283 research outputs found
Quality variation in comminuted forest fuels delivered during the winter in north Sweden
Swedish legislation stipulates the precision required for estimates of parameters used to determine the value of various forest fuels. The net energy value of fuel, as it is received, is often used to set the trade price. The estimate of energy content is based on the moisture content of samples taken from each truckload and the weight of the biomass; the ash content and net calorific value are measured a few times each year. Hence, it is necessary to know the variation in moisture content to ensure that a sufficient number of samples are taken, a number based on the allowed variation and precision of estimates, as defined in the legalization. In this study, the variation in moisture content was measured by taking samples from 18 truckloads of comminuted forest fuels during the winter. The results showed that the current sampling regime, i.e., manually taking four samples from each truckload, is sufficient for deliveries with 10 truckloads for logging residue chips and 4 for stem wood chips. The number of samples should be increased to 12-43, 8-21, and, 17-82 depending on assortment for what the measuring act defines as large deliveries (>=50 tonnes; >=3 truckloads), medium-sized deliveries (25 tonnes; 2 truckloads) and single truck deliveries (<= 25 tonnes; 1 truckload), respectively. Current research into fast online sampling and analysis methods could resolve this issue for small deliveries
Models for Galactic cosmic-ray propagation
A new numerical model of particle propagation in the Galaxy has been
developed, which allows the study of cosmic-ray and gamma-ray production and
propagation in 2D or 3D, including a full reaction network. This is a further
development of the code which has been used for studies of cosmic ray
reacceleration, Galactic halo size, antiprotons and positrons in cosmic rays,
the interpretation of diffuse continuum gamma rays, and dark matter. In this
paper we illustrate recent results focussing on B/C, sub-Fe/Fe, ACE radioactive
isotope data, source abundances and antiprotons. From the radioactive nuclei we
derive a range of 3-7 kpc for the height of the cosmic-ray halo.Comment: Invited talk at the 33rd COSPAR Scientific Assembly (Warsaw 2000); 10
pages including 10 ps-figures and 2 tables, latex2e, uses cospar.sty. To
appear in Advances in Space Research 2001. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Direct Constraints on Minimal Supersymmetry from Fermi-LAT Observations of the Dwarf Galaxy Segue 1
The dwarf galaxy Segue 1 is one of the most promising targets for the
indirect detection of dark matter. Here we examine what constraints 9 months of
Fermi-LAT gamma-ray observations of Segue 1 place upon the Constrained Minimal
Supersymmetric Standard Model (CMSSM), with the lightest neutralino as the dark
matter particle. We use nested sampling to explore the CMSSM parameter space,
simultaneously fitting other relevant constraints from accelerator bounds, the
relic density, electroweak precision observables, the anomalous magnetic moment
of the muon and B-physics. We include spectral and spatial fits to the Fermi
observations, a full treatment of the instrumental response and its related
uncertainty, and detailed background models. We also perform an extrapolation
to 5 years of observations, assuming no signal is observed from Segue 1 in that
time. Results marginally disfavour models with low neutralino masses and high
annihilation cross-sections. Virtually all of these models are however already
disfavoured by existing experimental or relic density constraints.Comment: 22 pages, 5 figures; added extra scans with extreme halo parameters,
expanded introduction and discussion in response to referee's comment
Evolution of associative learning in chemical networks
Organisms that can learn about their environment and modify their behaviour appropriately during their lifetime are more likely to survive and reproduce than organisms that do not. While associative learning – the ability to detect correlated features of the environment – has been studied extensively in nervous systems, where the underlying mechanisms are reasonably well understood, mechanisms within single cells that could allow associative learning have received little attention. Here, using in silico evolution of chemical networks, we show that there exists a diversity of remarkably simple and plausible chemical solutions to the associative learning problem, the simplest of which uses only one core chemical reaction. We then asked to what extent a linear combination of chemical concentrations in the network could approximate the ideal Bayesian posterior of an environment given the stimulus history so far? This Bayesian analysis revealed the ’memory traces’ of the chemical network. The implication of this paper is that there is little reason to believe that a lack of suitable phenotypic variation would prevent associative learning from evolving in cell signalling, metabolic, gene regulatory, or a mixture of these networks in cells
The fine-grained phase-space structure of Cold Dark Matter halos
We present a new and completely general technique for calculating the
fine-grained phase-space structure of dark matter throughout the Galactic halo.
Our goal is to understand this structure on the scales relevant for direct and
indirect detection experiments. Our method is based on evaluating the geodesic
deviation equation along the trajectories of individual DM particles. It
requires no assumptions about the symmetry or stationarity of the halo
formation process. In this paper we study general static potentials which
exhibit more complex behaviour than the separable potentials studied
previously. For ellipsoidal logarithmic potentials with a core, phase mixing is
sensitive to the resonance structure, as indicated by the number of independent
orbital frequencies. Regions of chaotic mixing can be identified by the very
rapid decrease in the real space density of the associated dark matter streams.
