1,156 research outputs found
A Study of the Orbits of the Logarithmic Potential for Galaxies
The logarithmic potential is of great interest and relevance in the study of
the dynamics of galaxies. Some small corrections to the work of Contopoulos &
Seimenis (1990) who used the method of Prendergast (1982) to find periodic
orbits and bifurcations within such a potential are presented. The solution of
the orbital radial equation for the purely radial logarithmic potential is then
considered using the p-ellipse (precessing ellipse) method pioneered by Struck
(2006). This differential orbital equation is a special case of the generalized
Burgers equation. The apsidal angle is also determined, both numerically as
well as analytically by means of the Lambert W and the Polylogarithm functions.
The use of these functions in computing the gravitational lensing produced by
logarithmic potentials is discussed.Comment: 12 pages, 4 figures. Accepted by MNRAS Sept 6 201
Classification Criteria for Intermediate Uveitis, Non–Pars Planitis Type
Purpose: To determine classification criteria for intermediate uveitis, non-pars planitis type (IU- NPP, also known as undifferentiated intermediate uveitis) / Design: Machine learning of cases with IU-NPP and 4 other intermediate uveitides. / Methods: Cases of intermediate uveitides were collected in an informatics-designed preliminary database, and a final database was constructed of cases achieving supermajority agreement on the diagnosis, using formal consensus techniques. Cases were split into a training set and a validation set. Machine learning using multinomial logistic regression was used on the training set to determine a parsimonious set of criteria that minimized the misclassification rate among the intermediate uveitides. The resulting criteria were evaluated on the validation set. / Results: Five hundred eighty-nine of cases of intermediate uveitides, including 114 cases of IU-NPP, were evaluated by machine learning. The overall accuracy for intermediate uveitides was 99.8% in the training set and 99.3% in the validation set (95% confidence interval 96.1, 99.9). Key criteria for IU-NPP included unilateral or bilateral intermediate uveitis with neither 1) snowballs in the vitreous nor 2) snowbanks on the pars plana. Other key exclusions included: 1) multiple sclerosis, 2) sarcoidosis, and 3) syphilis. The misclassification rates for pars planitis were 0 % in the training set and 0% in the validation set, respectively. / Conclusions: The criteria for IU-NPP had a low misclassification rate and appeared to perform well enough for use in clinical and translational research
Classification Criteria for Multiple Sclerosis-Associated Intermediate Uveitis
PURPOSE:
The purpose of this study was to determine classification criteria for multiple sclerosis-associated intermediate uveitis.
DESIGN:
Machine learning of cases with multiple sclerosis-associated intermediate uveitis and 4 other intermediate uveitides.
METHODS:
Cases of intermediate uveitides were collected in an informatics-designed preliminary database, and a final database was constructed of cases achieving supermajority agreement on the diagnosis, using formal consensus techniques. Cases were split into a training set and a validation set. Machine learning using multinomial logistic regression was used in the training set to determine a parsimonious set of criteria that minimized the misclassification rate among the intermediate uveitides. The resulting criteria were evaluated in the validation set.
RESULTS:
A total of 589 cases of intermediate uveitides, including 112 cases of multiple sclerosis-associated intermediate uveitis, were evaluated by machine learning. The overall accuracy for intermediate uveitides was 99.8% in the training set and 99.3% in the validation set (95% confidence interval: 96.1-99.9). Key criteria for multiple sclerosis-associated intermediate uveitis included unilateral or bilateral intermediate uveitis and multiple sclerosis diagnosed by the McDonald criteria. Key exclusions included syphilis and sarcoidosis. The misclassification rates for multiple sclerosis-associated intermediate uveitis were 0 % in the training set and 0% in the validation set.
