9,961 research outputs found
Photon propagation in a stationary warp drive space-time
We simplify the warp drive space-time so that it becomes stationary and the
distorsion becomes one-dimensional and static. We use this simplified warp
drive space-time as a background for a photon field. We shall especially use
the Drummond\&Hathrell action in order to investigate the velocity effects on
photons in this background. Finally, we discuss the limitations of this model.Comment: 10 pages Latex, Revised, major changes -no tunneling model
The Cosmic Ray Observatory Project: A Statewide Outreach and Education Experiment in Nebraska
The Cosmic Ray Observatory Project (CROP) is a statewide education and
research experiment involving Nebraska high school students, teachers and
university undergraduates in the study of extensive cosmic-ray air showers. A
network of high school teams construct, install, and operate school-based
detectors in coordination with University of Nebraska physics professors and
graduate students. The detector system at each school is an array of
scintillation counters recycled from the Chicago Air Shower Array in
weather-proof enclosures on the school roof, with a GPS receiver providing a
time stamp for cosmic-ray events. The detectors are connected to triggering
electronics and a data-acquisition PC inside the building. Students share data
via the Internet to search for time coincidences with other sites. Funded by
the National Science Foundation, CROP has enlisted 29 schools with the aim of
expanding to the 314 high schools in the state over several years. This report
highlights both the scientific and professional development achievements of the
project to date.Comment: 4 pages, 4 figures, submitted to the 2007 International Cosmic Ray
Conference (ICRC2007), Merida, Mexico, July 200
TraĆŸenje Higgsa i supersimetrije tevatronom
Results are presented of searches for Standard Model and MSSM Higgs, as well as supersymmetric particles. A broad variety of signatures were explored by the CDF and DĂ experiments, using approximately 2 fbâ1 of reconstructed ppÂŻ data from collisions at 1.96 TeV at Fermilabâs Tevatron. No significant deviation from background sources has been observed in any of the searches, and new limits have been set on the production cross sections.Predstavljamo ishode traĆŸenja Higgsa i supersimetriÄnih Äestica prema standardnom modelu i prema MSSM. Primjenjuje se ĆĄiroka lepeza znakova u CDF i DĂ eksperimentima, rabeÄi oko 2 fbâ1 analiziranih podataka za ppÂŻ sudare na 1.96 TeV u Tevatronu Fermilab-a. U tim traĆŸenjima nismo naĆĄli odstupanja od ĆĄuma, i postavili smo nove granice za udarne presjeke
Radio Emission and Particle Acceleration in SN 1993J
The radio light curves of SN 1993J are found to be well fit by a synchrotron
spectrum, suppressed by external free-free absorption and synchrotron
self-absorption. A standard r^-2 circumstellar medium is assumed, and found to
be adequate. The magnetic field and number density of relativistic electrons
behind the shock are determined. The strength of the magnetic field argues
strongly for turbulent amplification behind the shock. The ratio of the
magnetic and thermal energy density behind the shock is ~0.14. Synchrotron and
Coulomb cooling dominate the losses of the electrons. The injected electron
spectrum has a power law index -2.1, consistent with diffusive shock
acceleration, and the number density scales with the thermal electron energy
density. The total energy density of the relativistic electrons is, if
extrapolated to gamma ~ 1, ~ 5x10^-4 of the thermal energy density. The
free-free absorption required is consistent with previous calculations of the
circumstellar temperature of SN 1993J, T_e ~ (2-10)x10^5 K. The relative
importance of free-free absorption, Razin suppression, and the synchrotron
self-absorption effect for other supernovae are briefly discussed. Guidelines
for the modeling and interpretation of VLBI observations are given.Comment: accepted for Ap.
