707 research outputs found
Self-Organizing Maps Algorithm for Parton Distribution Functions Extraction
We describe a new method to extract parton distribution functions from hard
scattering processes based on Self-Organizing Maps. The extension to a larger,
and more complex class of soft matrix elements, including generalized parton
distributions is also discussed.Comment: 6 pages, 3 figures, to be published in the proceedings of ACAT 2011,
14th International Workshop on Advanced Computing and Analysis Techniques in
Physics Researc
Photobioreactors as a Dynamic Shading System Conceived for an Outdoor Workspace of the State Library of Queensland in Brisbane: Study of Daylighting Performances
In the field of responsive shading systems, the use of photobioreactors (PBRs) containing microalgae seems to be a promising technology. Within this framework, this paper presents a case study where a PBR was specifically conceived as a shading system for an external workspace located on an open terrace of the State Library of Queensland (SLQ) in Brisbane. The proposed shading system makes use of PBRs as translucent screening elements, capable of regulating natural light to provide adequate visual comfort for a multi-purpose workspace, despite exposure of the site to direct sunlight and fluctuating weather conditions. The microalgae (Scenedesmus Obliquus) cultivated within the shading elements contribute to improving the environment by bio-sequestrating CO2 and producing oxygen. They are also a valuable raw material containing bioactive compounds with various applications including nutraceutical products, livestock feed, and biofuel synthesis. In addition, their green pigmentation is visually engaging and allows for the filtration of solar radiation. The paper focuses on analyzing the daylight performance in the outdoor workplace under the PBR shade. The biomass density of the PBR is adjustable across a wide range of light transmittances (10%-80%) to counter the highly variable outdoor conditions. For different Tv values, illuminance values were calculated using the simulation tool DIVA-for-Rhino and then analyzed as both point-in-time illuminance in June, September and December, and through climate-based daylight metrics such as the Daylight Autonomy and the Useful Daylight Illuminance. As a further step, the daylighting performances were compared to two other climates (Turin, Italy, and Dubai, United Arab Emirates), to verify which setting of the PBR may be suitable for different boundary conditions, thus making the system more widely applicable. The results in Brisbane showed that for clear sky conditions, higher levels of biomass saturation are needed (Tv < 20%) to remain in the 300-3000 lx 'optimal' illuminance comfort range, with lower concentrations (Tv < 30%) to remain within the 300-6000 lx 'acceptable' illuminance comfort range. Differently, in overcast sky conditions, the optimal range of illuminance is easily obtained with reduced levels of biomass saturation (Tv < 80%)
Boundary conditions for hyperbolic formulations of the Einstein equations
In regards to the initial-boundary value problem of the Einstein equations,
we argue that the projection of the Einstein equations along the normal to the
boundary yields necessary and appropriate boundary conditions for a wide class
of equivalent formulations. We explicitly show that this is so for the
Einstein-Christoffel formulation of the Einstein equations in the case of
spherical symmetry.Comment: 15 pages; text added and typesetting errors corrected; to appear in
Classical and Quantum Gravit
Einstein boundary conditions for the 3+1 Einstein equations
In the 3+1 framework of the Einstein equations for the case of vanishing
shift vector and arbitrary lapse, we calculate explicitly the four boundary
equations arising from the vanishing of the projection of the Einstein tensor
along the normal to the boundary surface of the initial-boundary value problem.
Such conditions take the form of evolution equations along (as opposed to
across) the boundary for certain components of the extrinsic curvature and for
certain space-derivatives of the intrinsic metric. We argue that, in general,
such boundary conditions do not follow necessarily from the evolution equations
and the initial data, but need to be imposed on the boundary values of the
fundamental variables. Using the Einstein-Christoffel formulation, which is
strongly hyperbolic, we show how three of the boundary equations should be used
to prescribe the values of some incoming characteristic fields. Additionally,
we show that the fourth one imposes conditions on some outgoing fields.Comment: Revtex 4, 6 pages, text and references added, typos corrected, to
appear in Phys. Rev.
