371 research outputs found
production at hadron colliders in NNLO QCD
Charged gauge boson pair production at the Large Hadron Collider allows
detailed probes of the fundamental structure of electroweak interactions. We
present precise theoretical predictions for on-shell production that
include, for the first time, QCD effects up to next-to-next-to-leading order in
perturbation theory. As compared to next-to-leading order, the inclusive
cross section is enhanced by 9% at 7 TeV and 12% at 14 TeV. The
residual perturbative uncertainty is at the 3% level. The severe contamination
of the cross section due to top-quark resonances is discussed in
detail. Comparing different definitions of top-free production in the
four and five flavour number schemes, we demonstrate that top-quark resonances
can be separated from the inclusive cross section without significant
loss of theoretical precision.Comment: 7 pages, 3 figure
Fish schooling as a basis for vertical axis wind turbine farm design
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the
high power coefficient (mechanical power output divided by the power of the
free-stream air through the turbine cross-sectional area) of an isolated
turbine. However when in close proximity to neighbouring turbines, HAWTs suffer
from a reduced power coefficient. In contrast, previous research on vertical
axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience
only small decreases (or even increases) in an individual turbine's power
coefficient when placed in close proximity to neighbours, thus yielding much
higher power outputs for a given area of land. A potential flow model of
inter-VAWT interactions is developed to investigate the effect of changes in
VAWT spatial arrangement on the array performance coefficient, which compares
the expected average power coefficient of turbines in an array to a
spatially-isolated turbine. A geometric arrangement based on the configuration
of shed vortices in the wake of schooling fish is shown to significantly
increase the array performance coefficient based upon an array of 16x16 wind
turbines. Results suggest increases in power output of over one order of
magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note:
The technology described in this paper is protected under both US and
international pending patents filed by the California Institute of Technolog
Studying Free-Space Transmission Statistics and Improving Free-Space QKD in the Turbulent Atmosphere
The statistical fluctuations in free-space links in the turbulent atmosphere
are important for the distribution of quantum signals. To that end, we first
study statistics generated by the turbulent atmosphere in an entanglement based
free-space quantum key distribution (QKD) system. Using the insights gained
from this analysis, we study the effect of link fluctuations on the security
and key generation rate of decoy state QKD concluding that it has minimal
effect in the typical operating regimes. We then investigate the novel idea of
using these turbulent fluctuations to our advantage in QKD experiments. We
implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects
measurements during periods of low transmission efficiency, where the measured
quantum bit error rate (QBER) is temporarily elevated. Using this, we increase
the total secret key generated by the system from 78,009 bits to 97,678 bits,
representing an increase of 25.2% in the final secure key rate, generated from
the same raw signals. Lastly, we present simulations of a QKD exchange with an
orbiting LEO satellite and show that an SNRF will be extremely useful in such a
situation, allowing many more passes to extract a secret key than would
otherwise be possible.Comment: 9 pages, 6 figure
X-ray reflectivity measurement of interdiffusion in metallic multilayers during rapid heating
A technique for measuring interdiffusion in multilayer materials during rapid heating using X-ray reflectivity is described. In this technique the sample is bent to achieve a range of incident angles simultaneously, and the scattered intensity is recorded on a fast high-dynamic-range mixed-mode pixel array detector. Heating of the multilayer is achieved by electrical resistive heating of the silicon substrate, monitored by an infrared pyrometer. As an example, reflectivity data from Al/Ni heated at rates up to 200 K s^(−1) are presented. At short times the interdiffusion coefficient can be determined from the rate of decay of the reflectivity peaks, and it is shown that the activation energy for interdiffusion is consistent with a grain boundary diffusion mechanism. At longer times the simple analysis no longer applies because the evolution of the reflectivity pattern is complicated by other processes, such as nucleation and growth of intermetallic phases
Testing Leggett's Inequality Using Aharonov-Casher Effect
Bell's inequality is established based on local realism. The violation of
Bell's inequality by quantum mechanics implies either locality or realism or
both are untenable. Leggett's inequality is derived based on nonlocal realism.
The violation of Leggett's inequality implies that quantum mechanics is neither
local realistic nor nonlocal realistic. The incompatibility of nonlocal realism
and quantum mechanics has been urrently confirmed by photon experiments. In our
work, we propose to test Leggett's inequality using the Aharonov-Casher effect.
In our scheme, four entangled particles emitted from two sources manifest a
two-qubit-typed correlation that may result in the violation of the Leggett
inequality, while satisfying the no-signaling condition for spacelike
separation. Our scheme is tolerant to some local inaccuracies due to the
topological nature of the Aharonov-Casher phase. The experimental
implementation of our scheme can be possibly realized by a calcium atomic
polarization interferometer experiment.Comment: 7 pages, 2 figures. Accepted by Scientific Report
High-efficiency quantum interrogation measurements via the quantum Zeno effect
The phenomenon of quantum interrogation allows one to optically detect the
presence of an absorbing object, without the measuring light interacting with
it. In an application of the quantum Zeno effect, the object inhibits the
otherwise coherent evolution of the light, such that the probability that an
interrogating photon is absorbed can in principle be arbitrarily small. We have
implemented this technique, demonstrating efficiencies exceeding the 50%
theoretical-maximum of the original ``interaction-free'' measurement proposal.
