57,724 research outputs found
Overview of charmonium decays and production from Non-Relativistic QCD
I briefly review Non-Relativistic QCD and related effective theories, and
discuss applications to heavy quarkonium decay, and production in
electron-positron colliders.Comment: 8 pages, Invited talk at Charm 2010, Oct. 21-24, IHEP, Beijin
Theory of Electromagnetic Wave Transmission through Metallic Gratings of Subwavelength Slits
We present FDTD calculations for transmission of light and other
electromagnetic waves through periodic arrays of slits in a metallic slab. The
results show resonant, frequency dependent, transmittance peaks for
subwavelength widths of the slits which can be up to a factor of ten with
respect to those out of resonance. Although our conclusions agree with previous
work by Lezec and Thio as regards both the magnitude of the enhancement and the
lack of contribution of surface plasmon polaritons of the metal surface to this
effect, we derive an interpretation from a theory that deals with emerging
beam- Rayleigh anomalies of the grating, and with Fabry-Perot resonances of the
perforated slab considered as an effective medium.Comment: 12 pages 3 figure
Hall response of interacting bosonic atoms in strong gauge fields: from condensed to FQH states
Interacting bosonic atoms under strong gauge fields undergo a series of phase
transitions that take the cloud from a simple Bose-Einstein condensate all the
way to a family of fractional-quantum-Hall-type states [M. Popp, B. Paredes,
and J. I. Cirac, Phys. Rev. A 70, 053612 (2004)]. In this work we demonstrate
that the Hall response of the atoms can be used to locate the phase transitions
and characterize the ground state of the many-body state. Moreover, the same
response function reveals within some regions of the parameter space, the
structure of the spectrum and the allowed transitions to excited states. We
verify numerically these ideas using exact diagonalization for a small number
of atoms, and provide an experimental protocol to implement the gauge fields
and probe the linear response using a periodically driven optical lattice.
Finally, we discuss our theoretical results in relation to recent experiments
with condensates in artificial magnetic fields [ L. J. LeBlanc, K.
Jimenez-Garcia, R. A. Williams, M. C. Beeler, A. R. Perry, W. D. Phillips, and
I. B. Spielman, Proc. Natl. Acad. Sci. USA 109, 10811 (2012)] and we analyze
the role played by vortex states in the Hall response.Comment: 10 pages, 7 figure
Stochastic modelling of intermittent scrape-off layer plasma fluctuations
Single-point measurements of fluctuations in the scrape-off layer of
magnetized plasmas are generally found to be dominated by large-amplitude
bursts which are associated with radial motion of blob-like structures. A
stochastic model for these fluctuations is presented, with the plasma density
given by a random sequence of bursts with a fixed wave form. Under very general
conditions, this model predicts a parabolic relation between the skewness and
kurtosis moments of the plasma fluctuations. In the case of exponentially
distributed burst amplitudes and waiting times, the probability density
function for the fluctuation amplitudes is shown to be a Gamma distribution
with the scale parameter given by the average burst amplitude and the shape
parameter given by the ratio of the burst duration and waiting times.Comment: 11 pages, 1 figur
Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel
We theoretically investigate the process of coupling cold atoms into the core
of a hollow-core photonic-crystal optical fiber using a blue-detuned
Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam
to couple the atoms, the blue-detuned hollow-beam can confine cold atoms to the
darkest regions of the beam thereby minimizing shifts in the internal states
and making the guide highly robust to heating effects. This single optical beam
is used as both a funnel and guide to maximize the number of atoms into the
fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical
trap (MOT) above a vertically-oriented optical fiber. We observe a
gravito-optical trapping effect for atoms with high orbital momentum around the
trap axis, which prevents atoms from coupling to the fiber: these atoms lack
the kinetic energy to escape the potential and are thus trapped in the laser
funnel indefinitely. We find that by reducing the dipolar force to the point at
which the trapping effect just vanishes, it is possible to optimize the
coupling of atoms into the fiber. Our simulations predict that by using a
low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a
20-{\mu}m radius core hollow-fiber it is possible to couple 11% of the atoms
from a MOT 9 mm away from the fiber. When MOT is positioned further away,
coupling efficiencies over 50% can be achieved with larger core fibers.Comment: 11 pages, 12 figures, 1 tabl
Can we avoid dark energy?
The idea that we live near the centre of a large, nonlinear void has
attracted attention recently as an alternative to dark energy or modified
gravity. We show that an appropriate void profile can fit both the latest
cosmic microwave background and supernova data. However, this requires either a
fine-tuned primordial spectrum or a Hubble rate so low as to rule these models
out. We also show that measurements of the radial baryon acoustic scale can
provide very strong constraints. Our results present a serious challenge to
void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev.
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