754 research outputs found
The Neveu-Schwarz Five-Brane and its Dual Geometries
In this paper we discuss two aspects of duality transformations on the
Neveu-Schwarz (NS) 5-brane solutions in type II and heterotic string theories.
First we demonstrate that the non-extremal NS 5-brane background is U-dual to
its CGHS limit, a two-dimensional black hole times ; an
intermediate step is provided by the near horizon geometry which is given by
the three-dimensional black hole (being closely related to )
times . In the second part of the paper we discuss the T-duality
between NS 5-branes and the Taub-NUT spaces respectively ALE spaces, which
are related to the resolution of the singularities of the non-compact
orbifold . In particular in the framework of N=1
supersymmetric gauge theories related to brane box constructions we give the
metric dual to two sets of intersecting NS 5-branes. In this way we get a
picture of a dual orbifold background which is fibered
together out of two N=2 models (). Finally
we also discuss the intersection of NS 5-branes with D branes, which can serve
as probes of the dual background spaces.Comment: 18pp, added reference
Heterotic String Theory on non-Kaehler Manifolds with H-Flux and Gaugino Condensate
We discuss compactifications of heterotic string theory to four dimensions in
the presence of H-fluxes, which deform the geometry of the internal manifold,
and a gaugino condensate which breaks supersymmetry. We focus on the
compensation of the two effects in order to obtain vacua with zero cosmological
constant and we comment on the effective superpotential describing these vacua.Comment: 6 page
BPS Action and Superpotential for Heterotic String Compactifications with Fluxes
We consider N =1 compactifications to four dimensions of heterotic string
theory in the presence of fluxes. We show that up to order O(\alpha'^2) the
associated action can be written as a sum of squares of BPS-like quantities. In
this way we prove that the equations of motion are solved by backgrounds which
fulfill the supersymmetry conditions and the Bianchi identities. We also argue
for the expression of the related superpotential and discuss the radial modulus
stabilization for a class of examples.Comment: LaTeX, 28 pages. Minor changes, one more reference added. Final
version to appear on JHE
Duality Symmetries and Supersymmetry Breaking in String Compactifications
We discuss the spontaneous supersymetry breaking within the low-energy
effective supergravity action of four-dimensional superstrings. In particular,
we emphasize the non-universality of the soft supersymmetry breaking
parameters, the -problem and the duality symmetries.Comment: (invited talk to the 27th ICHEP, Glasgow, July 1994), 11 page
Multi-year Water Allocation: A Policy Analysis for Groundwater Management and Conservation for Irrigated Agriculture
Heavy withdrawals from the most dependable source of groundwater in the Texas Panhandle, the Ogallala Aquifer, create an impending need for implementing water conservation policies. This study evaluates the policy option of multi-year water allocation coupled with water use restriction in four water deficit counties of Castro, Deafsmith, Parmer and Swisher over a sixty year planning horizon. Results indicate that the water use in the study area declines with progressive restriction rates accompanied by a substantial decrease in the net present value of net returns over sixty years and therefore it is important to analyze the socio-economic effects of implementing such a policy alternative.Multi-year allocation, Ogallala Aquifer, Texas Panhandle, Water conservation, Environmental Economics and Policy, Farm Management, Resource /Energy Economics and Policy,
Black Hole Formation and Classicalization in Ultra-Planckian 2 -> N Scattering
We establish a connection between the ultra-Planckian scattering amplitudes
in field and string theory and unitarization by black hole formation in these
scattering processes. Using as a guideline an explicit microscopic theory in
which the black hole represents a bound-state of many soft gravitons at the
quantum critical point, we were able to identify and compute a set of
perturbative amplitudes relevant for black hole formation. These are the
tree-level N-graviton scattering S-matrix elements in a kinematical regime
(called classicalization limit) where the two incoming ultra-Planckian
gravitons produce a large number N of soft gravitons. We compute these
amplitudes by using the Kawai-Lewellen-Tye relations, as well as scattering
equations and string theory techniques. We discover that this limit reveals the
key features of the microscopic corpuscular black hole N-portrait. In
particular, the perturbative suppression factor of a N-graviton final state,
derived from the amplitude, matches the non-perturbative black hole entropy
when N reaches the quantum criticality value, whereas final states with
different value of N are either suppressed or excluded by non-perturbative
corpuscular physics. Thus we identify the microscopic reason behind the black
hole dominance over other final states including non-black hole classical
object. In the parameterization of the classicalization limit the scattering
equations can be solved exactly allowing us to obtain closed expressions for
the high-energy limit of the open and closed superstring tree-level scattering
amplitudes for a generic number N of external legs. We demonstrate matching and
complementarity between the string theory and field theory in different large-s
and large-N regimes.Comment: 55 pages, 7 figures, LaTeX; v2: typos removed; final version to
appear in Nucl. Phys.
