745 research outputs found
Fabricating Composite-Material Structures Containing SMA Ribbons
An improved method of designing and fabricating laminated composite-material (matrix/fiber) structures containing embedded shape-memory-alloy (SMA) actuators has been devised. Structures made by this method have repeatable, predictable properties, and fabrication processes can readily be automated. Such structures, denoted as shape-memory-alloy hybrid composite (SMAHC) structures, have been investigated for their potential to satisfy requirements to control the shapes or thermoelastic responses of themselves or of other structures into which they might be incorporated, or to control noise and vibrations. Much of the prior work on SMAHC structures has involved the use SMA wires embedded within matrices or within sleeves through parent structures. The disadvantages of using SMA wires as the embedded actuators include (1) complexity of fabrication procedures because of the relatively large numbers of actuators usually needed; (2) sensitivity to actuator/ matrix interface flaws because voids can be of significant size, relative to wires; (3) relatively high rates of breakage of actuators during curing of matrix materials because of sensitivity to stress concentrations at mechanical restraints; and (4) difficulty of achieving desirable overall volume fractions of SMA wires when trying to optimize the integration of the wires by placing them in selected layers only
Weak radiative hyperon decays, Hara's theorem and the diquark
Weak radiative hyperon decays are discussed in the diquark-level approach. It
is pointed out that in the general diquark formalism one may reproduce the
experimentally suggested pattern of asymmetries, while maintaining Hara's
theorem in the SU(3) limit. At present, however, no detailed quark-based model
of parity-violating diquark-photon coupling exists that would have the
necessary properties.Comment: 10 pages, LaTe
Comment on "Hara's theorem in the constituent quark model"
It is pointed out that current conservation alone does not suffice to prove
Hara's theorem as it was claimed recently. By explicit calculation we show that
the additional implicit assumption made in such "proofs" is that of a
sufficiently localized current.Comment: 8 pages, Late
Scoping the biosecurity risks and appropriate management relating to the freshwater ornamental aquarium trade across northern Australia
The current scoping study provides for a preliminary understanding of the numerous existing and potential pathways for incursions from the aquarium trade into freshwater ecosystems in tropical Australia, encompassing the lands and waterways north of the Tropic of Capricorn. A finding of this desktop project is the lack of consolidated nformation relevant to tropical Australia in terms of biotic incursion of even the better documented taxa (e.g. freshwater fishes) and a lack of information on most taxa in the aquarium trade (e.g. shrimps, molluscs, diseases in the wild). This is the consequence of the extent and scale of the aquatic biosecurity task as well as ad hoc biological sampling, under-resourcing of ecological surveys, monitoring and reporting in the grey or published literature, all of which is compounded by the remoteness of much of tropical northern Australia
A QCD Sum Rule Approach to the Contribution to the Radiative Decay
QCD sum rules are used to calculate the contribution of short-distance
single-quark transition , to the amplitudes of the
hyperon radiative decay, . We re-evaluate the
Wilson coefficient of the effective operator responsible for this transition.
We obtain a branching ratio which is comparable to the unitarity limit.Comment: 15 pages, Revtex, 13 figures available as a uuencoded, gz-compressed
ps fil
Long Distance Contribution to and Implications for and
We estimate the long distance (LD) contribution to the magnetic part of the
transition using the Vector Meson Dominance approximation
. We find that this contribution may be significantly
larger than the short distance (SD) contribution to and could
possibly saturate the present experimental upper bound on the decay rate, eV. For the decay , which is driven by as well, we obtain an upper bound on the branching ratio from . Barring the possibility that the Quantum Chromodynamics
coefficient be much smaller than 1, also implies the approximate relation .
