1,858 research outputs found
Cherenkov Radiation from Pairs and Its Effect on Induced Showers
We calculate the Cherenkov radiation from an pair at small
separations, as occurs shortly after a pair conversion. The radiation is
reduced (compared to that from two independent particles) when the pair
separation is smaller than the wavelength of the emitted light. We estimate the
reduction in light in large electromagnetic showers, and discuss the
implications for detectors that observe Cherenkov radiation from showers in the
Earth's atmosphere, as well as in oceans and Antarctic ice.Comment: Final version, with minor changes, to appear in PRD. 5 pages with 4
figure
Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars
This work investigates the short wavelength stability of the magnetopause
between a rapidly-rotating, supersonic, dense accretion disc and a
slowly-rotating low-density magnetosphere of a magnetized star. The
magnetopause is a strong shear layer with rapid changes in the azimuthal
velocity, the density, and the magnetic field over a short radial distance and
thus the Kelvin-Helmholtz (KH) instability may be important. The plasma
dynamics is treated using non-relativistic, compressible (isentropic)
magnetohydrodynamics. It is necessary to include the displacement current in
order that plasma wave velocities remain less than the speed of light. We focus
mainly on the case of a star with an aligned dipole magnetic field so that the
magnetic field is axial in the disc midplane and perpendicular to the disc flow
velocity. However, we also give results for cases where the magnetic field is
at an arbitrary angle to the flow velocity. For the aligned dipole case the
magnetopause is most unstable for KH waves propagating in the azimuthal
direction perpendicular to the magnetic field which tends to stabilize waves
propagating parallel to it. The wave phase velocity is that of the disc matter.
A quasi-linear theory of the saturation of the instability leads to a
wavenumber () power spectrum of the density and temperature
fluctuations of the magnetopause, and it gives the mass accretion and angular
momentum inflow rates across the magnetopause. For self-consistent conditions
this mass accretion rate will be equal to the disc accretion rate at large
distances from the magnetopause.Comment: 8 pages, 7 figure
Scalar radiation emitted from a source rotating around a black hole
We analyze the scalar radiation emitted from a source rotating around a
Schwarzschild black hole using the framework of quantum field theory at the
tree level. We show that for relativistic circular orbits the emitted power is
about 20% to 30% smaller than what would be obtained in Minkowski spacetime. We
also show that most of the emitted energy escapes to infinity. Our formalism
can readily be adapted to investigate similar processes.Comment: 19 pages (REVTEX), 5 figures, title slightly changed, extra
demonstration and minor improvements included. To appear in Class. Quant.
Gra
Slow group velocity and Cherenkov radiation
We theoretically study the effect of ultraslow group velocities on the
emission of Vavilov-Cherenkov radiation in a coherently driven medium. We show
that in this case the aperture of the group cone on which the intensity of the
radiation peaks is much smaller than that of the usual wave cone associated
with the Cherenkov coherence condition. We show that such a singular behaviour
may be observed in a coherently driven ultracold atomic gas.Comment: 4 pages, 4 figure
QCD in the nuclear medium and effects due to Cherenkov gluons
The equations of in-medium gluodynamics are proposed. Their classical lowest
order solution is explicitly shown for a color charge moving with constant
speed. For nuclear permittivity larger than 1 it describes emission of
Cherenkov gluons resembling results of classical electrodynamics. The values of
the real and imaginary parts of the nuclear permittivity are obtained from the
fits to experimental data on the double-humped structure around the away-side
jet obtained at RHIC. The dispersion of the nuclear permittivity is predicted
by comparing the RHIC, SPS and cosmic ray data. This is important for LHC
experiments. Cherenkov gluons may be responsible for the asymmetry of dilepton
mass spectra near rho-meson, observed in the SPS experiment with excess in the
low-mass wing of the resonance. This feature is predicted to be common for all
resonances. The "color rainbow" quantum effect might appear according to higher
order terms of in-medium QCD if the nuclear permittivity depends on color.Comment: 29 p., 4 figs; for "Phys. Atom. Nucl." volume dedicated to 80th
birthday of L.B. Okun; minor corrections on pp. 11 and 13 in v
Belowground DNA-based techniques: untangling the network of plant root interactions
Contains fulltext :
91591.pdf (publisher's version ) (Closed access)7 p
Theory of Cherenkov radiation in periodic dielectric media: Emission spectrum
The Cherenkov radiation is substantially modified in the presence of a medium
with a nontrivial dispersion relation. We consider Cherenkov emission spectra
of a point charge moving in general three- (3D) and two-dimensional (2D)
photonic crystals. Exact analytical expressions for the spectral distribution
of the radiated power are obtained in terms of the Bloch mode expansion. The
resulting expression reduces to a simple contour integral (3D case) or a
one-dimensional sum (2D case) over a small fraction of the reciprocal space,
which is defined by the generalized Cherenkov condition. We apply our method to
a specific case of an electron moving with different velocities in a 2D
square-lattice photonic crystal. Our method demonstrates an excellent agreement
with numerically rigorous finite-difference time-domain calculations while
being less demanding on computational resources.Comment: to appear in Phys. Rev.
