129 research outputs found
Correlation Between the Deuteron Characteristics and the Low-energy Triplet np Scattering Parameters
The correlation relationship between the deuteron asymptotic normalization
constant, , and the triplet np scattering length, , is
investigated. It is found that 99.7% of the asymptotic constant is
determined by the scattering length . It is shown that the linear
correlation relationship between the quantities and
provides a good test of correctness of various models of nucleon-nucleon
interaction. It is revealed that, for the normalization constant and
for the root-mean-square deuteron radius , the results obtained with the
experimental value recommended at present for the triplet scattering length
are exaggerated with respect to their experimental counterparts. By
using the latest experimental phase shifts of Arndt et al., we obtain, for the
low-energy scattering parameters (, , ) and for the
deuteron characteristics (, ), results that comply well with
experimental data.Comment: 19 pages, 1 figure, To be published in Physics of Atomic Nucle
Description of the two-nucleon system on the basis of the Bargmann representation of the S matrix
For the effective-range function , a pole approximation that
involves a small number of parameters is derived on the basis of the Bargmann
representation of the matrix. The parameters of this representation, which
have a clear physical meaning, are related to the parameters of the Bargmann
matrix by simple equations. By using a polynomial least-squares fit to the
function at low energies, the triplet low-energy parameters of
neutron-proton scattering are obtained for the latest experimental data of
Arndt et al. on phase shifts. The results are fm, fm, and fm. With allowance for the values found for the
low-energy scattering parameters and for the pole parameter, the pole
approximation of the function provides an excellent description
of the triplet phase shift for neutron-proton scattering over a wide energy
range (MeV), substantially improving the
description at low energies as well. For the experimental phase shifts of Arndt
et al., the triplet shape parameters of the effective-range expansion
are obtained by using the pole approximation. The description of the phase
shift by means of the effective-range expansion featuring values found for the
low-energy scattering parameters proves to be fairly accurate over a broad
energy region extending to energy values approximately equal to the energy at
which this phase shift changes sign, this being indicative of a high accuracy
and a considerable value of the effective-range expansion in describing
experimental data on nucleon-nucleon scattering. The properties of the deuteron
that were calculated by using various approximations of the effective-range
function comply well with their experimental values.Comment: 39 pages, 3 figure
Persistent and polarised global actin flow is essential for directionality during cell migration
Cell migration is hypothesized to involve a cycle of behaviours beginning with leading edge extension. However, recent evidence suggests that the leading edge may be dispensable for migration, raising the question of what actually controls cell directionality. Here, we exploit the embryonic migration of Drosophila macrophages to bridge the different temporal scales of the behaviours controlling motility. This approach reveals that edge fluctuations during random motility are not persistent and are weakly correlated with motion. In contrast, flow of the actin network behind the leading edge is highly persistent. Quantification of actin flow structure during migration reveals a stable organization and asymmetry in the cell-wide flowfield that strongly correlates with cell directionality. This organization is regulated by a gradient of actin network compression and destruction, which is controlled by myosin contraction and cofilin-mediated disassembly. It is this stable actin-flow polarity, which integrates rapid fluctuations of the leading edge, that controls inherent cellular persistence
Determinants of synaptic integration and heterogeneity in rebound firing explored with data-driven models of deep cerebellar nucleus cells
Significant inroads have been made to understand cerebellar cortical processing but neural coding at the output stage of the cerebellum in the deep cerebellar nuclei (DCN) remains poorly understood. The DCN are unlikely to just present a relay nucleus because Purkinje cell inhibition has to be turned into an excitatory output signal, and DCN neurons exhibit complex intrinsic properties. In particular, DCN neurons exhibit a range of rebound spiking properties following hyperpolarizing current injection, raising the question how this could contribute to signal processing in behaving animals. Computer modeling presents an ideal tool to investigate how intrinsic voltage-gated conductances in DCN neurons could generate the heterogeneous firing behavior observed, and what input conditions could result in rebound responses. To enable such an investigation we built a compartmental DCN neuron model with a full dendritic morphology and appropriate active conductances. We generated a good match of our simulations with DCN current clamp data we recorded in acute slices, including the heterogeneity in the rebound responses. We then examined how inhibitory and excitatory synaptic input interacted with these intrinsic conductances to control DCN firing. We found that the output spiking of the model reflected the ongoing balance of excitatory and inhibitory input rates and that changing the level of inhibition performed an additive operation. Rebound firing following strong Purkinje cell input bursts was also possible, but only if the chloride reversal potential was more negative than −70 mV to allow de-inactivation of rebound currents. Fast rebound bursts due to T-type calcium current and slow rebounds due to persistent sodium current could be differentially regulated by synaptic input, and the pattern of these rebounds was further influenced by HCN current. Our findings suggest that active properties of DCN neurons could play a crucial role for signal processing in the cerebellum
Low potency toxins reveal dense interaction networks in metabolism
Background
The chemicals of metabolism are constructed of a small set of atoms and bonds. This may be because chemical structures outside the chemical space in which life operates are incompatible with biochemistry, or because mechanisms to make or utilize such excluded structures has not evolved. In this paper I address the extent to which biochemistry is restricted to a small fraction of the chemical space of possible chemicals, a restricted subset that I call Biochemical Space. I explore evidence that this restriction is at least in part due to selection again specific structures, and suggest a mechanism by which this occurs.
