349 research outputs found
Colour Charges and the Anti-Screening Contribution to the Interquark Potential
Asymptotic freedom arises from the dominance of anti-screening over screening
in non-abelian gauge theories. In this paper we will present a simple and
physically appealing derivation of the anti-screening contribution to the
interquark potential. Our method allows us to identify the dominant gluonic
distribution around static quarks. Extensions are discussed.Comment: 7 pages, LaTe
Anti-Screening by Quarks and the Structure of the Inter-Quark Potential
The inter-quark potential is dominated by anti-screening effects which
underly asymptotic freedom. We calculate the order g^6 anti-screening
contribution from light fermions and demonstrate that these effects introduce a
non-local divergence. These divergences are shown to make it impossible to
define a coupling renormalisation scheme that renormalises this minimal,
anti-screening potential. Hence the beta function cannot be divided into
screening and anti-screening parts beyond lowest order. However, we then
demonstrate that renormalisation can be carried out in terms of the
anti-screening potential.Comment: 11 pages, some clarifications and typographical corrections, to
appear in Physics Letters
Factors determining the adoption and impact of a postharvest storage technology
This paper evaluates the determinants and impact of adopting the metal silo—a postharvest storage technology for staple grains—which was disseminated by the Swiss Agency for Development and Cooperation (SDC) from 1983 to 2003 in four Central American countries. The aim of the SDC program was to diminish smallholder farmers' postharvest losses by facilitating the manufacture and dissemination of metal silos and thereby to improve regional food security. Our empirical analysis is based on a unique data set obtained from a survey of 1,600 households from El Salvador, Guatemala, Honduras and Nicaragua. We employed a double-hurdle model to identify factors that contributed to the adoption of metal silos and used Tobit and standard regression models to assess the impact of adopting the silos on food security and well-being of households. Our results show that both the household demand for metal silos and the impact of their adoption varied across the four countries, demonstrating the relevance of regional policies for their adoption, as well as their impact. Furthermore, our results indicate that, in addition to achieving household self-sufficiency in maize, the main determinants of adoption were household socio-economic characteristics such as age, land ownership, completion of a training course and quality of basic infrastructure. Finally, when considering a group of economic and social indicators of household well-being, we found that, compared to the silo non-adopters, the adopter households experienced a significant improvement in their food security and well-being between 2005 and 200
Casimir Effect on the Radius Stabilization of the Noncommutative Torus
We evaluate the one-loop correction to the spectrum of Kaluza-Klein system
for the model on , where
dimensions are the ordinary flat Minkowski spacetimes and the extra dimensions
are the L two-dimensional noncommutative tori with noncommutativity .
The correction to the Kaluza-Klein mass spectrum is then used to compute the
Casimir energy. The results show that when the Casimir energy due to the
noncommutativity could give repulsive force to stabilize the extra
noncommutative tori in the cases of , with a positive integral.Comment: Latex, 11page
Bone Mineral Density and Secondary Hyperparathyroidism in Pulmonary Hypertension
BACKGROUND: Low bone mineral density (BMD) is common in chronic lung diseases and associated with reduced quality of life. Little is known about BMD in pulmonary hypertension (PH). METHODS: Steroid-naïve patients with PH (n=34; 19 idiopathic, 15 chronic thromboembolic) had BMD measured by DXA at the time of diagnostic right heart catheterization. Exercise capacity, quality of life and various parameters related to PH severity and bone metabolism were also assessed. 24 patients with left heart failure (LHF) were similarly assessed as controls. RESULTS: The prevalence of osteopenia was high both in PH (80%) and in controls with LHF (75%). Low BMD was associated with lean body mass, age, lower BMI, impaired exercise capacity and in PH with higher pulmonary vascular resistance. Serum parathyroid hormone (PTH) was elevated and considerably higher in PH than in LHF (above normal, in 55 vs 29%). Secondary hyperparathyroidism was not related to impaired renal function but possibly to low vitamin D status. CONCLUSIONS: Osteopenia is common in PH and in chronically ill patients with LHF. Osteopenia is associated with known risk factors but in PH also with disease severity. Preventive measures in an increasingly chronic ill PH population should be considered. Secondary hyperparathyroidism is highly prevalent in PH and might contribute to bone and possibly pulmonary vascular disease. Whether adequate vitamin D substitution could prevent low BMD in PH remains to be determined
Noncommutative Field Theory and Spontaneous Symmetry Breaking
We investigate the noncommutative analogue of the spontaneously broken linear
sigma model at the one-loop quantum level. In the commutative case,
renormalization of a theory with a spontaneously broken continuous global
symmetry depends on cancellations that enable the limited set of counterterms
consistent with that symmetry to remove the divergences even after its
spontaneous breaking, while preserving the masslessness of the associated
Goldstone modes. In the noncommutative case, we find that these cancellations
are violated, and the renormalized one-loop correction to the inverse pion
propagator explicitly yields a mass shift which depends on the ultraviolet
cutoff. Thus, we cannot naively take the ultraviolet cutoff to infinity first,
and then take the external momentum to zero to verify Nambu-Goldstone symmetry
realization. However, from the Wilsonian perspective where the cutoff is fixed
and physical, the zero external momentum limit of the inverse pion propagator
still vanishes, and implies the masslessness of the pion fields at one-loop.