This relation agrees quantitatively with a recent independent estimate of the
l.h.s. by Deshpande et al., confirming that the LD contributions to are small. We find that these amount to an increase of in
the magnitude of the transition amplitude, relative to the SD
contribution alone.Comment: 16 pages, LaTeX fil
Ants are the major agents of food resource removal from tropical rainforest floors
1. Ants are diverse and abundant, especially in tropical ecosystems. They are often cited as the agents of key ecological processes, but their precise contributions compared with other organisms have rarely been quantified. Through the removal of food resources from the forest floor and subsequent transport to nests, ants play an important role in the redistribution of nutrients in rainforests. This is an essential ecosystem process and a key energetic link between higher trophic levels, decomposers and primary producers. 2. We used the removal of carbohydrate, protein and seed baits as a proxy to quantify the contribution that ants, other invertebrates and vertebrates make to the redistribution of nutrients around the forest floor, and determined to what extent there is functional redundancy across ants, other invertebrate and vertebrate groups. 3. Using a large‐scale, field‐based manipulation experiment, we suppressed ants from c . 1 ha plots in a lowland tropical rainforest in Sabah, Malaysia. Using a combination of treatment and control plots, and cages to exclude vertebrates, we made food resources available to: (i) the whole foraging community, (ii) only invertebrates and (iii) only non‐ant invertebrates. This allowed us to partition bait removal into that taken by vertebrates, non‐ant invertebrates and ants. Additionally, we examined how the non‐ant invertebrate community responded to ant exclusion. 4. When the whole foraging community had access to food resources, we found that ants were responsible for 52% of total bait removal whilst vertebrates and non‐ant invertebrates removed the remaining 48%. Where vertebrates were excluded, ants carried out 61% of invertebrate‐mediated bait removal, with all other invertebrates removing the remaining 39%. Vertebrates were responsible for just 24% of bait removal and invertebrates (including ants) collectively removed the remaining 76%. There was no compensation in bait removal rate when ants and vertebrates were excluded, indicating low functional redundancy between these groups. 5. This study is the first to quantify the contribution of ants to the removal of food resources from rainforest floors and thus nutrient redistribution. We demonstrate that ants are functionally unique in this role because no other organisms compensated to maintain bait removal rate in their absence. As such, we strengthen a growing body of evidence establishing ants as ecosystem engineers, and provide new insights into the role of ants in maintaining key ecosystem processes. In this way, we further our basic understanding of the functioning of tropical rainforest ecosystems
Hyperon weak radiative decays in chiral perturbation theory
We investigate the leading-order amplitudes for weak radiative decays of
hyperons in chiral perturbation theory. We consistently include contributions
from the next-to-leading order weak-interaction Lagrangian. It is shown that
due to these terms Hara's theorem is violated. The data for the decays of
charged hyperons can be easily accounted for. However, at this order in the
chiral expansion, the four amplitudes for the decays of neutral hyperons
satisfy relations which are in disagreement with the data. The asymmetry
parameters for all the decays can not be accounted for without higher-order
terms. We shortly comment on the effect of the 27-plet part of the weak
interaction.Comment: 8 pages of REVTeX and using macro-package "feynman.tex" (available at
http://xxx.lanl.gov/ftp/hep-ph/papers/macros) for the 2 figure
Hara's Theorem and W-exchange in Hyperon Weak Radiative Decays
We reconsider Hara's theorem in its relation to the well-known properties of
beta-decay. All assumptions necessary for the theorem to be true are explicitly
formulated. Further, we study the W-exchange contribution to weak radiative
decays and show that it does not violate Hara's theorem. However, this
contribution reveals the essential role of particle mixing in symmetry
considerations and some peculiar features of gauge-invariant amplitudes under
perturbative expansion. Together they explain an effect, which was treated as
contradicting Hara's theorem, without any violation. The properties of
W-exchange we describe here may have more general importance and should be
taken into account in further detailed calculations of weak processes.Comment: 14 pages, LATEX, no figure
Local mean-field study of capillary condensation in silica aerogels
We apply local mean-field (i.e. density functional) theory to a lattice model
of a fluid in contact with a dilute, disordered gel network. The gel structure
is described by a diffusion-limited cluster aggregation model. We focus on the
influence of porosity on both the hysteretic and the equilibrium behavior of
the fluid as one varies the chemical potential at low temperature. We show that
the shape of the hysteresis loop changes from smooth to rectangular as the
porosity increases and that this change is associated to disorder-induced
out-of-equilibrium phase transitions that differ on adsorption and on
desorption. Our results provide insight in the behavior of He in silica
aerogels.Comment: 19 figure
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