4,717 research outputs found
A Class of N=1 Dual String Pairs and its Modular Superpotential
We compare the N=1 F-theory compactification of Donagi, Grassi and Witten
with modular superpotential - and some closely related models - to dual
heterotic models. We read of the F-theory spectrum from the cohomology of the
fourfold and discuss on the heterotic side the gauge bundle moduli sector
(including the spectral surface) and the necessary fivebranes. Then we consider
the N=1 superpotential and show how a heterotic superpotential matching the
F-theory computation is built up by worldsheet instantons. Finally we discuss
how the original modular superpotential should be corrected by an additional
modular correction factor, which on the F-theory side matches nicely with a
`curve counting function' for the del Pezzo surface. On the heterotic side we
derive the same factor demanding correct T-duality transformation properties of
the superpotential.Comment: 18 pages, Late
Illumina mate-paired DNA sequencing-library preparation using Cre-Lox recombination
Standard Illumina mate-paired libraries are constructed from 3- to 5-kb DNA fragments by a blunt-end circularization. Sequencing reads that pass through the junction of the two joined ends of a 3-5-kb DNA fragment are not easy to identify and pose problems during mapping and de novo assembly. Longer read lengths increase the possibility that a read will cross the junction. To solve this problem, we developed a mate-paired protocol for use with Illumina sequencing technology that uses Cre-Lox recombination instead of blunt end circularization. In this method, a LoxP sequence is incorporated at the junction site. This sequence allows screening reads for junctions without using a reference genome. Junction reads can be trimmed or split at the junction. Moreover, the location of the LoxP sequence in the reads distinguishes mate-paired reads from spurious paired-end reads. We tested this new method by preparing and sequencing a mate-paired library with an insert size of 3 kb from Saccharomyces cerevisiae. We present an analysis of the library quality statistics and a new bio-informatics tool called DeLoxer that can be used to analyze an IlluminaCre-Lox mate-paired data set. We also demonstrate how the resulting data significantly improves a de novo assembly of the S. cerevisiae genome
Gravitational wave detectors based on matter wave interferometers (MIGO) are no better than laser interferometers (LIGO)
We show that a recent claim that matter wave interferometers have a much
higher sensitivity than laser interferometers for a comparable physical setup
is unfounded. We point out where the mistake in the earlier analysis is made.
We also disprove the claim that only a description based on the geodesic
deviation equation can produce the correct physical result. The equations for
the quantum dynamics of non-relativistic massive particles in a linearly
perturbed spacetime derived here are useful for treating a wider class of
related physical problems. A general discussion on the use of atom
interferometers for the detection of gravitational waves is also provided.Comment: 16 pages, REVTeX4; minor changes, one figure and a few references
were added, an additional appendix was included where we explain why,
contrary to the claims in gr-qc/0409099, the effects due to the reflection
off the mirrors cancel out in the final result for the phase shif
Locating Star-Forming Regions in Quasar Host Galaxies
We present a study of the morphology and intensity of star formation in the
host galaxies of eight Palomar-Green quasars using observations with the Hubble
Space Telescope. Our observations are motivated by recent evidence for a close
relationship between black hole growth and the stellar mass evolution in its
host galaxy. We use narrow-band [O II] 3727, H, [O III]
5007 and Pa images, taken with the WFPC2 and NICMOS
instruments, to map the morphology of line-emitting regions, and, after
extinction corrections, diagnose the excitation mechanism and infer
star-formation rates. Significant challenges in this type of work are the
separation of the quasar light from the stellar continuum and the
quasar-excited gas from the star-forming regions. To this end, we present a
novel technique for image decomposition and subtraction of quasar light. Our
primary result is the detection of extended line-emitting regions with sizes
ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus,
powered primarily by star formation. We determine star-formation rates of order
a few tens of M/yr. The host galaxies of our target quasars have
stellar masses of order M and specific star formation rates
on a par with those of M82 and luminous infrared galaxies. As such they fall at
the upper envelope or just above the star-formation mass sequence in the
specific star formation vs stellar mass diagram. We see a clear trend of
increasing star formation rate with quasar luminosity, reinforcing the link
between the growth of the stellar mass of the host and the black hole mass
found by other authors.Comment: Accepted for publication in M.N.R.A.