Area-level deprivation and adiposity in children: is the relationship linear?
OBJECTIVE: It has been suggested that childhood obesity is inversely associated with deprivation, such that the prevalence is higher in more deprived groups. However, comparatively few studies actually use an area-level measure of deprivation, limiting the scope to assess trends in the association with obesity for this indicator. Furthermore, most assume a linear relationship. Therefore, the aim of this study was to investigate associations between area-level deprivation and three measures of adiposity in children: body mass index (BMI), waist circumference (WC) and waist-to-height ratio (WHtR). DESIGN: This is a cross-sectional study in which data were collected on three occasions a year apart (2005-2007). SUBJECTS: Data were available for 13,333 children, typically aged 11-12 years, from 37 schools and 542 lower super-output areas (LSOAs). MEASURES: Stature, mass and WC. Obesity was defined as a BMI and WC exceeding the 95th centile according to British reference data. WHtR exceeding 0.5 defined obesity. The Index of Multiple Deprivation affecting children (IDACI) was used to determine area-level deprivation. RESULTS: Considerable differences in the prevalence of obesity exist between the three different measures. However, for all measures of adiposity the highest probability of being classified as obese is in the middle of the IDACI range. This relationship is more marked in girls, such that the probability of being obese for girls living in areas at the two extremes of deprivation is around half that at the peak, occurring in the middle. CONCLUSION: These data confirm the high prevalence of obesity in children and suggest that the relationship between obesity and residential area-level deprivation is not linear. This is contrary to the 'deprivation theory' and questions the current understanding and interpretation of the relationship between obesity and deprivation in children. These results could help make informed decisions at the local level
Controlling crystallization and its absence: Proteins, colloids and patchy models
The ability to control the crystallization behaviour (including its absence)
of particles, be they biomolecules such as globular proteins, inorganic
colloids, nanoparticles, or metal atoms in an alloy, is of both fundamental and
technological importance. Much can be learnt from the exquisite control that
biological systems exert over the behaviour of proteins, where protein
crystallization and aggregation are generally suppressed, but where in
particular instances complex crystalline assemblies can be formed that have a
functional purpose. We also explore the insights that can be obtained from
computational modelling, focussing on the subtle interplay between the
interparticle interactions, the preferred local order and the resulting
crystallization kinetics. In particular, we highlight the role played by
``frustration'', where there is an incompatibility between the preferred local
order and the global crystalline order, using examples from atomic glass
formers and model anisotropic particles.Comment: 11 pages, 7 figure
One single dose of etomidate negatively influences adrenocortical performance for at least 24 h in children with meningococcal sepsis
Objective: To investigate the effect of one single bolus of etomidate used for intubation on adrenal function in children with meningococcal sepsis. Design: Retrospective study conducted between 1997 and 2004. Setting: University-affiliated paediatric intensive care unit (PICU). Patients and participants: Sixty children admitted to the PICU with meningococcal sepsis, not treated with steroids. Interventions: Adrenal hormone concentrations were determined as soon as possible after PICU admission, and after 12h and 24h. To assess disease severity, PRISM score and selected laboratory parameters were determined. Measurements and main results: On admission, before blood was drawn, 23 children had been intubated with etomidate, 8 without etomidate and 29 were not intubated. Children who were intubated had significantly higher disease severity parameters than those not intubated, whereas none of these parameters significantly differed between children intubated with or without etomidate. Children who received etomidate had significantly lower cortisol, higher ACTH and higher 11-deoxycortisol levels than those who did not receive etomidate. Arterial glucose levels were significantly lower in children who were intubated with etomidate than in non-intubated children. When children were intubated with etomidate, cortisol levels were 3.2 times lower for comparable 11-deoxycortisol levels. Eight children died, seven of whom had received etomidate. Within 24h cortisol/ACTH and cortisol/11-deoxycortisol ratios increased significantly in children who received etomidate, but not in children who did not, resulting in comparable cortisol/ACTH ratios with still significantly lowered cortisol/11-deoxycortisol ratios 24h after admission. Conclusions: Our data imply that even one single bolus of etomidate negatively influences adrenal function for at least 24h. It might therefore increase risk of death
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