111,508 research outputs found
Asymmetries arising from the space-filling nature of vascular networks
Cardiovascular networks span the body by branching across many generations of
vessels. The resulting structure delivers blood over long distances to supply
all cells with oxygen via the relatively short-range process of diffusion at
the capillary level. The structural features of the network that accomplish
this density and ubiquity of capillaries are often called space-filling. There
are multiple strategies to fill a space, but some strategies do not lead to
biologically adaptive structures by requiring too much construction material or
space, delivering resources too slowly, or using too much power to move blood
through the system. We empirically measure the structure of real networks (18
humans and 1 mouse) and compare these observations with predictions of model
networks that are space-filling and constrained by a few guiding biological
principles. We devise a numerical method that enables the investigation of
space-filling strategies and determination of which biological principles
influence network structure. Optimization for only a single principle creates
unrealistic networks that represent an extreme limit of the possible structures
that could be observed in nature. We first study these extreme limits for two
competing principles, minimal total material and minimal path lengths. We
combine these two principles and enforce various thresholds for balance in the
network hierarchy, which provides a novel approach that highlights the
trade-offs faced by biological networks and yields predictions that better
match our empirical data.Comment: 17 pages, 15 figure
Automated Markerless Extraction of Walking People Using Deformable Contour Models
We develop a new automated markerless motion capture system for the analysis of walking people. We employ global evidence gathering techniques guided by biomechanical analysis to robustly extract articulated motion. This forms a basis for new deformable contour models, using local image cues to capture shape and motion at a more detailed level. We extend the greedy snake formulation to include temporal constraints and occlusion modelling, increasing the capability of this technique when dealing with cluttered and self-occluding extraction targets. This approach is evaluated on a large database of indoor and outdoor video data, demonstrating fast and autonomous motion capture for walking people
The Formation of Cosmic Structures in a Light Gravitino Dominated Universe
We analyse the formation of cosmic structures in models where the dark matter
is dominated by light gravitinos with mass of eV -- 1 keV, as predicted
by gauge-mediated supersymmetry (SUSY) breaking models. After evaluating the
number of degrees of freedom at the gravitinos decoupling (), we compute
the transfer function for matter fluctuations and show that gravitinos behave
like warm dark matter (WDM) with free-streaming scale comparable to the galaxy
mass scale. We consider different low-density variants of the WDM model, both
with and without cosmological constant, and compare the predictions on the
abundances of neutral hydrogen within high-redshift damped Ly-- systems
and on the number density of local galaxy clusters with the corresponding
observational constraints. We find that none of the models satisfies both
constraints at the same time, unless a rather small value (\mincir
0.4) and a rather large Hubble parameter (\magcir 0.9) is assumed.
Furthermore, in a model with warm + hot dark matter, with hot component
provided by massive neutrinos, the strong suppression of fluctuation on scales
of \sim 1\hm precludes the formation of high-redshift objects, when the
low-- cluster abundance is required. We conclude that all different variants
of a light gravitino DM dominated model show strong difficulties for what
concerns cosmic structure formation.
This gives a severe cosmological constraint on the gauge-mediated SUSY
breaking scheme.Comment: 28 pages,Latex, submitted for publication to Phys.Rev.
Quark-Lepton Unification and Eight-Fold Ambiguity in the Left-Right Symmetric Seesaw Mechanism
In many extensions of the Standard Model, including a broad class of
left-right symmetric and Grand Unified theories, the light neutrino mass matrix
is given by the left-right symmetric seesaw formula , in which the right-handed neutrino mass
matrix and the triplet couplings are proportional to the same matrix
f. We propose a systematic procedure for reconstructing the solutions (in
the n-family case) for the matrix f as a function of the Dirac neutrino
couplings and of the light neutrino mass parameters, which can
be used in both analytical and numerical studies. We apply this procedure to a
particular class of supersymmetric SO(10) models with two 10-dimensional and a
pair of representations in the Higgs sector, and study the
properties of the corresponding 8 right-handed neutrino spectra. Then, using
the reconstructed right-handed neutrino and triplet parameters, we study
leptogenesis and lepton flavour violation in these models, and comment on
flavour effects in leptogenesis in the type I limit. We find that the mixed
solutions where both the type I and the type II seesaw mechanisms give a
significant contribution to neutrino masses provide new opportunities for
successful leptogenesis in SO(10) GUTs.Comment: 31 pages, 32 figures. Appendix augmented with useful analytic
formulae, a few typos corrected, 2 references adde
Multiscale 3D Shape Analysis using Spherical Wavelets
©2005 Springer. The original publication is available at www.springerlink.com:
http://dx.doi.org/10.1007/11566489_57DOI: 10.1007/11566489_57Shape priors attempt to represent biological variations within a population. When variations are global, Principal Component Analysis (PCA) can be used to learn major modes of variation, even from a limited training set. However, when significant local variations exist, PCA typically cannot represent such variations from a small training set. To address this issue, we present a novel algorithm that learns shape variations from data at multiple scales and locations using spherical wavelets and spectral graph partitioning. Our results show that when the training set is small, our algorithm significantly improves the approximation of shapes in a testing set over PCA, which tends to oversmooth data
Cosmic Microwave Background, Matter-Antimatter Asymmetry and Neutrino Masses
We study the implications of thermal leptogenesis for neutrino parameters.
Assuming that decays of N_1, the lightest of the heavy Majorana neutrinos,
initiate baryogenesis, we show that the final baryon asymmetry is determined by
only four parameters: the CP asymmetry epsilon_1, the heavy neutrino mass M_1,
the effective light neutrino mass \tilde{m}_1, and the quadratic mean \bar{m}
of the light neutrino masses. Imposing the CMB measurement of the baryon
asymmetry as constraint on the neutrino parameters, we show, in a model
independent way, that quasi-degenerate neutrinos are incompatible with thermal
leptogenesis. For maximal CP asymmetry epsilon_1, and neutrino masses in the
range from (\Delta m^2_{sol})^{1/2} to (\Delta m^2_{atm})^{1/2}, the
baryogenesis temperature is T_B = O(10^{10}) GeV.Comment: 28 pages, 14 figures included; v2: erratum added, M_1 lower bound in
the strong wash-out regime (see Eq. (63)) relaxed by a factor 2/
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