CONCLUSIONS:
The criteria for multiple sclerosis-associated intermediate uveitis had a low misclassification rate and appeared to perform sufficiently well enough for use in clinical and translational research
Pre-recombinational energy release and narrow features in the CMB spectrum
Energy release in the early Universe (z<~ 2x10^6) should lead to some broad
spectral distortion of the cosmic microwave background (CMB) radiation field,
which can be characterized as y-type distortion when the injection process
started at redshifts z<~ 5x10^4. Here we demonstrate that if energy was
released before the beginning of cosmological hydrogen recombination (z~1400),
closed loops of bound-bound and free-bound transitions in HI and HeII lead to
the appearance of (i) characteristic multiple narrow spectral features at dm
and cm wavelengths, and (ii) a prominent sub-millimeter feature consisting of
absorption and emission parts in the far Wien tail of CMB spectrum. The
additional spectral features are generated in the pre-recombinational epoch of
HI (z>~1800) and HeII (z>~7000), and therefore differ from those arising due to
normal cosmological recombination in the undisturbed CMB blackbody radiation
field. We present the results of numerical computations including 25 atomic
shells for both HI and HeII, and discuss the contributions of several
individual transitions in detail. As examples, we consider the case of
instantaneous energy release (e.g. due to phase transitions) and exponential
energy release because of long-lived decaying particles. Our computations show
that due to possible pre-recombinational atomic transitions the variability of
the CMB spectral distortion increases when comparing with the distortions
arising in the normal recombination epoch. The existence of these narrow
spectral features would open an unique way to separate y-distortions due to
pre-recombinational ($1400<~ z <~5x10^4) energy release from those arising in
the post-recombinational era at redshifts z<~800. (abridged)Comment: 17 pages, 12 Figures, 1 Table, submitted to A&
Measurement of single electron emission in two-phase xenon
We present the first measurements of the electroluminescence response to the
emission of single electrons in a two-phase noble gas detector. Single
ionization electrons generated in liquid xenon are detected in a thin gas layer
during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon
detector for WIMP dark matter searches. Both the pressure dependence and
magnitude of the single-electron response are in agreement with previous
measurements of electroluminescence yield in xenon. We discuss different
photoionization processes as possible cause for the sample of single electrons
studied in this work. This observation may have implications for the design and
operation of future large-scale two-phase systems.Comment: 11 pages, 6 figure
Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses
a 12kg two-phase xenon time projection chamber to search for the weakly
interacting massive particles (WIMPs) that may account for the dark matter of
our Galaxy. The detector measures both scintillation and ionisation produced by
radiation interacting in the liquid to differentiate between the nuclear
recoils expected from WIMPs and the electron recoil background signals down to
~10keV nuclear recoil energy. An analysis of 847kg.days of data acquired
between February 27th 2008 and May 20th 2008 has excluded a WIMP-nucleon
elastic scattering spin-independent cross-section above 8.1x10(-8)pb at
55GeV/c2 with a 90% confidence limit. It has also demonstrated that the
two-phase xenon technique is capable of better discrimination between electron
and nuclear recoils at low-energy than previously achieved by other xenon-based
experiments.Comment: 12 pages, 17 figure
WIMP-nucleon cross-section results from the second science run of ZEPLIN-III
We report experimental upper limits on WIMP-nucleon elastic scattering cross
sections from the second science run of ZEPLIN-III at the Boulby Underground
Laboratory. A raw fiducial exposure of 1,344 kg.days was accrued over 319 days
of continuous operation between June 2010 and May 2011. A total of eight events
was observed in the signal acceptance region in the nuclear recoil energy range
7-29 keV, which is compatible with background expectations. This allows the
exclusion of the scalar cross-section above 4.8E-8 pb near 50 GeV/c^2 WIMP mass
with 90% confidence. Combined with data from the first run, this result
improves to 3.9E-8 pb. The corresponding WIMP-neutron spin-dependent
cross-section limit is 8.0E-3 pb. The ZEPLIN programme reaches thus its
conclusion at Boulby, having deployed and exploited successfully three liquid
xenon experiments of increasing reach
Quenching Factor for Low Energy Nuclear Recoils in a Plastic Scintillator
Plastic scintillators are widely used in industry, medicine and scientific
research, including nuclear and particle physics. Although one of their most
common applications is in neutron detection, experimental data on their
response to low-energy nuclear recoils are scarce. Here, the relative
scintillation efficiency for neutron-induced nuclear recoils in a
polystyrene-based plastic scintillator (UPS-923A) is presented, exploring
recoil energies between 125 keV and 850 keV. Monte Carlo simulations,
incorporating light collection efficiency and energy resolution effects, are
used to generate neutron scattering spectra which are matched to observed
distributions of scintillation signals to parameterise the energy-dependent
quenching factor. At energies above 300 keV the dependence is reasonably
described using the semi-empirical formulation of Birks and a kB factor of
(0.014+/-0.002) g/MeVcm^2 has been determined. Below that energy the measured
quenching factor falls more steeply than predicted by the Birks formalism.Comment: 8 pages, 9 figure
Neutron Stars in Teleparallel Gravity
In this paper we deal with neutron stars, which are described by a perfect
fluid model, in the context of the teleparallel equivalent of general
relativity. We use numerical simulations to find the relationship between the
angular momentum of the field and the angular momentum of the source. Such a
relation was established for each stable star reached by the numerical
simulation once the code is fed with an equation of state, the central energy
density and the ratio between polar and equatorial radii. We also find a regime
where linear relation between gravitational angular momentum and moment of
inertia (as well as angular velocity of the fluid) is valid. We give the
spatial distribution of the gravitational energy and show that it has a linear
dependence with the squared angular velocity of the source.Comment: 19 pages, 14 figures. arXiv admin note: text overlap with
arXiv:1206.331
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