3,062 research outputs found
Inclusion of seasonal variation in river system microbial communities and phototroph activity increases environmental relevance of laboratory chemical persistence tests
Regulatory tests assess crop protection product environmental fate and toxicity before approval for commercial use. Although globally applied laboratory tests can assess biodegradation, they lack environmental complexity. Microbial communities are subject to temporal and spatial variation, but there is little consideration of these microbial dynamics in the laboratory. Here, we investigated seasonal variation in the microbial composition of water and sediment from a UK river across a two-year time course and determined its effect on the outcome of water-sediment (OECD 308) and water-only (OECD 309) biodegradation tests, using the fungicide isopyrazam. These OECD tests are performed under dark conditions, so test systems incubated under non-UV light:dark cycles were also included to determine the impact on both inoculum characteristics and biodegradation. Isopyrazam degradation was faster when incubated under non-UV light at all collection times in water-sediment microcosms, suggesting that phototrophic communities can metabolise isopyrazam throughout the year. Degradation rate varied seasonally between inoculum collection times only in microcosms incubated in the light, but isopyrazam mineralisation to 14CO2 varied seasonally under both light and dark conditions, suggesting that heterotrophic communities may also play a role in degradation. Bacterial and phototroph communities varied across time, but there was no clear link between water or sediment microbial composition and variation in degradation rate. During the test period, inoculum microbial community composition changed, particularly in non-UV light incubated microcosms. Overall, we show that regulatory test outcome is not influenced by temporal variation in microbial community structure; however, biodegradation rates from higher tier studies with improved environmental realism, e.g. through addition of non-UV light, may be more variable. These data suggest that standardised OECD tests can provide a conservative estimate of pesticide persistence end points and that additional tests including non-UV light could help bridge the gap between standard tests and field studies
Exciting dark matter in the galactic center
We reconsider the proposal of excited dark matter (DM) as an explanation for
excess 511 keV gamma rays from positrons in the galactic center. We
quantitatively compute the cross section for DM annihilation to nearby excited
states, mediated by exchange of a new light gauge boson with off-diagonal
couplings to the DM states. In models where both excited states must be heavy
enough to decay into e^+ e^- and the ground state, the predicted rate of
positron production is never large enough to agree with observations, unless
one makes extreme assumptions about the local circular velocity in the Milky
Way, or alternatively if there exists a metastable population of DM states
which can be excited through a mass gap of less than 650 keV, before decaying
into electrons and positrons.Comment: Dedicated to the memory of Lev Kofman; 16 pages, 9 figures; v3 added
refs, minor changes, accepted to PR
The Parameterized Post-Friedmann Framework for Theories of Modified Gravity: Concepts, Formalism and Examples
A unified framework for theories of modified gravity will be an essential
tool for interpreting the forthcoming deluge of cosmological data. We present
such a formalism, the Parameterized Post-Friedmann framework (PPF), which
parameterizes the cosmological perturbation theory of a wide variety of
modified gravity models. PPF is able to handle spin-0 degrees of freedom from
new scalar, vector and tensor fields, meaning that it is not restricted to
simple models based solely on cosmological scalar fields. A direct
correspondence is maintained between the parameterization and the underlying
space of theories, which allows us to build up a `dictionary' of modified
gravity theories and their PPF correspondences. In this paper we describe the
construction of the parameterization and demonstrate its use through a number
of worked examples relevant to the current literature. We indicate how the
formalism will be implemented numerically, so that the dictionary of modified
gravity can be pitted against forthcoming observations.Comment: 24 pages, updated to match version published in PRD. Discussion of
section 4 extended. Suggestions for the busy reader are given at the end of
section
Nonlinear dynamics of two coupled nano-electromechanical resonators
As a model of coupled nano-electromechanical resonantors we study two
nonlinear driven oscillators with an arbitrary coupling strength between them.