Results
Chemicals that contain structures that our outside Biochemical Space (UnBiological groups) are more likely to be toxic to a wide range of organisms, even though they have no specifically toxic groups and no obvious mechanism of toxicity. This correlation of UnBiological with toxicity is stronger for low potency (millimolar) toxins. I relate this to the observation that most chemicals interact with many biological structures at low millimolar toxicity. I hypothesise that life has to select its components not only to have a specific set of functions but also to avoid interactions with all the other components of life that might degrade their function.
Conclusions
The chemistry of life has to form a dense, self-consistent network of chemical structures, and cannot easily be arbitrarily extended. The toxicity of arbitrary chemicals is a reflection of the disruption to that network occasioned by trying to insert a chemical into it without also selecting all the other components to tolerate that chemical. This suggests new ways to test for the toxicity of chemicals, and that engineering organisms to make high concentrations of materials such as chemical precursors or fuels may require more substantial engineering than just of the synthetic pathways involved
Search For Charged Higgs Decays of the Top Quark Using Hadronic Decays of the Tau Lepton
This Letter describes a direct search for charged Higgs boson production in
proton-antiproton collisions at sqrt(s)=1.8 TeV recorded by the Collider
Detector at Fermilab. Two-Higgs-doublet extensions to the standard model
predict the existence of charged Higgs bosons. In such models, the branching
fraction for top quarks B(t --> H b --> tau nu b) can be large. This search
uses the hadronic decays of the tau lepton in this channel to significantly
extend previous limits on charged Higgs production.Comment: 6pages, 4 figures, 1 table; LaTeX; submitted to PR
Limits on and couplings from and production in collisions at TeV
Direct limits are set on and three-boson couplings in a
search for and production with high transverse momentum in
collisions at TeV, using the Collider Detector
at Fermilab. The results are in agreement with the SU(2) U(1) model of
electroweak interactions. Assuming Standard Model coupling, the the
limits are interpreted as direct evidence for a non-zero coupling at
subprocess energies near 500 GeV. Alternatively, assumiong identical and
couplings, bounds and are obtained at CL for a form factor scale 1000 GeV.Comment: 16 pages, submitted to PRL, URL:
http://www-cdf.fnal.gov/physics/pub95/cdf2951_vvprl.p
Measurement of the Meson Differential Cross Section, , in Collisions at TeV
This paper presents the first direct measurement of the meson
differential cross section, , in collisions at
TeV using a sample of pb accumulated by
the Collider Detector at Fermilab (CDF). The cross section is measured in the
central rapidity region GeV/ by fully
reconstructing the meson decays and , where and .
A comparison is made to the theoretical QCD prediction calculated at
next-to-leading order.Comment: 14 pages. Submitted to Phys. Rev. Lett. The postscript file is at
http://www-cdf.fnal.gov/physics/pub95/cdf2893_bexcl_xsection.p
Analysis of a fragmentary diatom record from Lake Van (Turkey) reveals substantial lake-level variability during previous interglacials MIS7 and MIS5e
Global gradients of coral exposure to environmental stresses and implications for local management
Background: The decline of coral reefs globally underscores the need for a spatial assessment of their exposure to multiple
environmental stressors to estimate vulnerability and evaluate potential counter-measures.
Methodology/Principal Findings: This study combined global spatial gradients of coral exposure to radiation stress factors
(temperature, UV light and doldrums), stress-reinforcing factors (sedimentation and eutrophication), and stress-reducing
factors (temperature variability and tidal amplitude) to produce a global map of coral exposure and identify areas where
exposure depends on factors that can be locally managed. A systems analytical approach was used to define interactions
between radiation stress variables, stress reinforcing variables and stress reducing variables. Fuzzy logic and spatial
ordinations were employed to quantify coral exposure to these stressors. Globally, corals are exposed to radiation and
reinforcing stress, albeit with high spatial variability within regions. Based on ordination of exposure grades, regions group
into two clusters. The first cluster was composed of severely exposed regions with high radiation and low reducing stress
scores (South East Asia, Micronesia, Eastern Pacific and the central Indian Ocean) or alternatively high reinforcing stress
scores (the Middle East and the Western Australia). The second cluster was composed of moderately to highly exposed
regions with moderate to high scores in both radiation and reducing factors (Caribbean, Great Barrier Reef (GBR), Central
Pacific, Polynesia and the western Indian Ocean) where the GBR was strongly associated with reinforcing stress.
Conclusions/Significance: Despite radiation stress being the most dominant stressor, the exposure of coral reefs could be
reduced by locally managing chronic human impacts that act to reinforce radiation stress. Future research and management
efforts should focus on incorporating the factors that mitigate the effect of coral stressors until long-term carbon reductions
are achieved through global negotiations
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