Production Profitability of Ethanol from Alternative Feedstocks in the Texas Panhandle
The potential of three feedstocks: grain sorghum, sweet sorghum, and switchgrass for ethanol production in the top 26 counties of the Texas Panhandle Region is analyzed using yield and production costs of feedstock, processing cost of feedstock, final demand for ethanol, farm to wholesale marketing margin, and the derived demand price of feedstock. The calculated economic returns per acre of grain sorghum, sweet sorghum, and switchgrass are -410.19, and -38.25, -29.04 respectively under dryland condition. The evaluation in this study demonstrates that ethanol production from grain sorghum, sweet sorghum, and switchgrass in the Texas Panhandle Region is not economically feasible given the current price for ethanol in Texas. This is consistent with the status of the ethanol industry in the Texas Panhandle.Ethanol production, Texas Panhandle, Grain sorghum, Sweet sorghum, and Switchgrass, Feedstock, Crop Production/Industries, Production Economics, Resource /Energy Economics and Policy, Q16, Q25, Q27, and Q42,
Evaluating Dryland Crop/Livestock System Alternatives for Risk Management under Declining Irrigation in the Texas Panhandle
Production budgets for dryland crop and crop/livestock systems are developed to estimate yields, costs and returns for dryland wheat and sorghum and for alternative dryland crop/livestock systems. A crop simulation model aids yield estimation. The yield and return distributions are used to estimate risk and relative risk for included alternatives.Relative Risk, Ogallala Aquifer, Crop-Livestock Systems, Wheat, Sorghum, Crop Production/Industries, Farm Management, Livestock Production/Industries, Production Economics, Productivity Analysis,
Intercellular signaling as a cause of cell death in cyclically impacted cartilage explants
AbstractRecently, in vitro cartilage studies have shown that impact loading can produce structural damage and osteoarthritis-like changes, including tissue swelling, collagen denaturation, and cell death.Objective This study was to determine whether a signal for cell death moves through the cartilage matrix, resulting in the spread of cell death over time from impacted to unimpacted regions.Design Cyclic impacts were applied to the 2mm core of 4mm cartilage discs. Post-impact culturing extended for 3, 6 or 21 days and occurred in one of two ways. In one, discs were cultured intact. In the second, cores were removed immediately after cessation of impact and cores and rings cultured separately. Cells in apoptosis and later stage necrosis were monitored using the TUNEL assay.Results The extent of cell death in impacted samples increased with increased duration of post-impact culturing. At the early time, the majority of cell death was located in the regions of direct impact whereas after extended culture, the extent of cell death was similar in the surrounding unimpacted regions and in the impacted core region. However, the physical separation of the impacted core from the surrounding, non-impacted ring regions immediately after impact, and prior to independent culture, kept the level of cell death in the surrounding ring close to control levels, even after 21 days of incubation.Discussion These findings indicate that soluble intercellular signalling is involved in the spreading of cell death through the cartilage matrix, and that its effects can be prevented by physical isolation of the surrounding ring from the impacted core
Structured Random Matrices
Random matrix theory is a well-developed area of probability theory that has
numerous connections with other areas of mathematics and its applications. Much
of the literature in this area is concerned with matrices that possess many
exact or approximate symmetries, such as matrices with i.i.d. entries, for
which precise analytic results and limit theorems are available. Much less well
understood are matrices that are endowed with an arbitrary structure, such as
sparse Wigner matrices or matrices whose entries possess a given variance
pattern. The challenge in investigating such structured random matrices is to
understand how the given structure of the matrix is reflected in its spectral
properties. This chapter reviews a number of recent results, methods, and open
problems in this direction, with a particular emphasis on sharp spectral norm
inequalities for Gaussian random matrices.Comment: 46 pages; to appear in IMA Volume "Discrete Structures: Analysis and
Applications" (Springer
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