Indomethacin decreases viscosity of gallbladder bile in patients with cholesterol gallstone disease
There is experimental evidence that inhibition of cyclooxygenase with nonsteroidal anti-inflammatory drugs may decrease cholesterol gall-stone formation and mitigate biliary pain in gall-stone patients. The mechanisms by which NSAIDs exert these effect are unclear. In a prospective, controlled clinical trial we examined the effects of oral indomethacin on the composition of human gall-bladder bile. The study included 28 patients with symptomatic cholesterol or mixed gallstones. Of these, 8 were treated with 3 à 25 mg indomethacin daily for 7 days prior to elective cholecystectomy while 20 received no treatment and served as controls. Bile and tissue samples from the gallbladder were obtained during cholecystectomy. Indomethacin tissue levels in the gallbladder mucosa, as assessed by HPLC, were 1.05±0.4 ng/mg wet weight, a concentration known to inhibit effectively cyclooxygenase activity. Nevertheless, no differences between the treated and untreated groups were found in the concentrations of biliary mucus glycoprotein (0.94±0.27 versus 0.93±0.32 mg/ml) or total protein (5.8±0.9 versus 6.4±1.3 mg/ml), cholesterol saturation (1.3±0.2 versus 1.5±0.2), or nucleation time (2.0±3.0 versus 1.5±2.0 days). However, biliary viscosity, measured using a low-shear rotation viscosimeter, was significantly lower in patients receiving indomethacin treatment (2.9±0.6 versus 5.6±1.2 mPa.s; P < 0.02). In conclusion, in man oral indomethacin decreases bile viscosity without alteration of bile lithogenicity or biliary mucus glycoprotein content. Since mucus glycoproteins are major determinants of bile viscosity, an alteration in mucin macromolecular composition may conceivably cause the indomethacin-induced decrease in biliary viscosity and explain the beneficial effects of nonsteroidal anti-inflammatory drugs in gallstone disease
Lattice Universes in 2+1-dimensional gravity
Lattice universes are spatially closed space-times of spherical topology in
the large, containing masses or black holes arranged in the symmetry of a
regular polygon or polytope. Exact solutions for such spacetimes are found in
2+1 dimensions for Einstein gravity with a non-positive cosmological constant.
By means of a mapping that preserves the essential nature of geodesics we
establish analogies between the flat and the negative curvature cases. This map
also allows treatment of point particles and black holes on a similar footing.Comment: 14 pages 7 figures, to appear in Festschrift for Vince Moncrief (CQG
Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?
Recent neutron scattering experiments on aqueous salt solutions of
amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508
(2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of
t-butanol pairs, bridged by a chloride ion via
hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic
butanol-butanol contacts. Here we present a joint experimental/theoretical
study on the same system, using a combination of molecular dynamics simulations
and nuclear magnetic relaxation measurements. Both theory and experiment
clearly support the more intuitive scenario of an enhanced number of
hydrophobic contacts in the presence of the salt, as it would be expected for
purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although
our conclusions arrive at a structurally completely distinct scenario, the
molecular dynamics simulation results are within the experimental errorbars of
the Bowron and Finney work.Comment: 15 pages twocolumn revtex, 11 figure
Interrelation of structural and electronic properties of InGaN/GaN quantum dots using an eight-band k.p model
We present an eight-band k.p model for the calculation of the electronic
structure of wurtzite semiconductor quantum dots (QDs) and its application to
indium gallium nitride (InGaN) QDs formed by composition fluctuations in InGaN
layers. The eight-band k.p model accounts for strain effects, piezoelectric and
pyroelectricity, spin-orbit and crystal field splitting. Exciton binding
energies are calculated using the self-consistent Hartree method. Using this
model, we studied the electronic properties of InGaN QDs and their dependence
on structural properties, i.e., their chemical composition, height, and lateral
diameter. We found a dominant influence of the built-in piezoelectric and
pyroelectric fields, causing a spatial separation of the bound electron and
hole states and a redshift of the exciton transition energies. The
single-particle energies as well as the exciton energies depend heavily on the
composition and geometry of the QDs
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