Analytical expressions are derived for the oscillation amplitudes as a function
of the driving frequency and for the energy transfer rate between the two
oscillators. The nonlinear restoring forces induce the expected nonlinear
resonance structures in the amplitude-frequency characteristics with asymmetric
resonance peaks. The corresponding multistable behavior is shown to be an
efficient tool to control the energy transfer arising from the sensitive
response to small changes in the driving frequency. Our results imply that the
nonlinear response can be exploited to design precise sensors for mass or force
detection experiments based on nano-electromechanical resonators.Comment: 19 pages, 2 figure
Sixth-Order Vacuum-Polarization Contribution to the Lamb Shift of the Muonic Hydrogen
The sixth-order electron-loop vacuum-polarization contribution to the
Lamb shift of the muonic hydrogen ( bound
state) has been evaluated numerically. Our result is 0.007608(1) meV. This
eliminates the largest uncertainty in the theoretical calculation. Combined
with the proposed precision measurement of the Lamb shift it will lead to a
very precise determination of the proton charge radius.Comment: 4 pages, 5 figures the totoal LS number is change
On the combination of omics data for prediction of binary outcomes
Enrichment of predictive models with new biomolecular markers is an important
task in high-dimensional omic applications. Increasingly, clinical studies
include several sets of such omics markers available for each patient,
measuring different levels of biological variation. As a result, one of the
main challenges in predictive research is the integration of different sources
of omic biomarkers for the prediction of health traits. We review several
approaches for the combination of omic markers in the context of binary outcome
prediction, all based on double cross-validation and regularized regression
models. We evaluate their performance in terms of calibration and
discrimination and we compare their performance with respect to single-omic
source predictions. We illustrate the methods through the analysis of two real
datasets. On the one hand, we consider the combination of two fractions of
proteomic mass spectrometry for the calibration of a diagnostic rule for the
detection of early-stage breast cancer. On the other hand, we consider
transcriptomics and metabolomics as predictors of obesity using data from the
Dietary, Lifestyle, and Genetic determinants of Obesity and Metabolic syndrome
(DILGOM) study, a population-based cohort, from Finland
On the regularization scheme and gauge choice ambiguities in topologically massive gauge theories
It is demonstrated that in the (2+1)-dimensional topologically massive gauge
theories an agreement of the Pauli-Villars regularization scheme with the other
schemes can be achieved by employing pairs of auxiliary fermions with the
opposite sign masses. This approach does not introduce additional violation of
discrete (P and T) symmetries. Although it breaks the local gauge symmetry only
in the regulator fields' sector, its trace disappears completely after removing
the regularization as a result of superrenormalizability of the model. It is
shown also that analogous extension of the Pauli-Villars regularization in the
vector particle sector can be used to agree the arbitrary covariant gauge
results with the Landau ones. The source of ambiguities in the covariant gauges
is studied in detail. It is demonstrated that in gauges that are softer in the
infrared region (e.g. Coulomb or axial) nonphysical ambiguities inherent to the
covariant gauges do not arise.Comment: Latex, 13 pages. Replaced mainly to change preprint references to
journal one
A 4 Ga record of granitic heat production: Implications for geodynamic evolution and crustal composition of the early Earth
The radiogenic heat produced by granites has a significant influence on the thermal state of the crust due to both their relatively high heat production with respect to most rock types and high abundance. However, the variations in present day heat production with age are generally based on relatively few measurements that are poorly distributed geographically. In this study, we construct a global model for the heat production of granitic rocks for the past 4 Ga using 13,400 geochemical analyses. We observe a nearly monotonic increase in radiogenic heat production from 4.0 to 2.0 Ga, which mirrors a shift from more TTG-like calcic to more alkalic compositions. This shift towards high-heat-producing granites post-2.0 Ga is often attributed to enrichment related to reworking and/or erosion. However, there is a strong correlation between granitic heat production with that of similarly-aged basalts and gabbros, which suggests a dominant mantle-level component to granite generation rather than crustal reworking. Secular cooling and mantle depletion may affect heat production, but the signal is complex and cannot easily explain the heat production with age profile. The most likely mechanism to describe the observed heat production–age pattern is one of selective preservation as a consequence of thermal stability. High heat producing terranes that were not stable during the Archean become increasingly stable towards the present. This selective preservation model has significant implications for the growth and composition of the continental crust. Ferroan, alkalic and felsic compositions were less thermally stable in the Archean due to their generally higher heat production and thus may have been more common in the early Earth than assumed by most compositional models. The temporal heat production model determined in this study can be used to improve geotherm models, particularly within ancient terranes.D.Hasterok, M.Gard, G.Cox, M.Han
What are the Best Hierarchical Descriptors for Complex Networks?
This work reviews several hierarchical measurements of the topology of
complex networks and then applies feature selection concepts and methods in
order to quantify the relative importance of each measurement with respect to
the discrimination between four representative theoretical network models,
namely Erd\"{o}s-R\'enyi, Barab\'asi-Albert, Watts-Strogatz as well as a
geographical type of network. The obtained results confirmed that the four
models can be well-separated by using a combination of measurements. In
addition, the relative contribution of each considered feature for the overall
discrimination of the models was quantified in terms of the respective weights
in the canonical projection into two dimensions, with the traditional
clustering coefficient, hierarchical clustering coefficient and neighborhood
clustering coefficient resulting particularly effective. Interestingly, the
average shortest path length and hierarchical node degrees contributed little
for the separation of the four network models.Comment: 9 pages, 4 figure
The Distribution of Constituent Charm Quarks in the Hadron
Using a statistical approach in the framework of non-covariant perturbation
theory the distributions for light and charmed quarks in the hadron have been
derived, taking into account the mass of the charmed quark. The parameters of
the model have been extracted from the comparison with NA3 data on
hadroproduction of J/psi particles. A reanalysis of the EMC data on charm
production in muon-nucleon scattering has been performed. It has been found in
comparison with the conventional source of charmed quarks from photon-gluon
fusion, that the EMC data indicate the presence of an additional contribution
from deep-inelastic scattering on charmed quarks at large x. The resulting
admixture of the Fock states, containing charmed quarks in the decomposition of
the proton wave function is of the order of 1%. The approach presented for the
excitation of the Fock states with charmed quarks can also be applied to states
with beauty quarks as well as to the hadronic component of the virtual photon
(resolved photon component).Comment: 23 pages, 4 PostScript figures, Latex2e. In revised version in
comparison with the original one all (?) mistypings have been corrected, one
more thank has been added and the comparison of the pion and the proton J/psi
production is described in more detai
- …