6,579 research outputs found
Wedges, Cones, Cosmic Strings, and the Reality of Vacuum Energy
One of J. Stuart Dowker's most significant achievements has been to observe
that the theory of diffraction by wedges developed a century ago by Sommerfeld
and others provided the key to solving two problems of great interest in
general-relativistic quantum field theory during the last quarter of the
twentieth century: the vacuum energy associated with an infinitely thin,
straight cosmic string, and (after an interchange of time with a space
coordinate) the apparent vacuum energy of empty space as viewed by an
accelerating observer. In a sense the string problem is more elementary than
the wedge, since Sommerfeld's technique was to relate the wedge problem to that
of a conical manifold by the method of images. Indeed, Minkowski space, as well
as all cone and wedge problems, are related by images to an infinitely sheeted
master manifold, which we call Dowker space. We review the research in this
area and exhibit in detail the vacuum expectation values of the energy density
and pressure of a scalar field in Dowker space and the cone and wedge spaces
that result from it. We point out that the (vanishing) vacuum energy of
Minkowski space results, from the point of view of Dowker space, from the
quantization of angular modes, in precisely the way that the Casimir energy of
a toroidal closed universe results from the quantization of Fourier modes; we
hope that this understanding dispels any lingering doubts about the reality of
cosmological vacuum energy.Comment: 28 pages, 16 figures. Special volume in honor of J. S. Dowke
Fast transform decoding of nonsystematic Reed-Solomon codes
A Reed-Solomon (RS) code is considered to be a special case of a redundant residue polynomial (RRP) code, and a fast transform decoding algorithm to correct both errors and erasures is presented. This decoding scheme is an improvement of the decoding algorithm for the RRP code suggested by Shiozaki and Nishida, and can be realized readily on very large scale integration chips
Crossing Symmetry Violation of Unitarized Pion-Pion Amplitude in the Resonance Region
Pion-pion scattering amplitude obtained from one-loop Chiral Perturbation
Theory (ChPT) is crossing symmetric, however the corresponding partial wave
amplitudes do not respect exact unitarity relation. There are different
approaches to get unitarized partial wave amplitudes from ChPT. Here we
consider the inverse amplitude method (IAM) that is often used to fit pion-pion
phase shifts to experimental data, by adjusting free parameters. We measure the
amount of crossing symmetry violation (CSV) in this case and we show that
crossing symmetry is badly violated by the IAM unitarized ChPT amplitude in the
resonance region. Important CSV also occurs when all free parameters are set
equal to zero.Comment: 6 pages, 4 figure
Insights into the Carbon chemistry of Mon R2
Aiming to learn about the chemistry of the dense PDR around the ultracompact
(UC) HII region in Mon R2, we have observed a series of mm-wavelength
transitions of C3H2 and C2H. In addition, we have traced the distribution of
other molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together with
the reactive ions recently detected, have been considered to determine the
physical conditions and to model the PDR chemistry. We then identified two kind
of molecules. The first group, formed by the reactive ions (CO+, HOC+) and
small hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and is
presumably exposed to a high UV field (hence we called it as "high UV", or
HUV). HUV species is expected to dominate for visual absorptions 2 < Av < 5
mag. A second group (less exposed to the UV field, and hence called "low UV",
or LUV) includes HCO and SiO, and is mainly present at the edges of the PDR (Av
> 5 mag). While the abundances of the HUV molecules can be explained by gas
phase models, this is not the case for the studied LUV ones. Although some
efficient gas-phase reactions might be lacking, grain chemistry sounds like a
probable mechanism able to explain the observed enhancement of HCO and SiO.
Within this scenario, the interaction of UV photons with grains produces an
important effect on the molecular gas chemistry and constitutes the first
evidence of an ionization front created by the UC HII region carving its host
molecular cloud. The physical conditions and kinematics of the gas layer which
surrounds the UC HII region were derived from the HUV molecules. Molecular
hydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations.