We also study the evolution of stream density in ellipsoidal NFW halos with
radially varying isopotential shape, showing that if such a model is applied to
the Galactic halo, at least streams are expected near the Sun. The most
novel aspect of our approach is that general non-static systems can be studied
through implementation in a cosmological N-body code. Such an implementation
allows a robust and accurate evaluation of the enhancements in annihilation
radiation due to fine-scale structure such as caustics. We embed the scheme in
the current state-of-the-art code GADGET-3 and present tests which demonstrate
that N-body discreteness effects can be kept under control in realistic
configurations.Comment: 20 pages, 24 figures, submitted to MNRA
Antideuterons as a Signature of Supersymmetric Dark Matter
Once the energy spectrum of the secondary component is well understood,
measurements of the antiproton cosmic-ray flux at the Earth will be a powerful
way to indirectly probe for the existence of supersymmetric relics in the
galactic halo. Unfortunately, it is still spoilt by considerable theoretical
uncertainties. As shown in this work, searches for low-energy antideuterons
appear in the mean time as a plausible alternative, worth being explored. Above
a few GeV/n, a dozen spallation antideuterons should be collected by the future
AMS experiment on board ISSA. For energies less than about 3 GeV/n, the
antideuteron spallation component becomes negligible and may be supplanted by a
potential supersymmetric signal. If a few low-energy antideuterons are
discovered, this should be seriously taken as a clue for the existence of
massive neutralinos in the Milky Way.Comment: 16 pages, 9 figure
Further investigation of a relic neutralino as a possible origin of an annual-modulation effect in WIMP direct search
We analyze the annual-modulation effect, measured by the DAMA Collaboration
with the new implementation of a further two-years running, in the context of a
possible interpretation in terms of relic neutralinos. We impose over the set
of supersymmetric configurations, selected by the annual-modulation data, the
constraints derived from WIMP indirect measurements, and discuss the features
of the ensuing relic neutralinos. We critically discuss the sources of the main
theoretical uncertainties in the analysis of event rates for direct and
indirect WIMP searches.Comment: 29 pages, 12 figures, typeset with ReVTeX. In order to reduce size,
the version on the archive has low resolution figures. A full version of the
paper can be found at http://www.to.infn.it/~fornengo/papers
Bayesian Strong Gravitational-Lens Modeling on Adaptive Grids: Objective Detection of Mass Substructure in Galaxies
We introduce a new adaptive and fully Bayesian grid-based method to model
strong gravitational lenses with extended images. The primary goal of this
method is to quantify the level of luminous and dark-mass substructure in
massive galaxies, through their effect on highly-magnified arcs and Einstein
rings. The method is adaptive on the source plane, where a Delaunay
tessellation is defined according to the lens mapping of a regular grid onto
the source plane. The Bayesian penalty function allows us to recover the best
non-linear potential-model parameters and/or a grid-based potential correction
and to objectively quantify the level of regularization for both the source and
the potential. In addition, we implement a Nested-Sampling technique to
quantify the errors on all non-linear mass model parameters -- ... -- and allow
an objective ranking of different potential models in terms of the marginalized
evidence. In particular, we are interested in comparing very smooth lens mass
models with ones that contain mass-substructures. The algorithm has been tested
on a range of simulated data sets, created from a model of a realistic lens
system. One of the lens systems is characterized by a smooth potential with a
power-law density profile, twelve include a NFW dark-matter substructure of
different masses and at different positions and one contains two NFW dark
substructures with the same mass but with different positions. Reconstruction
of the source and of the lens potential for all of these systems shows the
method is able, in a realistic scenario, to identify perturbations with masses
>=10^7 solar mass when located on the Einstein ring. For positions both inside
and outside of the ring, masses of at least 10^9 solar mass are required (...).Comment: 21 pages, 15 figures, 4 tables; accepted for publication in MNRA
The phase-space structure of a dark-matter halo: Implications for dark-matter direct detection experiments
We study the phase-space structure of a dark-matter halo formed in a high
resolution simulation of a Lambda CDM cosmology. Our goal is to quantify how
much substructure is left over from the inhomogeneous growth of the halo, and
how it may affect the signal in experiments aimed at detecting the dark matter
particles directly. If we focus on the equivalent of ``Solar vicinity'', we
find that the dark-matter is smoothly distributed in space. The probability of
detecting particles bound within dense lumps of individual mass less than 10^7
M_\sun h^{-1} is small, less than 10^{-2}. The velocity ellipsoid in the Solar
neighbourhood deviates only slightly from a multivariate Gaussian, and can be
thought of as a superposition of thousands of kinematically cold streams. The
motions of the most energetic particles are, however, strongly clumped and
highly anisotropic. We conclude that experiments may safely assume a smooth
multivariate Gaussian distribution to represent the kinematics of dark-matter
particles in the Solar neighbourhood. Experiments sensitive to the direction of
motion of the incident particles could exploit the expected anisotropy to learn
about the recent merging history of our Galaxy.Comment: 13 pages, 13 figures, Phys. Rev. D in press. Postscript version with
high resolution figures available from
http://www.mpa-garching.mpg.de/~ahelmi/research/lcdm_dm.html; some changes in
the text; constraints on the effect of bound dark-matter lumps revised;
remaining conclusions unchange
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