We have also predicted and experimentally verified a previously unsuspected
dependence on loss; efficiencies of up to 73% were observed and the feasibility
of efficiencies up to 85% was demonstrated.Comment: 4 pages, 3 postscript figures. To appear in Phys. Rev. Lett;
submitted June 11, 199
Extreme events in total ozone over Arosa – Part 2: Fingerprints of atmospheric dynamics and chemistry and effects on mean values and long-term changes
In this study the frequency of days with extreme low (termed ELOs) and extreme high (termed EHOs) total ozone values and their influence on mean values and trends are analyzed for the world's longest total ozone record (Arosa, Switzerland). The results show (a) an increase in ELOs and (b) a decrease in EHOs during the last decades and (c) that the overall trend during the 1970s and 1980s in total ozone is strongly dominated by changes in these extreme events. After removing the extremes, the time series shows a strongly reduced trend (reduction by a factor of 2.5 for trend in annual mean). Excursions in the frequency of extreme events reveal "fingerprints" of dynamical factors such as ENSO or NAO, and chemical factors, such as cold Arctic vortex ozone losses, as well as major volcanic eruptions of the 20th century (Gunung Agung, El Chichón, Mt. Pinatubo). Furthermore, atmospheric loading of ozone depleting substances leads to a continuous modification of column ozone in the Northern Hemisphere also with respect to extreme values (partly again in connection with polar vortex contributions). Application of extreme value theory allows the identification of many more such "fingerprints" than conventional time series analysis of annual and seasonal mean values. The analysis shows in particular the strong influence of dynamics, revealing that even moderate ENSO and NAO events have a discernible effect on total ozone. Overall the approach to extremal modelling provides new information on time series properties, variability, trends and the influence of dynamics and chemistry, complementing earlier analyses focusing only on monthly (or annual) mean values
A Zoology of Bell inequalities resistant to detector inefficiency
We derive both numerically and analytically Bell inequalities and quantum
measurements that present enhanced resistance to detector inefficiency. In
particular we describe several Bell inequalities which appear to be optimal
with respect to inefficient detectors for small dimensionality d=2,3,4 and 2 or
more measurement settings at each side. We also generalize the family of Bell
inequalities described in Collins et all (Phys. Rev. Lett. 88, 040404) to take
into account the inefficiency of detectors. In addition we consider the
possibility for pairs of entangled particles to be produced with probability
less than one. We show that when the pair production probability is small, one
must in general use different Bell inequalities than when the pair production
probability is high.Comment: 12 pages, revtex. Appendix completed, minor revision
Quantum Physics from A to Z
This is a collection of statements gathered on the occasion of the Quantum
Physics of Nature meeting in Vienna.Comment: 3 pages, Quantum Physics of Nature (QUPON) Conference, Vienna,
Austria, May 22nd-26th, 2005; v4: more contribution
Extreme events in total ozone over Arosa – Part 1: Application of extreme value theory
In this study ideas from extreme value theory are for the first time applied in the field of stratospheric ozone research, because statistical analysis showed that previously used concepts assuming a Gaussian distribution (e.g. fixed deviations from mean values) of total ozone data do not adequately address the structure of the extremes. We show that statistical extreme value methods are appropriate to identify ozone extremes and to describe the tails of the Arosa (Switzerland) total ozone time series. In order to accommodate the seasonal cycle in total ozone, a daily moving threshold was determined and used, with tools from extreme value theory, to analyse the frequency of days with extreme low (termed ELOs) and high (termed EHOs) total ozone at Arosa. The analysis shows that the Generalized Pareto Distribution (GPD) provides an appropriate model for the frequency distribution of total ozone above or below a mathematically well-defined threshold, thus providing a statistical description of ELOs and EHOs. The results show an increase in ELOs and a decrease in EHOs during the last decades. The fitted model represents the tails of the total ozone data set with high accuracy over the entire range (including absolute monthly minima and maxima), and enables a precise computation of the frequency distribution of ozone mini-holes (using constant thresholds). Analyzing the tails instead of a small fraction of days below constant thresholds provides deeper insight into the time series properties. Fingerprints of dynamical (e.g. ENSO, NAO) and chemical features (e.g. strong polar vortex ozone loss), and major volcanic eruptions, can be identified in the observed frequency of extreme events throughout the time series. Overall the new approach to analysis of extremes provides more information on time series properties and variability than previous approaches that use only monthly averages and/or mini-holes and mini-highs
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