Such high densities suggest that the HII region could be pressure-confined by
the surrounding high density molecular gas.Comment: 32 pages, 8 figures. Accepted by Astrophysical Journa
Direct observation of dynamic surface acoustic wave controlled carrier injection into single quantum posts using phase-resolved optical spectroscopy
A versatile stroboscopic technique based on active phase-locking of a surface
acoustic wave to picosecond laser pulses is used to monitor dynamic
acoustoelectric effects. Time-integrated multi-channel detection is applied to
probe the modulation of the emission of a quantum well for different
frequencies of the surface acoustic wave. For quantum posts we resolve
dynamically controlled generation of neutral and charged excitons and
preferential injection of holes into localized states within the nanostructure.Comment: 10 pages, 4 figure
Particle Acceleration in Advection-Dominated Accretion Disks with Shocks: Green's Function Energy Distribution
The distribution function describing the acceleration of relativistic
particles in an advection-dominated accretion disk is analyzed using a
transport formalism that includes first-order Fermi acceleration, advection,
spatial diffusion, and the escape of particles through the upper and lower
surfaces of the disk. When a centrifugally-supported shock is present in the
disk, the concentrated particle acceleration occurring in the vicinity of the
shock channels a significant fraction of the binding energy of the accreting
gas into a population of relativistic particles. These high-energy particles
diffuse vertically through the disk and escape, carrying away both energy and
entropy and allowing the remaining gas to accrete. The dynamical structure of
the disk/shock system is computed self-consistently using a model previously
developed by the authors that successfully accounts for the production of the
observed relativistic outflows (jets) in M87 and \SgrA. This ensures that the
rate at which energy is carried away from the disk by the escaping relativistic
particles is equal to the drop in the radial energy flux at the shock location,
as required for energy conservation. We investigate the influence of advection,
diffusion, and acceleration on the particle distribution by computing the
nonthermal Green's function, which displays a relatively flat power-law tail at
high energies. We also obtain the energy distribution for the particles
escaping from the disk, and we conclude by discussing the spectrum of the
observable secondary radiation produced by the escaping particles.Comment: Published in Ap
Sentinel surveillance of HIV-1 transmitted drug resistance, acute infection and recent infection.
BackgroundHIV-1 acute infection, recent infection and transmitted drug resistance screening was integrated into voluntary HIV counseling and testing (VCT) services to enhance the existing surveillance program in San Francisco. This study describes newly-diagnosed HIV cases and characterizes correlates associated with infection.Methodology/principal findingsA consecutive sample of persons presenting for HIV VCT at the municipal sexually transmitted infections (STI) clinic from 2004 to 2006 (N = 9,868) were evaluated by standard enzyme-linked immunoassays (EIA). HIV antibody-positive specimens were characterized as recent infections using a less-sensitive EIA. HIV-RNA pooled testing was performed on HIV antibody-negative specimens to identify acute infections. HIV antibody-positive and acute infection specimens were evaluated for drug resistance by sequence analysis. Multivariable logistic regression was performed to evaluate associations. The 380 newly-diagnosed HIV cases included 29 acute infections, 128 recent infections, and 47 drug-resistant cases, with no significant increases or decreases in prevalence over the three years studied. HIV-1 transmitted drug resistance prevalence was 11.0% in 2004, 13.4% in 2005 and 14.9% in 2006 (p = 0.36). Resistance to non-nucleoside reverse transcriptase inhibitors (NNRTI) was the most common pattern detected, present in 28 cases of resistance (59.6%). Among MSM, recent infection was associated with amphetamine use (AOR = 2.67; p<0.001), unprotected anal intercourse (AOR = 2.27; p<0.001), sex with a known HIV-infected partner (AOR = 1.64; p = 0.02), and history of gonorrhea (AOR = 1.62; p = 0.03).ConclusionsNew HIV diagnoses, recent infections, acute infections and transmitted drug resistance prevalence remained stable between 2004 and 2006. Resistance to NNRTI comprised more than half of the drug-resistant cases, a worrisome finding given its role as the backbone of first-line antiretroviral therapy in San Francisco as well as worldwide. The integration of HIV-1 drug resistance, recent infection, and acute infection testing should be considered for existing HIV/STI surveillance and prevention activities, particularly in an era of enhanced efforts for early diagnosis and treatment
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