9,535 research outputs found
Parametrization of the Driven Betatron Oscillation
An AC dipole is a magnet which produces a sinusoidally oscillating dipole
field and excites coherent transverse beam motion in a synchrotron. By
observing this coherent motion, the optical parameters can be directly measured
at the beam position monitor locations. The driven oscillation induced by an AC
dipole will generate a phase space ellipse which differs from that of the free
oscillation. If not properly accounted for, this difference can lead to a
misinterpretation of the actual optical parameters, for instance, of 6% or more
in the cases of the Tevatron, RHIC, or LHC. The effect of an AC dipole on the
linear optics parameters is identical to that of a thin lens quadrupole. By
introducing a new amplitude function to describe this new phase space ellipse,
the motion produced by an AC dipole becomes easier to interpret. Beam position
data taken under the influence of an AC dipole, with this new interpretation in
mind, can lead to more precise measurements of the normal Courant-Snyder
parameters. This new parameterization of the driven motion is presented and is
used to interpret data taken in the FNAL Tevatron using an AC dipole.Comment: 8 pages, 8 figures, and 1 tabl
Group velocity and causality in standard relativistic resistive magnetohydrodynamics
Group velocity of electromagnetic waves in plasmas derived by standard
relativistic resistive MHD (resistive RMHD) equations is superluminal. If we
assume that the group velocity represents the propagation velocity of a signal,
we have to worry about the causality problem. That is, some acausal phenomena
may be induced, such that information transportation to the absolute past and
spontaneous decrease in the entropy. Here, we tried to find the acausal
phenomena using standard resistive RMHD numerical simulations in the suggested
situation of the acausal phenomena. The calculation results showed that even in
such situations no acausal effect happens. The numerical result with respect to
the velocity limit of the information transportation is consistent with a
linear theory of wave train propagation. Our results assure that we can use
these equations without problems of acausal phenomena.Comment: 28 pages, 10 figure
Naked Singularity Explosion in Higher Dimensions
Motivated by the recent argument that in the TeV-scale gravity
trans-Planckian domains of spacetime as effective naked singularities would be
generated by high-energy particle (and black-hole) collisions, we investigate
the quantum particle creation by naked-singularity formation in general
dimensions. Background spacetime is simply modeled by the self-similar Vaidya
solution, describing the spherical collapse of a null dust fluid. In a generic
case the emission power is found to be proportional to the quadratic inverse of
the remaining time to a Cauchy horizon, as known in four dimensions. On the
other hand, the power is proportional to the quartic inverse for a critical
case in which the Cauchy horizon is `degenerate'. According to these results,
we argue that the backreaction of the particle creation to gravity will be
important in particle collisions, in contrast to the gravitational collapse of
massive stellar objects, since the bulk of energy is carried away by the
quantum radiation even if a quantum gravitational effect cutoff the radiation
just before the appearance of naked singularity.Comment: 19 pages, 2 figures; v2: typos fixe
Passive spiral formation from halo gas starvation: Gradual transformation into S0s
Recent spectroscopic and high resolution -imaging observations have
revealed significant numbers of ``passive'' spiral galaxies in distant
clusters, with all the morphological hallmarks of a spiral galaxy (in
particular, spiral arm structure), but with weak or absent star formation.
Exactly how such spiral galaxies formed and whether they are the progenitors of
present-day S0 galaxies is unclear. Based on analytic arguments and numerical
simulations of the hydrodynamical evolution of a spiral galaxy's halo gas
(which is a likely candidate for the source of gas replenishment for star
formation in spirals), we show that the origin of passive spirals may well be
associated with halo gas stripping. Such stripping results mainly from the
hydrodynamical interaction between the halo gas and the hot intracluster gas.
Our numerical simulations demonstrate that even if a spiral orbits a cluster
with a pericenter distance 3 times larger than the cluster core radius,
80 % of the halo gas is stripped within a few Gyr and, accordingly,
cannot be accreted by the spiral. Furthermore, our study demonstrates that this
dramatic decline in the gaseous infall rate leads to a steady increase in the
parameter for the disk, with the spiral arm structure, although persisting,
becoming less pronounced as the star formation rate gradually decreases. These
results suggest that passive spirals formed in this way, gradually evolve into
red cluster S0s.Comment: 13 pages 4 figures (fig.1 = jpg format), accepted by Ap
Identification of novel clostridium perfringens type E strains that carry an iota toxin plasmid with a functional enterotoxin gene
Clostridium perfringens enterotoxin (CPE) is a major virulence factor for human gastrointestinal diseases, such as food poisoning and antibiotic associated diarrhea. The CPE-encoding gene (cpe) can be chromosomal or plasmid-borne. Recent development of conventional PCR cpe-genotyping assays makes it possible to identify cpe location (chromosomal or plasmid) in type A isolates. Initial studies for developing cpe genotyping assays indicated that all cpe-positive strains isolated from sickened patients were typable by cpe-genotypes, but surveys of C. perfringens environmental strains or strains from feces of healthy people suggested that this assay might not be useful for some cpe-carrying type A isolates. In the current study, a pulsed-field gel electrophoresis Southern blot assay showed that four cpe-genotype untypable isolates carried their cpe gene on a plasmid of ~65 kb. Complete sequence analysis of the ~65 kb variant cpe-carrying plasmid revealed no intact IS elements and a disrupted cytosine methyltransferase (dcm) gene. More importantly, this plasmid contains a conjugative transfer region, a variant cpe gene and variant iota toxin genes. The toxin genes encoded by this plasmid are expressed based upon the results of RT-PCR assays. The ~65 kb plasmid is closely related to the pCPF4969 cpe plasmid of type A isolates. MLST analyses indicated these isolates belong to a unique cluster of C. perfringens. Overall, these isolates carrying a variant functional cpe gene and iota toxin genes represent unique type E strains. © 2011 Miyamoto et al
Long-Lived Double-Barred Galaxies: Critical Mass and Length Scales
A substantial fraction of disk galaxies is double-barred. We analyze the
dynamical stability of such nested bar systems by means of Liapunov
exponents,by fixing a generic model and varying the inner (secondary) bar mass.