This is another demonstration of the failure of ultraviolet and infrared limits
to commute in noncommutative field theories, and signals the incompatibility of
Nambu-Goldstone symmetry realization with the continuum renormalization of
these theories.Comment: 17 pages, LaTeX, uses feynmp package; analyzes general case using all
orderings of star product in quartic vertices; uses symmetrized Feynman
vertex rules; revised combinatorics; reference added; conclusions unaltered;
diagrams now appear correctl
A Precise Determination of the Running Coupling in the SU(3) Yang-Mills Theory
A non-perturbative finite-size scaling technique is used to study the
evolution of the running coupling (in a certain adapted scheme) in the SU(3)
Yang-Mills theory. At low energies contact is made with the fundamental
dynamical scales, such as the string tension K, while at larger energies the
coupling is shown to evolve according to perturbation theory. In that regime
the coupling in the MS-bar scheme of dimensional regularization is obtained
with an estimated total error of a few percent.Comment: pages 0-27, ps-file 255491 bytes, preprint DESY 93-114 (CERN-TH
6996/93
The Trouble with de Sitter Space
In this paper we assume the de Sitter Space version of Black Hole
Complementarity which states that a single causal patch of de Sitter space is
described as an isolated finite temperature cavity bounded by a horizon which
allows no loss of information. We discuss the how the symmetries of de Sitter
space should be implemented. Then we prove a no go theorem for implementing the
symmetries if the entropy is finite. Thus we must either give up the finiteness
of the de Sitter entropy or the exact symmetry of the classical space. Each has
interesting implications for the very long time behavior. We argue that the
lifetime of a de Sitter phase can not exceed the Poincare recurrence time. This
is supported by recent results of Kachru, Kallosh, Linde and Trivedi.Comment: 15 pages, 1 figure. v2: added fifth section with comments on long
time stability of de Sitter space, in which we argue that the lifetime can
not exceed the Poincare recurrence time. v3: corrected a minor error in the
appendi
The heavy quark decomposition of the S-matrix and its relation to the pinch technique
We propose a decomposition of the S-matrix into individually gauge invariant
sub-amplitudes, which are kinematically akin to propagators, vertices, boxes,
etc. This decompsition is obtained by considering limits of the S-matrix when
some or all of the external particles have masses larger than any other
physical scale. We show at the one-loop level that the effective gluon
self-energy so defined is physically equivalent to the corresponding gauge
independent self-energy obtained in the framework of the pinch technique. The
generalization of this procedure to arbitrary gluonic -point functions is
briefly discussed.Comment: 11 uuencoded pages, NYU-TH-94/10/0
On-farm comparison of different postharvest storage technologies for effectiveness in pest management in a maize farming system of Tanzania Central Corridor: Presentation
Seven methods for storing maize were compared with traditional practice of storing maize in polypropylene bags. Twenty farmers managed the experiment under their prevailing conditions for 30 weeks. Stored grain was assessed for damage every six weeks. The dominant storage insect pests identified were the Maize weevil (Sitophilus zeamais) and the Red flour beetle (Tribolium castaneum). There was no significant difference (F = 87.09; P < 0.0001) in insect control and grain damage between hermetic storage and fumigation with insecticides. However, the insecticide treated polypropylene yarn (ZeroFly®) did not control insect infestation of grain for the experimental period under farmers’ management. Grain damage was significantly lower in hermetic storage and fumigated grain than ZeroFly® and polypropylene bags without fumigation. No significant difference in grain damage was found between airtight treatment alone and when combined with the use of insecticides. During storage, S. zeamais was predominant and could be of more economic importance than T. castaneum as far as maize damage is concerned. Even though ZeroFly®, and polypropylene bags without grain treatment did not control storage pests, farmers still prefered this cheap technology. Hermetic storage techniques can be recommended to farmers without the use of insecticides provided they are inexpensive, and the proper application of technologies is ensured.Seven methods for storing maize were compared with traditional practice of storing maize in polypropylene bags. Twenty farmers managed the experiment under their prevailing conditions for 30 weeks. Stored grain was assessed for damage every six weeks. The dominant storage insect pests identified were the Maize weevil (Sitophilus zeamais) and the Red flour beetle (Tribolium castaneum). There was no significant difference (F = 87.09; P < 0.0001) in insect control and grain damage between hermetic storage and fumigation with insecticides. However, the insecticide treated polypropylene yarn (ZeroFly®) did not control insect infestation of grain for the experimental period under farmers’ management. Grain damage was significantly lower in hermetic storage and fumigated grain than ZeroFly® and polypropylene bags without fumigation. No significant difference in grain damage was found between airtight treatment alone and when combined with the use of insecticides. During storage, S. zeamais was predominant and could be of more economic importance than T. castaneum as far as maize damage is concerned. Even though ZeroFly®, and polypropylene bags without grain treatment did not control storage pests, farmers still prefered this cheap technology. Hermetic storage techniques can be recommended to farmers without the use of insecticides provided they are inexpensive, and the proper application of technologies is ensured
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