Lattice Boltzmann simulations of fluid flow in continental carbonate reservoir rocks and in upscaled rock models generated with multiple-point geostatistics
Microcomputed tomography (mu CT) and Lattice Boltzmann Method (LBM) simulations were applied to continental carbonates to quantify fluid flow. Fluid flow characteristics in these complex carbonates with multiscale pore networks are unique and the applied method allows studying their heterogeneity and anisotropy. 3D pore network models were introduced to single-phase flow simulations in Palabos, a software tool for particle-based modelling of classic computational fluid dynamics. In addition, permeability simulations were also performed on rock models generated with multiple-point geostatistics (MPS). This allowed assessing the applicability of MPS in upscaling high-resolution porosity patterns into large rock models that exceed the volume limitations of the mu CT. Porosity and tortuosity control fluid flow in these porous media. Micro-and mesopores influence flow properties at larger scales in continental carbonates. Upscaling with MPS is therefore necessary to overcome volume-resolution problems of CT scanning equipment. The presented LBM-MPS workflow is applicable to other lithologies, comprising different pore types, shapes, and pore networks altogether. The lack of straightforward porosity-permeability relationships in complex carbonates highlights the necessity for a 3D approach. 3D fluid flow studies provide the best understanding of flow through porous media, which is of crucial importance in reservoir modelling
LabView Interface for School-Network DAQ Card
A low-cost DAQ card has been developed for school-network cosmic ray detector
projects, providing digitized data from photomultiplier tubes via a standard
serial interface. To facilitate analysis of these data and to provide students
with a starting point for custom readout systems, a model interface has been
developed using the National Instruments LabVIEW(R) system. This user-friendly
interface allows one to initialize the trigger coincidence conditions for
data-taking runs and to monitor incoming or pre-recorded data sets with
updating singles- and coincidence-rate plots and other user-selectable
histograms.Comment: 4 pages, 6 figures. Presented as Paper NS26-119 at IEEE-NSS 2003,
Portland, OR, by R. J. Wilke
Using the Uncharged Kerr Black Hole as a Gravitational Mirror
We extend the study of the possibility to use the Schwarzschild black hole as
a gravitational mirror to the more general case of an uncharged Kerr black
hole. We use the null geodesic equation in the equatorial plane to prove a
theorem concerning the conditions the impact parameter has to satisfy if there
shall exist boomerang photons. We derive an equation for these boomerang
photons and an equation for the emission angle. Finally, the radial null
geodesic equation is integrated numerically in order to illustrate boomerang
photons.Comment: 11 pages Latex, 3 Postscript figures, uufiles to compres
A strand specific high resolution normalization method for chip-sequencing data employing multiple experimental control measurements
Background: High-throughput sequencing is becoming the standard tool for investigating protein-DNA interactions or epigenetic modifications. However, the data generated will always contain noise due to e. g. repetitive regions or non-specific antibody interactions. The noise will appear in the form of a background distribution of reads that must be taken into account in the downstream analysis, for example when detecting enriched regions (peak-calling). Several reported peak-callers can take experimental measurements of background tag distribution into account when analysing a data set. Unfortunately, the background is only used to adjust peak calling and not as a preprocessing step that aims at discerning the signal from the background noise. A normalization procedure that extracts the signal of interest would be of universal use when investigating genomic patterns. Results: We formulated such a normalization method based on linear regression and made a proof-of-concept implementation in R and C++. It was tested on simulated as well as on publicly available ChIP-seq data on binding sites for two transcription factors, MAX and FOXA1 and two control samples, Input and IgG. We applied three different peak-callers to (i) raw (un-normalized) data using statistical background models and (ii) raw data with control samples as background and (iii) normalized data without additional control samples as background. The fraction of called regions containing the expected transcription factor binding motif was largest for the normalized data and evaluation with qPCR data for FOXA1 suggested higher sensitivity and specificity using normalized data over raw data with experimental background. Conclusions: The proposed method can handle several control samples allowing for correction of multiple sources of bias simultaneously. Our evaluation on both synthetic and experimental data suggests that the method is successful in removing background noise
Erratum: Observational constraints on supermassive dark stars
No abstract is available for this article
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