We show that there exists a critical mass below which the secondary bar cannot
sustain its own orbital structure, and above which it progressively destroys
the outer (primary) bar-supporting orbits. In this critical state, a large
fraction of the trajectories (regular and chaotic) are aligned with either bar,
suggesting the plausibility of long-lived dynamical states when
secondary-to-primary bar mass ratio is of the order of a few percent.
Qualitatively similar results are obtained by varying the size of the secondary
bar, within certain limits, while keeping its mass constant. In both cases, an
important role appears to be played by chaotic trajectories which are trapped
around (especially) the primary bar for long periods of time.Comment: 7 pages, 1 figure, to be published in Astrophysical Journal Letters
(Vol. 595, 9/20/03 issue). Replaced by revised figure and corrected typo
The potential investment impact of improved access to accelerated approval on the development of treatments for low prevalence rare diseases
<p>Abstract</p> <p>Background</p> <p>Over 95% of rare diseases lack treatments despite many successful treatment studies in animal models. To improve access to treatments, the Accelerated Approval (AA) regulations were implemented allowing the use of surrogate endpoints to achieve drug approval and accelerate development of life-saving therapies. Many rare diseases have not utilized AA due to the difficulty in gaining acceptance of novel surrogate endpoints in untreated rare diseases.</p> <p>Methods</p> <p>To assess the potential impact of improved AA accessibility, we devised clinical development programs using proposed clinical or surrogate endpoints for fifteen rare disease treatments.</p> <p>Results</p> <p>We demonstrate that better AA access could reduce development costs by approximately 60%, increase investment value, and foster development of three times as many rare disease drugs for the same investment.</p> <p>Conclusion</p> <p>Our research brings attention to the need for well-defined and practical qualification criteria for the use of surrogate endpoints to allow more access to the AA approval pathway in clinical trials for rare diseases.</p
Stellar Bar Evolution in Cuspy and Flat-Cored Triaxial CDM Halos
We analyze the evolution of stellar bars in galactic disks in mildly triaxial
flat-core and cuspy CDM halos. We use tailored simulations of rigid and live
halos which include the feedback from disk/bar onto the halo in order to test
the work by El-Zant & Shlosman (2002). The latter used the Liapunov exponents
to analyze the fate of bars in analytical asymmetric halos. We find: (1) The
bar growth is similar in all rigid axisymmetric and triaxial halos. (2) Bars in
live models vertically buckle and form a pseudobulge with a boxy/peanut shape.
(3) In live axisymmetric halos, the bar strength varies little during the
secular evolution. The bar pattern speed anticorrelates with the halo core
size. The bar strength is larger for smaller disk-to-halo mass ratios within
disk radii, the bar size correlates with the halo core sizes, and the bar
pattern speeds -- with the halo central mass concentration. Bars embedded in
live triaxial halos have a starkly different fate: they dissolve on ~1.5-5 Gyr
due to the onset of chaos over continuous zones, leaving behind a weak oval
distortion. The onset of chaos is related to the halo triaxiality, the fast
rotating bar and the halo cuspiness. Before the bar dissolves, the region
outside it develops strong spiral structures, especially in the live triaxial
halos. (4) More angular momentum is absorbed by the triaxial halos as compared
to the axisymmetric models and its exchange is mediated by resonances. (5)
Cuspy halos are more susceptible than flat-core halos to having their
prolateness washed out by the bar. We analyze these results in terms of the
stability of trajectories and development of chaos. We set constraints on the
triaxiality of DM halos by comparing our predictions to recent observations of
bars out to z~1.Comment: 17 pages, 14 figures, Astrophysical Journal, in press, Vol. 637.
Updated version (text, references
Fitting orbits to tidal streams
Recent years have seen the discovery of many tidal streams through the
Galaxy. Relatively straightforward observations of a stream allow one to deduce
three phase-space coordinates of an orbit. An algorithm is presented that
reconstructs the missing phase-space coordinates from these data. The
reconstruction starts from assumed values of the Galactic potential and a
distance to one point on the orbit, but with noise-free data the condition that
energy be conserved on the orbit enables one to reject incorrect assumptions.
The performance of the algorithm is investigated when errors are added to the
input data that are comparable to those in published data for the streams of
Pal 5. It is found that the algorithm returns distances and proper motions that
are accurate to of order one percent, and enables one to reject quite
reasonable but incorrect trial potentials. In practical applications it will be
important to minimize errors in the input data, and there is considerable scope
for doing this.Comment: 6 pages MNRAS in pres
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