediate phase reaction
One of the more striking problems for the synthesis of the Y1Ba2Cu3Ox compound is the high-temperature decomposition of the BaCO3. This compound is present as raw material or as an intermediate compound in chemical processes such as amorphous citrate, coprecipitation oxalate, sol-gel process, acetate pyrolisis, etc. This fact makes difficult the total formation reaction of the Y1Ba2Cu3Ox phase and leads to the presence of undesirable phases such as the BaCuO2 phase, the 'green phase', Y2BaCuO5 and others. Here, a new procedure to overcome this difficulty is studied. The barium cation is previously combined with yttrium and/or copper to form intermediate compounds which can react between them to give Y1Ba2Cu3Ox. BaY2O4 and BaCu2O3 react according to the equation BaY2O4+3BaCu2O3 yields 2Y1Ba2Cu3Ox. BaY2O4 is a stable compound of the Y2O3-BaO system; BaCu2O3 is an intimate mixture of BaCuO2 and uncombined CuO. The reaction kinetics of these phases have been established between 860 and 920 C. The phase evolution has been determined. The crystal structure of the Y1Ba2Cu3Ox obtained powder was studied. According to the results obtained from the kinetics study the Y1Ba2Cu3Ox the synthesis was performed at temperatures of 910 to 920 C for short treatment times (1 to 2 hours). Pure Y1Ba2Cu3Ox was prepared, which develops orthorombic type I structure despite of the cooling cycle. Superconducting transition took place at 91 K. The sintering behavior and the superconducting properties of sintered samples were studied. Density, microstructure and electrical conductivity were measured. Sintering densities higher than 95 percent D(sub th) were attained at temperatures below 940 C. Relatively fine grained microstructure was observed, and little or no-liquid phase was detected
Palomar 13: a velocity dispersion inflated by binaries ?
Recently, combining radial velocities from Keck/HIRES echelle spectra with
published proper motion membership probabilities, Cote et al (2002) observed a
sample of 21 stars, probable members of Palomar 13, a globular cluster in the
Galactic halo. Their projected velocity dispersion sigma_p = 2.2 +/-0.4 km/s
gives a mass-to-light ratio M/L_V = 40 +24/-17, about one order of magnitude
larger than the usual estimate for globular clusters. We present here radial
velocities measured from three different CCD frames of commissioning
observations obtained with the new ESO/VLT instrument FLAMES (Fibre Large Array
Multi Element Spectrograph). From these data, now publicly available, we
measure the homogeneous radial velocities of eight probable members of this
globular cluster. A new projected velocity dispersion sigma_p = 0.6-0.9 +/-0.3
km/s implies Palomar 13 mass-to-light ratio M/L_V = 3-7, similar to the usual
value for globular clusters. We discuss briefly the two most obvious reasons
for the previous unusual mass-to-light ratio finding: binaries, now clearly
detected, and more homogeneous data from the multi-fibre FLAMES spectrograph.Comment: 9 pages, 2 Postscript figure
The Effects of Seagrass Species and Trophic Interactions in Experimental Seagrass Communities
Mechanism for the failure of the Edwards hypothesis in the SK spin glass
The dynamics of the SK model at T=0 starting from random spin configurations
is considered. The metastable states reached by such dynamics are atypical of
such states as a whole, in that the probability density of site energies,
, is small at . Since virtually all metastable states
have a much larger , this behavior demonstrates a qualitative failure of
the Edwards hypothesis. We look for its origins by modelling the changes in the
site energies during the dynamics as a Markov process. We show how the small
arises from features of the Markov process that have a clear physical
basis in the spin-glass, and hence explain the failure of the Edwards
hypothesis.Comment: 5 pages, new title, modified text, additional reference
In search of the forgotten rare earth
This is the author accepted manuscript. The final version is available from the Geological Society via the DOI in this recordScandium (Sc) has been called a ‘miracle metal’. When alloyed with aluminium, it produces super-strong but lightweight materials, just right for use in the next generation of aeroplane manufacture and other high-tech applications. By definition, Sc is a member of the 17-strong, rare earth element (REE) family. Like the other REEs, Sc isn’t actually rare in terms of distribution—it can be found throughout Earth’s crust and has a similar abundance to lead—but unlike lead, economic concentrations of Sc are very rare, making it one of the most expensive elements in the world. Indeed, concentrations of Sc are usually so low that Sc is often excluded from geological assessments of REE—it is the forgotten rare earth.
But this may be about to change. Several research groups are turning their attention to Sc, using new data from exploration projects to identify a number of promising sources that could be mined. Here we discuss the behaviour of Sc and the distribution of these deposits worldwide. We argue that with improved understanding of how Sc associates with clinopyroxene and how it concentrates within weathering and waste products, such as laterites and red muds, these newly identified deposits could create a virtuous circle of raw materials supply and new high-tech uses
On Maximal Unbordered Factors
Given a string of length , its maximal unbordered factor is the
longest factor which does not have a border. In this work we investigate the
relationship between and the length of the maximal unbordered factor of
. We prove that for the alphabet of size the expected length
of the maximal unbordered factor of a string of length~ is at least
(for sufficiently large values of ). As an application of this result, we
propose a new algorithm for computing the maximal unbordered factor of a
string.Comment: Accepted to the 26th Annual Symposium on Combinatorial Pattern
Matching (CPM 2015
Exact solutions for models of evolving networks with addition and deletion of nodes
There has been considerable recent interest in the properties of networks,
such as citation networks and the worldwide web, that grow by the addition of
vertices, and a number of simple solvable models of network growth have been
studied. In the real world, however, many networks, including the web, not only
add vertices but also lose them. Here we formulate models of the time evolution
of such networks and give exact solutions for a number of cases of particular
interest. For the case of net growth and so-called preferential attachment --
in which newly appearing vertices attach to previously existing ones in
proportion to vertex degree -- we show that the resulting networks have
power-law degree distributions, but with an exponent that diverges as the
growth rate vanishes. We conjecture that the low exponent values observed in
real-world networks are thus the result of vigorous growth in which the rate of
addition of vertices far exceeds the rate of removal. Were growth to slow in
the future, for instance in a more mature future version of the web, we would
expect to see exponents increase, potentially without bound.Comment: 9 pages, 3 figure
Determinants of a transcriptionally competent environment at the GM-CSF promoter
Granulocyte macrophage-colony stimulating factor
(GM-CSF) is produced by T cells, but not B cells,
in response to immune signals. GM-CSF gene
activation in response to T-cell stimulation requires
remodelling of chromatin associated with the
gene promoter, and these changes do not occur in
B cells. While the CpG methylation status of the
murine GM-CSF promoter shows no correlation with
the ability of the gene to respond to activation, we
find that the basal chromatin environment of the
gene promoter influences its ability to respond to
immune signals. In unstimulated T cells but not B
cells, the GM-CSF promoter is selectively marked
by enrichment of histone acetylation, and association
of the chromatin-remodelling protein BRG1.
BRG1 is removed from the promoter upon activation
concomitant with histone depletion and BRG1
is required for efficient chromatin remodelling and
transcription. Increasing histone acetylation at
the promoter in T cells is paralleled by increased
BRG1 recruitment, resulting in more rapid chromatin
remodelling, and an associated increase in GM-CSF
mRNA levels. Furthermore, increasing histone
acetylation in B cells removes the block in chromatin
remodelling and transcriptional activation
of the GM-CSF gene. These data are consistent
with a model in which histone hyperacetylation
and BRG1 enrichment at the GM-CSF promoter,
generate a chromatin environment competent
to respond to immune signals resulting in gene
activation
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