65,017 research outputs found
Dynamics of WIMPs in the solar system and implications for direct and indirect detection
Semi-analytic treatments of the evolution of orbits of weakly interacting
massive particles (WIMPs) in the solar system suggest that the WIMPs bound to
the solar system may enhance the direct detection rate relative to that of the
unbound population by up to a factor of order unity, and boost the flux of
neutrinos from WIMP annihilation in the Earth by up to two orders of magnitude.
To test these important but uncertain results, we perform a suite of numerical
orbit integrations to explore the properties of the bound WIMP population as a
function of the WIMP mass and the scattering cross section with baryonic
matter. For regions of WIMP parameter space presently allowed by experiments,
we find that (i) the bound WIMP population enhances the direct detection rate
by at most ~1% relative to the rate from unbound halo WIMPs; (ii) it is
unlikely that planned km^3-scale neutrino telescopes will detect neutrinos from
WIMP annihilation in the Earth; (iii) the event rate from neutrinos produced by
WIMP annihilation in the Sun may be much smaller than implied by the usual
calculations, which assume that WIMPs scattered onto bound orbits are rapidly
thermalized in the Sun.Comment: 4 pages, 1 figure, to appear in the IDM2008 conference proceeding
NMR Probing Spin Excitations in the Ring-Like Structure of a Two-Subband System
Resistively detected nuclear magnetic resonance (NMR) is observed inside the
ring-like structure, with a quantized Hall conductance of 6e^2/h, in the phase
diagram of a two subband electron system. The NMR signal persists up to 400 mK
and is absent in other states with the same quantized Hall conductance. The
nuclear spin-lattice relaxation time, T1, is found to decrease rapidly towards
the ring center. These observations are consistent with the assertion of the
ring-like region being a ferromagnetic state that is accompanied by collective
spin excitations.Comment: 4 pages, 4 figure
Evolutionary dynamics of incubation periods
The incubation period of a disease is the time between an initiating
pathologic event and the onset of symptoms. For typhoid fever, polio, measles,
leukemia and many other diseases, the incubation period is highly variable.
Some affected people take much longer than average to show symptoms, leading to
a distribution of incubation periods that is right skewed and often
approximately lognormal. Although this statistical pattern was discovered more
than sixty years ago, it remains an open question to explain its ubiquity. Here
we propose an explanation based on evolutionary dynamics on graphs. For simple
models of a mutant or pathogen invading a network-structured population of
healthy cells, we show that skewed distributions of incubation periods emerge
for a wide range of assumptions about invader fitness, competition dynamics,
and network structure. The skewness stems from stochastic mechanisms associated
with two classic problems in probability theory: the coupon collector and the
random walk. Unlike previous explanations that rely crucially on heterogeneity,
our results hold even for homogeneous populations. Thus, we predict that two
equally healthy individuals subjected to equal doses of equally pathogenic
agents may, by chance alone, show remarkably different time courses of disease.Comment: 24 pages, 8 figures, 1 tabl
A Predictive Model for Convective Flows Induced by Surface Reactivity Contrast
Concentration gradients in a fluid along a reactive surface due to contrast
in surface reactivity generate convective flows. These flows result from
contributions by electro and diffusio osmotic phenomena. In this study we have
analyzed reactive patterns that release and consume protons, analogous to
bimetallic catalytic conversion of peroxide. Here, we present a simple
analytical model that accurately predicts the induced potentials and consequent
velocities in such systems over a wide range of input parameters. Our model is
tested against direct numerical solutions to the coupled Poisson,
Nernst-Planck, and Navier-Stokes equations. Our analysis can be used to predict
enhancement of mass transport and the resulting impact on overall catalytic
conversion, and is also applicable to predicting the speed of catalytic
nanomotors
Photoionization cross sections of O II, O III, O IV, and O V: benchmarking R-matrix theory and experiments
For crucial tests between theory and experiment, ab initio close coupling
calculations are carried out for photoionization of O II, O III, O IV, O V. The
relativistic fine structure and resonance effects are studied using the
R-matrix and its relativistic variant the Breit Pauli R-matrix (BPRM)
approximation. Detailed comparison is made with high resolution experimental
measurements carried out in three different set-ups: Advanced Light Source at
Berkeley, and synchrotron radiation experiments at University of Aarhus and
University of Paris-Sud. The comparisons illustrate physical effects in
photoionization such as (i) fine structure, (ii) resolution, and (iii)
metastable components. Photoionization cross sections sigma{PI} of the ground
and a few low lying excited states of these ions obtained in the experimental
spectrum include combined features of these states. Theoretically calculated
resonances need to be resolved with extremely fine energy mesh for precise
comparison. In addition, prominent resonant features are observed in the
measured spectra from transitions allowed with relativistic fine structure, but
not in LS coupling. The sigma_{PI} are obtained for ground and metastable (i)
2s^22p^3(^4S^o, ^2D^o, ^2P^o) states of O II, (ii) 2s^22p^2(^3P,^1D,^1S) and
2s2p^3(^5S^o) states of O III, (iii) 2s^22p(^2P^o_J) and 2s2p^2(^4P_J) levels
of O IV, and (iv) 2s^2(^1S) and 2s2p(^3P^o,^1P^o) states of O V. It is found
that resonances in ground and metastable cross sections can be a diagnostic of
experimental beam composition, with potential ap plications to astrophysical
and laboratory plasma environments.Comment: 27 pages, 7 figs., submitted to Phys. Rev. A., text with high
resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/Oions.p
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