Closed geodesics and billiards on quadrics related to elliptic KdV solutions
We consider algebraic geometrical properties of the integrable billiard on a
quadric Q with elastic impacts along another quadric confocal to Q. These
properties are in sharp contrast with those of the ellipsoidal Birkhoff
billiards. Namely, generic complex invariant manifolds are not Abelian
varieties, and the billiard map is no more algebraic. A Poncelet-like theorem
for such system is known. We give explicit sufficient conditions both for
closed geodesics and periodic billiard orbits on Q and discuss their relation
with the elliptic KdV solutions and elliptic Calogero systemComment: 23 pages, Latex, 1 figure Postscrip
First-order quasilinear canonical representation of the characteristic formulation of the Einstein equations
We prescribe a choice of 18 variables in all that casts the equations of the
fully nonlinear characteristic formulation of general relativity in
first--order quasi-linear canonical form. At the analytical level, a
formulation of this type allows us to make concrete statements about existence
of solutions. In addition, it offers concrete advantages for numerical
applications as it now becomes possible to incorporate advanced numerical
techniques for first order systems, which had thus far not been applicable to
the characteristic problem of the Einstein equations, as well as in providing a
framework for a unified treatment of the vacuum and matter problems. This is of
relevance to the accurate simulation of gravitational waves emitted in
astrophysical scenarios such as stellar core collapse.Comment: revtex4, 7 pages, text and references added, typos corrected, to
appear in Phys. Rev.
A candidate for a background independent formulation of M theory
A class of background independent membrane field theories are studied, and
several properties are discovered which suggest that they may play a role in a
background independent form of M theory. The bulk kinematics of these theories
are described in terms of the conformal blocks of an algebra G on all oriented,
finite genus, two-surfaces. The bulk dynamics is described in terms of causal
histories in which time evolution is specified by giving amplitudes to certain
local changes of the states. Holographic observables are defined which live in
finite dimensional states spaces associated with boundaries in spacetime. We
show here that the natural observables in these boundary state spaces are, when
G is chosen to be Spin(D) or a supersymmetric extension of it, generalizations
of matrix model coordinates in D dimensions. In certain cases the bulk dynamics
can be chosen so the matrix model dynamics is recoverd for the boundary
observables. The bosonic and supersymmetric cases in D=3 and D=9 are studied,
and it is shown that the latter is, in a certain limit, related to the matrix
model formulation of M theory. This correspondence gives rise to a conjecture
concerning a background independent form of M theory in terms of which
excitations of the background independent membrane field theory that correspond
to strings and D0 branes are identified.Comment: Latex 46 pages, 21 figures, new results included which lead to a
modification of the statement of the basic conjecture. Presentation improve
Gravitational Waves from a Fissioning White Hole
We present a fully nonlinear calculation of the waveform of the gravitational
radiation emitted in the fission of a vacuum white hole. At early times, the
waveforms agree with close-approximation perturbative calculations but they
reveal dramatic time and angular dependence in the nonlinear regime. The
results pave the way for a subsequent computation of the radiation emitted
after a binary black hole merger.Comment: 11 pages, 6 figures, RevTeX
Wave-train induced unpinning of weakly anchored vortices in excitable media
A free vortex in excitable media can be displaced and removed by a
wave-train. However, simple physical arguments suggest that vortices anchored
to large inexcitable obstacles cannot be removed similarly. We show that
unpinning of vortices attached to obstacles smaller than the core radius of the
free vortex is possible through pacing. The wave-train frequency necessary for
unpinning increases with the obstacle size and we present a geometric
explanation of this dependence. Our model-independent results suggest that
decreasing excitability of the medium can facilitate pacing-induced removal of
vortices in cardiac tissue.Comment: Published versio
Employment and earning differences in the early career of ethnic minority British graduates: the importance of university career, parental background and area characteristics
Ethnic minorities in the U.K. are more likely than the white majority to gain university qualifications, but experience worse labour market outcomes on average. This paper compares employment and earnings of British graduates from ethnic minorities to those of white British graduates to analyse whether ethnic labour market
differences exist among the highly qualified, and whether they can be explained by differences in parental background, local area characteristics or differences in university careers. These factors account for a substantial part of persistent ethnic differences in
earnings, but explain very little of the differences in employment. Compared to the literature estimating ethnic labour market inequalities on people with any level of qualification, we find smaller ethnic differences in employment and almost no differences in earnings among graduates entering the labour market. The results are robust to various changes in model specification
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