2,246 research outputs found
Far-infrared absorption in parallel quantum wires with weak tunneling
We study collective and single-particle intersubband excitations in a system
of quantum wires coupled via weak tunneling. For an isolated wire with
parabolic confinement, the Kohn's theorem guarantees that the absorption
spectrum represents a single sharp peak centered at the frequency given by the
bare confining potential. We show that the effect of weak tunneling between two
parabolic quantum wires is twofold: (i) additional peaks corresponding to
single-particle excitations appear in the absorption spectrum, and (ii) the
main absorption peak acquires a depolarization shift. We also show that the
interplay between tunneling and weak perpendicular magnetic field drastically
enhances the dispersion of single-particle excitations. The latter leads to a
strong damping of the intersubband plasmon for magnetic fields exceeding a
critical value.Comment: 18 pages + 6 postcript figure
An improved constraint satisfaction adaptive neural network for job-shop scheduling
Copyright @ Springer Science + Business Media, LLC 2009This paper presents an improved constraint satisfaction adaptive neural network for job-shop scheduling problems. The neural network is constructed based on the constraint conditions of a job-shop scheduling problem. Its structure and neuron connections can change adaptively according to the real-time constraint satisfaction situations that arise during the solving process. Several heuristics are also integrated within the neural network to enhance its convergence, accelerate its convergence, and improve the quality of the solutions produced. An experimental study based on a set of benchmark job-shop scheduling problems shows that the improved constraint satisfaction adaptive neural network outperforms the original constraint satisfaction adaptive neural network in terms of computational time and the quality of schedules it produces. The neural network approach is also experimentally validated to outperform three classical heuristic algorithms that are widely used as the basis of many state-of-the-art scheduling systems. Hence, it may also be used to construct advanced job-shop scheduling systems.This work was supported in part by the Engineering and Physical Sciences Research Council (EPSRC) of UK under Grant EP/E060722/01 and in part by the National Nature Science Fundation of China under Grant 60821063 and National Basic Research Program of China under Grant 2009CB320601
Restricted Isometries for Partial Random Circulant Matrices
In the theory of compressed sensing, restricted isometry analysis has become
a standard tool for studying how efficiently a measurement matrix acquires
information about sparse and compressible signals. Many recovery algorithms are
known to succeed when the restricted isometry constants of the sampling matrix
are small. Many potential applications of compressed sensing involve a
data-acquisition process that proceeds by convolution with a random pulse
followed by (nonrandom) subsampling. At present, the theoretical analysis of
this measurement technique is lacking. This paper demonstrates that the th
order restricted isometry constant is small when the number of samples
satisfies , where is the length of the pulse.
This bound improves on previous estimates, which exhibit quadratic scaling
Specific Heat of Liquid Helium in Zero Gravity very near the Lambda Point
We report the details and revised analysis of an experiment to measure the
specific heat of helium with subnanokelvin temperature resolution near the
lambda point. The measurements were made at the vapor pressure spanning the
region from 22 mK below the superfluid transition to 4 uK above. The experiment
was performed in earth orbit to reduce the rounding of the transition caused by
gravitationally induced pressure gradients on earth. Specific heat measurements
were made deep in the asymptotic region to within 2 nK of the transition. No
evidence of rounding was found to this resolution. The optimum value of the
critical exponent describing the specific heat singularity was found to be a =
-0.0127+ - 0.0003. This is bracketed by two recent estimates based on
renormalization group techniques, but is slightly outside the range of the
error of the most recent result. The ratio of the coefficients of the leading
order singularity on the two sides of the transition is A+/A- =1.053+ - 0.002,
which agrees well with a recent estimate. By combining the specific heat and
superfluid density exponents a test of the Josephson scaling relation can be
made. Excellent agreement is found based on high precision measurements of the
superfluid density made elsewhere. These results represent the most precise
tests of theoretical predictions for critical phenomena to date.Comment: 27 Pages, 20 Figure
The Palomar Testbed Interferometer Calibrator Catalog
The Palomar Testbed Interferometer (PTI) archive of observations between 1998
and 2005 is examined for objects appropriate for calibration of optical
long-baseline interferometer observations - stars that are predictably
point-like and single. Approximately 1,400 nights of data on 1,800 objects were
examined for this investigation. We compare those observations to an
intensively studied object that is a suitable calibrator, HD217014, and
statistically compare each candidate calibrator to that object by computing
both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis
is that the frequency distribution of visibility data associated with
calibrator stars differs from non-calibrator stars such as binary stars.
Spectroscopic binaries resolved by PTI, objects known to be unsuitable for
calibrator use, are similarly tested to establish detection limits of this
approach. From this investigation, we find more than 350 observed stars
suitable for use as calibrators (with an additional being
rejected), corresponding to sky coverage for PTI. This approach
is noteworthy in that it rigorously establishes calibration sources through a
traceable, empirical methodology, leveraging the predictions of spectral energy
distribution modeling but also verifying it with the rich body of PTI's on-sky
observations.Comment: 100 pages, 7 figures, 7 tables; to appear in the May 2008ApJS, v176n
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A Search for Dark Higgs Bosons
Recent astrophysical and terrestrial experiments have motivated the proposal
of a dark sector with GeV-scale gauge boson force carriers and new Higgs
bosons. We present a search for a dark Higgs boson using 516 fb-1 of data
collected with the BABAR detector. We do not observe a significant signal and
we set 90% confidence level upper limits on the product of the Standard
Model-dark sector mixing angle and the dark sector coupling constant.Comment: 7 pages, 5 postscript figures, published version with improved plots
for b/w printin
Gravitational wave extraction and outer boundary conditions by perturbative matching
We present a method for extracting gravitational radiation from a
three-dimensional numerical relativity simulation and, using the extracted
data, to provide outer boundary conditions. The method treats dynamical
gravitational variables as nonspherical perturbations of Schwarzschild
geometry. We discuss a code which implements this method and present results of
tests which have been performed with a three dimensional numerical relativity
code
Search for anomalous t t-bar production in the highly-boosted all-hadronic final state
A search is presented for a massive particle, generically referred to as a
Z', decaying into a t t-bar pair. The search focuses on Z' resonances that are
sufficiently massive to produce highly Lorentz-boosted top quarks, which yield
collimated decay products that are partially or fully merged into single jets.
The analysis uses new methods to analyze jet substructure, providing
suppression of the non-top multijet backgrounds. The analysis is based on a
data sample of proton-proton collisions at a center-of-mass energy of 7 TeV,
corresponding to an integrated luminosity of 5 inverse femtobarns. Upper limits
in the range of 1 pb are set on the product of the production cross section and
branching fraction for a topcolor Z' modeled for several widths, as well as for
a Randall--Sundrum Kaluza--Klein gluon. In addition, the results constrain any
enhancement in t t-bar production beyond expectations of the standard model for
t t-bar invariant masses larger than 1 TeV.Comment: Submitted to the Journal of High Energy Physics; this version
includes a minor typo correction that will be submitted as an erratu
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV
A search for new physics is presented based on an event signature of at least
three jets accompanied by large missing transverse momentum, using a data
sample corresponding to an integrated luminosity of 36 inverse picobarns
collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector
at the LHC. No excess of events is observed above the expected standard model
backgrounds, which are all estimated from the data. Exclusion limits are
presented for the constrained minimal supersymmetric extension of the standard
model. Cross section limits are also presented using simplified models with new
particles decaying to an undetected particle and one or two jets
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