342 research outputs found
Green's Dyadic Approach of the Self-Stress on a Dielectric-Diamagnetic Cylinder with Non-Uniform Speed of Light
We present a Green's dyadic formulation to calculate the Casimir energy for a
dielectric-diamagnetic cylinder with the speed of light differing on the inside
and outside. Although the result is in general divergent, special cases are
meaningful. It is pointed out how the self-stress on a purely dielectric
cylinder vanishes through second order in the deviation of the permittivity
from its vacuum value, in agreement with the result calculated from the sum of
van der Waals forces.Comment: 8 pages, submitted to proceedings of QFEXT0
Surface Divergences and Boundary Energies in the Casimir Effect
Although Casimir, or quantum vacuum, forces between distinct bodies, or
self-stresses of individual bodies, have been calculated by a variety of
different methods since 1948, they have always been plagued by divergences.
Some of these divergences are associated with the volume, and so may be more or
less unambiguously removed, while other divergences are associated with the
surface. The interpretation of these has been quite controversial. Particularly
mysterious is the contradiction between finite total self-energies and surface
divergences in the local energy density. In this paper we clarify the role of
surface divergences.Comment: 8 pages, 1 figure, submitted to proceedings of QFEXT0
Chiral condensate thermal evolution at finite baryon chemical potential within Chiral Perturbation Theory
We present a model independent study of the chiral condensate evolution in a
hadronic gas, in terms of temperature and baryon chemical potential. The
meson-meson interactions are described within Chiral Perturbation Theory and
the pion-nucleon interaction by means of Heavy Baryon Chiral Perturbation
Theory, both at one loop, and nucleon-nucleon interactions can be safely
neglected within our hadronic gas domain of validity. Together with the virial
expansion, this provides a systematic expansion at low temperatures and
chemical potentials, which includes the physical quark masses. This can serve
as a guideline for further studies on the lattice. We also obtain estimates of
the critical line of temperature and chemical potential where the chiral
condensate melts, which systematically lie somewhat higher than recent lattice
calculations but are consistent with several hadronic models. We have also
estimated uncertainties due to chiral parameters, heavier hadrons and higher
orders through unitarized Chiral Perturbation Theory.Comment: 15 pages, 15 figures, 3 tables, ReVTeX. Version to appear in Phys.
Rev. D. References added. More conservative estimate of applicability domain,
with new figure. More detailed explanation of final results with two more
figures. Results unchange
Floating Bare Tether as Upper Atmosphere Probe
Use of a (bare) conductive tape electrically floating in LEO as an effective e-beam source that produces artificial auroras, and is free of problems that have marred standard beams, is considered. Ambient ions impacting the tape with KeV energies over most of its length liberate secondary electrons, which race down the magnetic field and excite neutrals in the E-layer, resulting in auroral emissions. The tether would operate at night-time with both a power supply and a plasma contactor off; power and contactor would be on at daytime for reboost. The optimal tape thickness yielding a minimum mass for an autonomous system is determined; the alternative use of an electric thruster for day reboost, depending on mission duration, is discussed. Measurements of emission brightness from the spacecraft could allow
determination of the (neutral) density vertical profile in the critical E-layer; the flux and energy in the beam, varying along the tether, allow imaging line-of-sight integrated emissions that mix effects with altitude-dependent neutral density and lead to a brightness
peak in the beam footprint at the E-layer. Difficulties in tomographic inversion, to determine the density profile, result from beam broadening, due to elastic collisions, which flattens the peak, and to the highly nonlinear functional dependency of line-of-sight brightness. Some
dynamical issues are discussed
K -> 3 pi Final State Interactions at NLO in CHPT and Cabibbo's Proposal to Measure a_0-a_2
We present the analytical results for the K -> 3 pi final state interactions
at next-to-leading order (NLO) in CHPT. We also study the recent Cabibbo's
proposal to measure the pi-pi scattering lenghts combination a_0-a_2 from the
cusp effect in the pi^0-pi^0 energy spectrum at threshold for K^+ -> pi^0 pi^0
pi^+ and K_L -> pi^0 pi^0 pi^0$, and give the relevant formulas to describe it
at NLO. For that, we use the NLO CHPT expression to fit the real part of K -> 3
pi to data while the pi-pi scattering lenghts are treated non-perturbatively.
Using them, we make a quantitative estimate of the theoretical uncertaintity of
the a_0-a_2 determination at NLO in our approach and obtain that it is not
smaller than 5 % if added quadratically and 7 % if linearly for K^+ -> pi^0
pi^0 pi^+. One gets similar theoretical uncertainties if the neutral K_L ->
pi^0 pi^0 pi^0 decay data below threshold are used instead. For this decay,
there are very large theoretical uncertainties above threshold due to
cancellations and data above threshold cannot be used to get the scattering
lenghts. All the numbers we present are in the isospin limit apart of two-pion
phase space factors which are physical. We compare our results for the cusp
effect with Cabibbo and Isidori's results and discuss the differences and
agreements.
