56,713 research outputs found
Calculation of epsilon'/epsilon
I shortly review the present status of the theoretical calculations of
epsilon'/epsilon and the comparison with the present experimental results. I
discuss the role of higher order chiral corrections and in general of
non-factorizable contributions for the explanation of the Delta I = 1/2
selection rule and direct CP violation in kaon decays. Still lacking reliable
lattice calculations, analytic methods and phenomenological approaches are
helpful in understanding correlations among theoretical effects and the
experimental data. Substantial progress from lattice QCD is expected in the
coming years.Comment: 10 pages, Invited talk at the 9th Int. Symposium on Heavy Flavour
Physics, Pasadena, CA, 10-13 September 2001 Comments added on Table
Estimating Graphlet Statistics via Lifting
Exploratory analysis over network data is often limited by the ability to
efficiently calculate graph statistics, which can provide a model-free
understanding of the macroscopic properties of a network. We introduce a
framework for estimating the graphlet count---the number of occurrences of a
small subgraph motif (e.g. a wedge or a triangle) in the network. For massive
graphs, where accessing the whole graph is not possible, the only viable
algorithms are those that make a limited number of vertex neighborhood queries.
We introduce a Monte Carlo sampling technique for graphlet counts, called {\em
Lifting}, which can simultaneously sample all graphlets of size up to
vertices for arbitrary . This is the first graphlet sampling method that can
provably sample every graphlet with positive probability and can sample
graphlets of arbitrary size . We outline variants of lifted graphlet counts,
including the ordered, unordered, and shotgun estimators, random walk starts,
and parallel vertex starts. We prove that our graphlet count updates are
unbiased for the true graphlet count and have a controlled variance for all
graphlets. We compare the experimental performance of lifted graphlet counts to
the state-of-the art graphlet sampling procedures: Waddling and the pairwise
subgraph random walk
2D materials and van der Waals heterostructures
The physics of two-dimensional (2D) materials and heterostructures based on
such crystals has been developing extremely fast. With new 2D materials, truly
2D physics has started to appear (e.g. absence of long-range order, 2D
excitons, commensurate-incommensurate transition, etc). Novel heterostructure
devices are also starting to appear - tunneling transistors, resonant tunneling
diodes, light emitting diodes, etc. Composed from individual 2D crystals, such
devices utilize the properties of those crystals to create functionalities that
are not accessible to us in other heterostructures. We review the properties of
novel 2D crystals and how their properties are used in new heterostructure
devices
Large-scale bias in the Universe: bispectrum method
Evidence that the Universe may be close to the critical density, required for
its expansion eventually to be halted, comes principally from dynamical studies
of large-scale structure. These studies either use the observed peculiar
velocity field of galaxies directly, or indirectly by quantifying its
anisotropic effect on galaxy clustering in redshift surveys. A potential
difficulty with both such approaches is that the density parameter
is obtained only in the combination , if linear
perturbation theory is used. The determination of the density parameter
is therefore compromised by the lack of a good measurement of the
bias parameter , which relates the clustering of sample galaxies to the
clustering of mass.
In this paper, we develop an idea of Fry (1994), using second-order
perturbation theory to investigate how to measure the bias parameter on large
scales. The use of higher-order statistics allows the degeneracy between
and to be lifted, and an unambiguous determination of
then becomes possible. We apply a likelihood approach to the bispectrum, the
three-point function in Fourier space. This paper is the first step in turning
the idea into a practical proposition for redshift surveys, and is principally
concerned with noise properties of the bispectrum, which are non-trivial. The
calculation of the required bispectrum covariances involves the six-point
function, including many noise terms, for which we have developed a generating
functional approach which will be of value in calculating high-order statistics
in general.Comment: 12 pages, latex, 7 postscript figures included. Accepted by MNRAS.
(Minor numerical typesetting errors corrected: results unchanged
Two-dimensional Bragg grating lasers defined by electron-beam lithography
Two-dimensional Bragg grating (2DBG) lasers with two quarter-wave slip line defects have been designed and fabricated by electron-beam lithography and reactive ion etching. Unlike conventional two-dimensional photonic crystal defect lasers, which use a large refractive index perturbation to confine light in a plane, the 2DBG structures described here selectively control the longitudinal and transverse wave vector components using a weak index perturbation. Two line defects perpendicular to each other are introduced in the 2DBG to define the optical resonance condition in the longitudinal and transverse directions. In this article, we describe the lithography process used to pattern these devices. The 2DBG lasers were defined using polymethylmethacrylate resist exposed in a Leica Microsystems EBPG 5000+ electron-beam writer at 100 kV. A proximity correction code was used to obtain a uniform pattern distribution over a large area, and a dosage matrix was used to optimize the laser design parameters. Measurements of electrically pumped 2DBG lasers showed modal selection in both the longitudinal and transverse directions due to proper design of the grating and defects, making them promising candidates for single-mode, high power, high efficiency, large-area lasers
Dynamics of aeolian sand ripples
We analyze theoretically the dynamics of aeolian sand ripples. In order to
put the study in the context we first review existing models. We argue on the
local character of sand ripple formation. Using a hydrodynamical model we
derive a nonlinear equation for the sand profile. We show how the
hydrodynamical model may be modified to recover the missing terms that are
dictated by symmetries. The symmetry and conservation arguments are powerful in
that the form of the equation is model-independent. We then present an
extensive numerical and analytical analysis of the generic sand ripple
equation. We find that at the initial stage the wavelength of the ripple is
that corresponding to the linearly most dangerous mode. At later stages the
profile undergoes a coarsening process leading to a significant increase of the
wavelength. We find that including the next higher order nonlinear term in the
equation, leads naturally to a saturation of the local slope. We analyze both
analytically and numerically the coarsening stage, in terms of a dynamical
exponent for the mean wavelength increase. We discuss some future lines of
investigations.Comment: 22 pages and 10 postscript figure
Does the UK Local Finance Improvement Trust (Lift)Initiative Improve Risk Management in Public-Private Procurement?
The UK government introduced the Private Finance Initiative (PFI) and, latterly, the Local Improvement Finance Trust (LIFT) in an attempt to improve public service provision. As a variant of PFI, LIFT seeks to create a framework for the effective provision of primary care facilities. Like conventional PFI procurement, LIFT projects involve long-term contracts, complex multi-party interactions and thus create various risks to public sector clients. This paper investigates the advantages and disadvantages of LIFT with a focus on how this approach facilitates or impedes risk management from the public sector client perspective. Our paper concludes that LIFT has a potential for creating additional problems, including the further reduction of public sector control, conflicts of interest, the inappropriate use of enabling funds, and higher than market rental costs affecting the uptake of space in the buildings by local health care providers. However, there is also evidence that LIFT has facilitated new investment and that Primary Care Trusts (PCTs) have themselves started addressing some of the weaknesses of this procurement format through the bundling of projects and other forms of regional co-operation
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