451 research outputs found
Continuously-variable survival exponent for random walks with movable partial reflectors
We study a one-dimensional lattice random walk with an absorbing boundary at
the origin and a movable partial reflector. On encountering the reflector, at
site x, the walker is reflected (with probability r) to x-1 and the reflector
is simultaneously pushed to x+1. Iteration of the transition matrix, and
asymptotic analysis of the probability generating function show that the
critical exponent delta governing the survival probability varies continuously
between 1/2 and 1 as r varies between 0 and 1. Our study suggests a mechanism
for nonuniversal kinetic critical behavior, observed in models with an infinite
number of absorbing configurations.Comment: 5 pages, 3 figure
An alternative approach to the galactic dark matter problem
We discuss scenarios in which the galactic dark matter in spiral galaxies is
described by a long range coherent field which settles in a stationary
configuration that might account for the features of the galactic rotation
curves. The simplest possibility is to consider scalar fields, so we discuss in
particular, two mechanisms that would account for the settlement of the scalar
field in a non-trivial configuration in the absence of a direct coupling of the
field with ordinary matter: topological defects, and spontaneous scalarization.Comment: 36 pages, 12 figures, Revtex, a brief discussion added, accepted for
publication in PR
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Genomics and Proteomics of Mycobacteriophage Patience, an Accidental Tourist in the Mycobacterium Neighborhood
ABSTRACT Newly emerging human viruses such as Ebola virus, severe acute respiratory syndrome (SARS) virus, and HIV likely originate within an extant population of viruses in nonhuman hosts and acquire the ability to infect and cause disease in humans. Although several mechanisms preventing viral infection of particular hosts have been described, the mechanisms and constraints on viral host expansion are ill defined. We describe here mycobacteriophage Patience, a newly isolated phage recovered using Mycobacterium smegmatis mc2155 as a host. Patience has genomic features distinct from its M. smegmatis host, including a much lower GC content (50.3% versus 67.4%) and an abundance of codons that are rarely used in M. smegmatis. Nonetheless, it propagates well in M. smegmatis, and we demonstrate the use of mass spectrometry to show expression of over 75% of the predicted proteins, to identify new genes, to refine the genome annotation, and to estimate protein abundance. We propose that Patience evolved primarily among lower-GC hosts and that the disparities between its genomic profile and that of M. smegmatis presented only a minimal barrier to host expansion. Rapid adaptions to its new host include recent acquisition of higher-GC genes, expression of out-of-frame proteins within predicted genes, and codon selection among highly expressed genes toward the translational apparatus of its new host
ISO--LWS observations of the two nearby spiral galaxies: NGC6946 and NGC1313
(Abridged) We present the analysis of the main FIR fine structure lines
emission in NGC1313 and NGC6946. We calculate that a component probably
associated with the diffuse disks contributes <~40% in N6946 and ~30 % in N1313
to the total [CII] emission. The main PDR physical parameters responsible for
the neutral atomic gas emission in N1313 and N6946 do not significantly differ
from what Malhotra etal (2001) found by modelling the integrated emission of a
sample of 60 normal galaxies,although there are evidences for a beam averaged
contribution of a less active component inside NGC6946 higher than its
contribution in the integrated emission of normal galaxies. CO and [CII] in
N6946 are well correlated with a mean [CII]/CO ratio similar to that of the
normal galaxies sample. In N1313 the [CII]/CO seems to systematically increase
from the North to the south, along the S-shaped spiral arm, indicating much
more inhomogeneous conditions than in N6946. HI and [CII] in N6946 are
completely de-correlated, probably because they arise from different gas
components. In N1313 we successfully detect two distinct gas components: a
cirrus-like component where HI and [CII] are weakly correlated as observed in
our Galaxy, and a component associated with dense PDRs completely de-correlated
from HI as observed in N6946.Finally, we find that the HI residing in dense
PDRs and presumably recently photo-dissociated, constitutes a few % of the
total HI. In turn, this dense gas component produces most of the [CII] emission
emitted by the atomic neutral medium.Comment: Latex, 100 pages, 11 Figures, 11 Tables. Accepted for publication in
A
Minimum information and guidelines for reporting a Multiplexed Assay of Variant Effect
Multiplexed Assays of Variant Effect (MAVEs) have emerged as a powerful
approach for interrogating thousands of genetic variants in a single
experiment. The flexibility and widespread adoption of these techniques across
diverse disciplines has led to a heterogeneous mix of data formats and
descriptions, which complicates the downstream use of the resulting datasets.
To address these issues and promote reproducibility and reuse of MAVE data, we
define a set of minimum information standards for MAVE data and metadata and
outline a controlled vocabulary aligned with established biomedical ontologies
for describing these experimental designs
The role of biophysical cohesion on subaqueous bed form size
Biologically active, fine-grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud-sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross-stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record
A Precise Measurement of the Neutron Magnetic Form Factor GMn in the Few-GeV2 Region
The neutron elastic magnetic form factor GMn has been extracted from
quasielastic electron scattering data on deuterium with the CEBAF Large
Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic coverage of the
measurement is continuous from Q2=1 GeV2 to 4.8 GeV2. High precision was
achieved by employing a ratio technique in which many uncertainties cancel, and
by a simultaneous in-situ calibration of the neutron detection efficiency, the
largest correction to the data. Neutrons were detected using the CLAS
electromagnetic calorimeters and the time-of-flight scintillators. Data were
taken at two different electron beam energies, allowing up to four
semi-independent measurements of GMn to be made at each value of Q2. The dipole
parameterization is found to provide a good description of the data over the
measured Q2 range.Comment: 14 pages, 5 figures, revtex4, submitted to Physical Review Letters,
Revised version has changes recommended by journal referee
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