We also comment on the apperance of the singularity at the K -> 3 pi
pseudo-threshold s=(m_K-m_pi)^2 in the discontinuity that defines the cusp.Comment: 31 pages, 8 figures. v2=v3 Added the full contributions to the cusp
from the real part of the discontinuity. v4 Improved text. Matches published
versio
Using radio astronomical receivers for molecular spectroscopic characterization in astrochemical laboratory simulations: A proof of concept
We present a proof of concept on the coupling of radio astronomical receivers
and spectrometers with chemical reactorsand the performances of the resulting
setup for spectroscopy and chemical simulations in laboratory astrophysics.
Several experiments including cold plasma generation and UV photochemistry were
performed in a 40\,cm long gas cell placed in the beam path of the Aries 40\,m
radio telescope receivers operating in the 41-49 GHz frequency range interfaced
with fast Fourier transform spectrometers providing 2 GHz bandwidth and 38 kHz
resolution.
The impedance matching of the cell windows has been studied using different
materials. The choice of the material and its thickness was critical to obtain
a sensitivity identical to that of standard radio astronomical observations.
Spectroscopic signals arising from very low partial pressures of CH3OH,
CH3CH2OH, HCOOH, OCS,CS, SO2 (<1E-03 mbar) were detected in a few seconds. Fast
data acquisition was achieved allowing for kinetic measurements in
fragmentation experiments using electron impact or UV irradiation. Time
evolution of chemical reactions involving OCS, O2 and CS2 was also observed
demonstrating that reactive species, such as CS, can be maintained with high
abundance in the gas phase during these experiments.Comment: Accepted for publication in Astronomy and Astrophysics in September
21, 2017. 16 pages, 18 figure
Evaluation of the Casimir Force for a Dielectric-diamagnetic Cylinder with Light Velocity Conservation Condition and the Analogue of Sellmeir's Dispersion Law
We study the Casimir pressure for a dielectric-diamagnetic cylinder subject
to light velocity conservation and with a dispersion law analogous to
Sellmeir's rule. Similarities to and differences from the spherical case are
pointed out.Comment: 19 pages Latex, no figures; discussion expanded. To appear in Physica
Script
Effect of cocoa's theobromine on intestinal microbiota of rats
SCOPE: To establish the role of cocoa theobromine on gut microbiota composition and fermentation products after cocoa consumption in rats. METHODS AND RESULTS: Lewis rats were fed either a standard diet (RF diet), a diet containing 10% cocoa (CC diet) or a diet including 0.25% theobromine (TB diet) for 15 days. Gut microbiota (fluorescence in situ hybridization coupled to flow cytometry and metagenomics analysis), SCFA and IgA-coated bacteria were analyzed in fecal samples. CC and TB diets induced lower counts of E. coli whereas TB diet led to lower counts of Bifidobacterium spp., Streptococcus spp. and Clostridium histolyticum-C. perfingens group compared to RF diet. Metagenomics analysis also revealed a different microbiota pattern among the studied groups. The SCFA content was higher after both CC and TB diets, which was mainly due to enhanced butyric acid production. Furthermore, both diets decreased the proportion of IgA-coated bacteria. CONCLUSION: Cocoa's theobromine plays a relevant role in some effects related to cocoa intake, such as the lower proportion of IgA-coated bacteria. Moreover, theobromine modifies gut microbiota although other cocoa compounds could also act on intestinal bacteria, attenuating or enhancing the theobromine effects
On the precision of the theoretical predictions for pi pi scattering
In a recent paper, Pelaez and Yndurain evaluate some of the low energy
observables of pi pi scattering and obtain flat disagreement with our earlier
results. The authors work with unsubtracted dispersion relations, so that their
results are very sensitive to the poorly known high energy behaviour of the
scattering amplitude. They claim that the asymptotic representation we used is
incorrect and propose an alternative one. We repeat their calculations on the
basis of the standard, subtracted fixed-t dispersion relations, using their
asymptotics. The outcome fully confirms our earlier findings. Moreover, we show
that the Regge parametrization proposed by these authors for the region above
1.4 GeV violates crossing symmetry: Their ansatz is not consistent with the
behaviour observed at low energies.Comment: Added more material, mostly in Sects. 7, 8 and 9, in support of the
same conclusions. Latex, 28 pages, 3 figure
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