81 research outputs found
UNITED STATES-LATIN AMERICAN TRADE RELATIONS: IMPORTANCE TO U.S. AGRICULTURAL POLICY DECISIONS
International Relations/Trade,
On Conformal Infinity and Compactifications of the Minkowski Space
Using the standard Cayley transform and elementary tools it is reiterated
that the conformal compactification of the Minkowski space involves not only
the "cone at infinity" but also the 2-sphere that is at the base of this cone.
We represent this 2-sphere by two additionally marked points on the Penrose
diagram for the compactified Minkowski space. Lacks and omissions in the
existing literature are described, Penrose diagrams are derived for both,
simple compactification and its double covering space, which is discussed in
some detail using both the U(2) approach and the exterior and Clifford algebra
methods. Using the Hodge * operator twistors (i.e. vectors of the
pseudo-Hermitian space H_{2,2}) are realized as spinors (i.e., vectors of a
faithful irreducible representation of the even Clifford algebra) for the
conformal group SO(4,2)/Z_2. Killing vector fields corresponding to the left
action of U(2) on itself are explicitly calculated. Isotropic cones and
corresponding projective quadrics in H_{p,q} are also discussed. Applications
to flat conformal structures, including the normal Cartan connection and
conformal development has been discussed in some detail.Comment: 38 pages, 8 figures, late
Self-consistent solution of the Schwinger-Dyson equations for the nucleon and meson propagators
The Schwinger-Dyson equations for the nucleon and meson propagators are
solved self-consistently in an approximation that goes beyond the Hartree-Fock
approximation. The traditional approach consists in solving the nucleon
Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon
vertices; the corrections to the meson propagators are calculated using the
bare nucleon propagator and bare nucleon-meson vertices. It is known that such
an approximation scheme produces the appearance of ghost poles in the
propagators. In this paper the coupled system of Schwinger-Dyson equations for
the nucleon and the meson propagators are solved self-consistently including
vertex corrections. The interplay of self-consistency and vertex corrections on
the ghosts problem is investigated. It is found that the self-consistency does
not affect significantly the spectral properties of the propagators. In
particular, it does not affect the appearance of the ghost poles in the
propagators.Comment: REVTEX, 7 figures (available upon request), IFT-P.037/93,
DOE/ER/40427-12-N9
Nuclear Alpha-Particle Condensates
The -particle condensate in nuclei is a novel state described by a
product state of 's, all with their c.o.m. in the lowest 0S orbit. We
demonstrate that a typical -particle condensate is the Hoyle state
( MeV, state in C), which plays a crucial role for
the synthesis of C in the universe. The influence of antisymmentrization
in the Hoyle state on the bosonic character of the particle is
discussed in detail. It is shown to be weak. The bosonic aspects in the Hoyle
state, therefore, are predominant. It is conjectured that -particle
condensate states also exist in heavier nuclei, like O,
Ne, etc. For instance the state of O at MeV
is identified from a theoretical analysis as being a strong candidate of a
condensate. The calculated small width (34 keV) of ,
consistent with data, lends credit to the existence of heavier Hoyle-analogue
states. In non-self-conjugated nuclei such as B and C, we discuss
candidates for the product states of clusters, composed of 's,
triton's, and neutrons etc. The relationship of -particle condensation
in finite nuclei to quartetting in symmetric nuclear matter is investigated
with the help of an in-medium modified four-nucleon equation. A nonlinear order
parameter equation for quartet condensation is derived and solved for
particle condensation in infinite nuclear matter. The strong qualitative
difference with the pairing case is pointed out.Comment: 71 pages, 41 figures, review article, to be published in "Cluster in
Nuclei (Lecture Notes in Physics) - Vol.2 -", ed. by C. Beck,
(Springer-Verlag, Berlin, 2011
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Commercial Actors and the Governing of Education: the case of academy school sponsors in England.
grant number ESIO 1943X/1This article explores the ways in which commercial actors are operating in state education by focusing on the case study of England’s academies policy. First of all the discussion outlines the development of academies over time and the way in which the policy has provided opportunities for private actors to become involved in the state schooling system. The second part of the article focuses on empirical findings from interviews with academy sponsors as a way of understanding the nature of the role commercial actors play in academies. Commercial sponsor involvement in academies demonstrates a blurring of the boundaries between ‘public’ and ‘private’, and the article suggests that commercial actors need to be understood not merely as participants in this process, but as being active in the reconstruction of ‘public—private’ categories. Also, it is underlined that the commercial sponsors in this study cannot be disconnected from the locality in which they operate. In light of this, the article calls for greater attention to be devoted to understanding local assemblages of policy when attempting to explore the role played by commercial actors in the governance of education
Buoyancy Effects on Upward Brine Displacement Caused by CO2 Injection
Upward displacement of brine from deep reservoirs driven by pressure increases resulting from CO{sub 2} injection for geologic carbon sequestration may occur through improperly sealed abandoned wells, through permeable faults, or through permeable channels between pinch-outs of shale formations. The concern about upward brine flow is that, upon intrusion into aquifers containing groundwater resources, the brine may degrade groundwater. Because both salinity and temperature increase with depth in sedimentary basins, upward displacement of brine involves lifting fluid that is saline but also warm into shallower regions that contain fresher, cooler water. We have carried out dynamic simulations using TOUGH2/EOS7 of upward displacement of warm, salty water into cooler, fresher aquifers in a highly idealized two-dimensional model consisting of a vertical conduit (representing a well or permeable fault) connecting a deep and a shallow reservoir. Our simulations show that for small pressure increases and/or high-salinity-gradient cases, brine is pushed up the conduit to a new static steady-state equilibrium. On the other hand, if the pressure rise is large enough that brine is pushed up the conduit and into the overlying upper aquifer, flow may be sustained if the dense brine is allowed to spread laterally. In this scenario, dense brine only contacts the lower-most region of the upper aquifer. In a hypothetical case in which strong cooling of the dense brine occurs in the upper reservoir, the brine becomes sufficiently dense that it flows back down into the deeper reservoir from where it came. The brine then heats again in the lower aquifer and moves back up the conduit to repeat the cycle. Parameter studies delineate steady-state (static) and oscillatory solutions and reveal the character and period of oscillatory solutions. Such oscillatory solutions are mostly a curiosity rather than an expected natural phenomenon because in nature the geothermal gradient prevents the cooling in the upper aquifer that occurs in the model. The expected effect of upward brine displacement is either establishment of a new hydrostatic equilibrium or sustained upward flux into the bottom-most region of the upper aquifer
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