14,523 research outputs found
Regge Behaviour from an Environmentally Friendly Renormalization Group
The asymptotic behaviour of cubic field theories is investigated in the Regge
limit using the techniques of environmentally friendly renormalization,
environmentally friendly in the present context meaning asymmetric in its
momentum dependence. In particular we consider the crossover between large and
small energies at fixed momentum transfer for a model scalar theory of the type
phi^2 psi. The asymptotic forms of the crossover scaling functions are
exhibited for all two particle scattering processes in this channel to one loop
in a renormalization group improved perturbation theory.Comment: 9 pages text, one figure, LaTeX, uses psfig.sty. Revised version
submitted to Phys. Lett. B., besides minor changes a figure to illustrate the
conventions and a discussion of the full crossover function have been adde
Radiative diffusivity factors in cirrus and stratocumulus clouds: Application to two-stream models
A diffusion-like description of radiative transfer in clouds and the free atmosphere is often used. The two stream model is probably the best known example of such a description. The main idea behind the approach is that only the first few moments of radiance are needed to describe the radiative field correctly. Integration smooths details of the angular distribution of specific intensity and it is assumed that the closure parameters of the theory (diffusivity factors) are only weakly dependent on the distribution. The diffusivity factors are investigated using the results obtained from both Stratocumulus and Cirrus phases of FIRE experiment. A new theoretical framework is described in which two (upwards and downwards) diffusivity factors are used and a detailed multistream model is used to provide further insight about both the diffusivity factors and their dependence on scattering properties of clouds
Selective Michael additions to alkylidenemalonates using thiourea-based bifunctional organocatalysts
Bifunctional thiourea catalysts have been found to be excellent promoters of the challenging Michael addition to alkylidenemalonates giving high yields of up to 99%. Substrate structure was important for enantiodiscrimination, with aryl alkylidenemalonate acceptors furnishing products with ee values of up to 73%
Self-organization, scaling and collapse in a coupled automaton model of foragers and vegetation resources with seed dispersal
We introduce a model of traveling agents ({\it e.g.} frugivorous animals) who
feed on randomly located vegetation patches and disperse their seeds, thus
modifying the spatial distribution of resources in the long term. It is assumed
that the survival probability of a seed increases with the distance to the
parent patch and decreases with the size of the colonized patch. In turn, the
foraging agents use a deterministic strategy with memory, that makes them visit
the largest possible patches accessible within minimal travelling distances.
The combination of these interactions produce complex spatio-temporal patterns.
If the patches have a small initial size, the vegetation total mass (biomass)
increases with time and reaches a maximum corresponding to a self-organized
critical state with power-law distributed patch sizes and L\'evy-like movement
patterns for the foragers. However, this state collapses as the biomass sharply
decreases to reach a noisy stationary regime characterized by corrections to
scaling. In systems with low plant competition, the efficiency of the foraging
rules leads to the formation of heterogeneous vegetation patterns with
frequency spectra, and contributes, rather counter-intuitively,
to lower the biomass levels.Comment: 11 pages, 5 figure
Septic shock: the changing Zeitgeist of management
Most interventions in critically unwell patients with septic shock are poorly supported by evidence, in part reflecting the difficulty of conducting trials in this heterogeneous group. Four important clinical trials in 2001-2 appeared to demonstrate mortality benefits associated with early goal-directed resuscitation, intensive glycaemic control, physiological-dose steroid replacement and activated protein C. However, recent evidence has not confirmed the beneficial effect of these interventions
Spin accumulation in forward-biased MnAs/GaAs Schottky diodes
We describe a new means for electrically creating spin polarization in
semiconductors. In contrast to spin injection of electrons by tunneling through
a reverse-biased Schottky barrier, we observe spin accumulation at the
metal/semiconductor interface of forward-biased ferromagnetic Schottky diodes,
which is consistent with a theory of spin-dependent reflection off the
interface. Spatiotemporal Kerr microscopy is used to image the electron spin
and the resulting dynamic nuclear polarization that arises from the non
equilibrium carrier polarization.Comment: 13 pages, 4 figures, submitted for publicatio
A Consumer Test of Canned Seasoned Salad Tomatoes
Consumer, Canned Tomatoes, Salad Tomatoes, Seasoned Tomatoes, Consumer/Household Economics, Food Consumption/Nutrition/Food Safety,
Asymmetric quantum error correction via code conversion
In many physical systems it is expected that environmental decoherence will
exhibit an asymmetry between dephasing and relaxation that may result in qubits
experiencing discrete phase errors more frequently than discrete bit errors. In
the presence of such an error asymmetry, an appropriately asymmetric quantum
code - that is, a code that can correct more phase errors than bit errors -
will be more efficient than a traditional, symmetric quantum code. Here we
construct fault tolerant circuits to convert between an asymmetric subsystem
code and a symmetric subsystem code. We show that, for a moderate error
asymmetry, the failure rate of a logical circuit can be reduced by using a
combined symmetric asymmetric system and that doing so does not preclude
universality.Comment: 5 pages, 8 figures, presentation revised, figures and references
adde
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