9,037 research outputs found
Brane Worlds in Collision
We obtain an exact solution of the supergravity equations of motion in which
the four-dimensional observed universe is one of a number of colliding
D3-branes in a Calabi-Yau background. The collision results in the
ten-dimensional spacetime splitting into disconnected regions, bounded by
curvature singularities. However, near the D3-branes the metric remains static
during and after the collision. We also obtain a general class of solutions
representing -brane collisions in arbitrary dimensions, including one in
which the universe ends with the mutual annihilation of a positive-tension and
negative-tension 3-brane.Comment: RevTex, 4 pages, 1 figure, typos and minor errors correcte
Topological Quantum Computing with Only One Mobile Quasiparticle
In a topological quantum computer, universal quantum computation is performed
by dragging quasiparticle excitations of certain two dimensional systems around
each other to form braids of their world lines in 2+1 dimensional space-time.
In this paper we show that any such quantum computation that can be done by
braiding identical quasiparticles can also be done by moving a single
quasiparticle around n-1 other identical quasiparticles whose positions remain
fixed.Comment: 4 pages, 5 figure
The B Neutrino Spectrum
Knowledge of the energy spectrum of B neutrinos is an important
ingredient for interpreting experiments that detect energetic neutrinos from
the Sun. The neutrino spectrum deviates from the allowed approximation because
of the broad alpha-unstable Be final state and recoil order corrections to
the beta decay. We have measured the total energy of the alpha particles
emitted following the beta decay of B. The measured spectrum is
inconsistent with some previous measurements, in particular with a recent
experiment of comparable precision. The beta decay strength function for the
transition from B to the accessible excitation energies in Be is fit to
the alpha energy spectrum using the R-matrix approach. Both the positron and
neutrino energy spectra, corrected for recoil order effects, are constructed
from the strength function. The positron spectrum is in good agreement with a
previous direct measurement. The neutrino spectrum disagrees with previous
experiments, particularly for neutrino energies above 12 MeV.Comment: 15 pages, 13 figures, 4 tables, submitted to Phys. Rev. C, typos
correcte
Neutrino mass constraint from CMB and its degeneracy with other cosmological parameters
We show that the cosmic microwave background (CMB) data of WMAP can give
subelectronvolt limit on the neutrino mass: m_nu < 0.63 eV (95% CL). We also
investigate its degeneracy with other cosmological parameters. In particular,
we show the Hubble constant derived from the WMAP data decreases considerably
when the neutrino mass is a few times 0.1 eV.Comment: 3 pages, 2 figures, prepared for the TAUP2007 Proceeding
pp Wave Big Bangs: Matrix Strings and Shrinking Fuzzy Spheres
We find pp wave solutions in string theory with null-like linear dilatons.
These provide toy models of big bang cosmologies. We formulate Matrix String
Theory in these backgrounds. Near the big bang ``singularity'', the string
theory becomes strongly coupled but the Yang-Mills description of the matrix
string is weakly coupled. The presence of a second length scale allows us to
focus on a specific class of non-abelian configurations, viz. fuzzy cylinders,
for a suitable regime of parameters. We show that, for a class of pp waves,
fuzzy cylinders which start out big at early times dynamically shrink into
usual strings at sufficiently late times.Comment: 29 pages, ReVTeX and AMSLaTeX. 4 Figures. v2: Typo corrected and
reference adde
Topological Quantum Compiling
A method for compiling quantum algorithms into specific braiding patterns for
non-Abelian quasiparticles described by the so-called Fibonacci anyon model is
developed. The method is based on the observation that a universal set of
quantum gates acting on qubits encoded using triplets of these quasiparticles
can be built entirely out of three-stranded braids (three-braids). These
three-braids can then be efficiently compiled and improved to any required
accuracy using the Solovay-Kitaev algorithm.Comment: 20 pages, 20 figures, published versio
Flowing with Eight Supersymmetries in M-Theory and F-theory
We consider holographic RG flow solutions with eight supersymmetries and
study the geometry transverse to the brane. For both M2-branes and for
D3-branes in F-theory this leads to an eight-manifold with only a four-form
flux. In both settings there is a natural four-dimensional hyper-Kahler slice
that appears on the Coulomb branch. In the IIB theory this hyper-Kahler
manifold encodes the Seiberg-Witten coupling over the Coulomb branch of a U(1)
probe theory. We focus primarily upon a new flow solution in M-theory. This
solution is first obtained using gauged supergravity and then lifted to eleven
dimensions. In this new solution, the brane probes have an Eguchi-Hanson moduli
space with the M2-branes spread over the non-trivial 2-sphere. It is also shown
that the new solution is valid for a class of orbifold theories. We discuss how
the hyper-Kahler structure on the slice extends to some form of G-structure in
the eight-manifold, and describe how this can be computed.Comment: 29 pages, 1 figure, harvma
Photometric Recovery of Crowded Stellar Fields Observed with HST/WFPC2 and the Effects of Confusion Noise on the Extragalactic Distance Scale
We explore the limits of photometric reductions of crowded stellar fields
observed with the Wide Field and Planetary Camera 2 on board the Hubble Space
Telescope. Two photometric procedures, based on the DoPHOT and DAOPHOT/ALLFRAME
programs are tested, and the effects of crowding, complex sky background and
cosmic-ray contamination are discussed using an extensive set of artificial
star simulations. As a specific application of the results presented in this
paper, we assess the magnitude of photometric biases on programs aimed at
finding Cepheids and determining distances. We find that while the photometry
in individual images can be biased too bright by up to 0.2 mag in the most
crowded fields due to confusion noise, the effects on distance measurements
based on Cepheid variables are insignificant, less than 0.02 mag (1% in
distance) even in the most problematic cases. This result, which is at odds
with claims recently surfaced in the literature, is due to the strict criteria
applied in the selection of the variable stars, and the photometric cross
checks made possible by the availability of multiple exposures in different
filters which characterizes Cepheid observations.Comment: Accepted for publication in PASP. 41 pages, 18 figures, 8 tables. The
figures included with this submission are very low quality bitmap postscript,
please see http://www.astro.ucla.edu/~laura/pub.htm for the full size image
Archeops: an instrument for present and future cosmology
Archeops is a balloon-borne instrument dedicated to measure the cosmic
microwave background (CMB) temperature anisotropies. It has, in the millimetre
domain (from 143 to 545 GHz), a high angular resolution (about 10 arcminutes)
in order to constrain high l multipoles, as well as a large sky coverage
fraction (30%) in order to minimize the cosmic variance. It has linked, before
WMAP, Cobe large angular scales to the first acoustic peak region. From its
results, inflation motivated cosmologies are reinforced with a flat Universe
(Omega_tot=1 within 3%). The dark energy density and the baryonic density are
in very good agreement with other independent estimations based on supernovae
measurements and big bang nucleosynthesis. Important results on galactic dust
emission polarization and their implications for Planck are also addressed.Comment: 4 pages, 2 figures, to appear in Proceedings of the Multiwavelength
Cosmology Conference, June 2003, Mykonos Island, Greec
Incidence of Postpartum Endomyometritis Following Single-Dose Antibiotic Prophylaxis With Either Ampicillin/Sulbactam, Cefazolin, or Cefotetan in High-Risk Cesarean Section Patients
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