12,566 research outputs found
Hidden supersymmetry of domain walls and cosmologies
We show that all domain-wall solutions of gravity coupled to scalar fields
for which the worldvolume geometry is Minkowski or anti-de Sitter admit Killing
spinors, and satisfy corresponding first-order equations involving a
superpotential determined by the solution. By analytic continuation, all flat
or closed FLRW cosmologies are shown to satisfy similar first-order equations
arising from the existence of ``pseudo-Killing'' spinors.Comment: 4 pages, v2:minor improvements, refs added, version to appear in PR
Force reflecting hand controller
A universal input device for interfacing a human operator with a slave machine such as a robot or the like includes a plurality of serially connected mechanical links extending from a base. A handgrip is connected to the mechanical links distal from the base such that a human operator may grasp the handgrip and control the position thereof relative to the base through the mechanical links. A plurality of rotary joints is arranged to connect the mechanical links together to provide at least three translational degrees of freedom and at least three rotational degrees of freedom of motion of the handgrip relative to the base. A cable and pulley assembly for each joint is connected to a corresponding motor for transmitting forces from the slave machine to the handgrip to provide kinesthetic feedback to the operator and for producing control signals that may be transmitted from the handgrip to the slave machine. The device gives excellent kinesthetic feedback, high-fidelity force/torque feedback, a kinematically simple structure, mechanically decoupled motion in all six degrees of freedom, and zero backlash. The device also has a much larger work envelope, greater stiffness and responsiveness, smaller stowage volume, and better overlap of the human operator's range of motion than previous designs
Sulfur amino acid requirements of broilers from two to five weeks of age
Phase-feeding (PF) in broiler chickens has been researched as a way to reduce feed costs without reducing growth performance and yield. Predicted amino acid requirements for PF are generated using linear regression equations derived from best estimates of lysine (Lys), sulfur amino acid (SAA), and threonine (Thr) requirements. During the late starter and early grower periods, predicted requirements for the SAA methionine (Met) and cysteine (Cys) are higher than levels recommended by the National Research Council (NRC), and previous research suggests that SAA may be lowered during the grower period without sacrificing growth performance or yield. The objective of this study was to estimate Met and Cys requirements for broilers from 2 to 5 weeks of age. In Experiment 1, a Met-deficient corn-peanut meal diet was formulated to contain excess Cys, so that supplemental Met was not utilized for Cys synthesis. The basal diet for Experiment 2 met the Met requirement but was deficient in Cys. Graded levels of Met (0, 0.045, 0.09, 0.135, and 0.225%) and Cys (0, 0.035, 0.070, 0.105, 0.140, 0.175%) were added in Experiments 1 and 2, respectively, and diets were fed to five replicates of five broilers per pen. Broken-line analysis was used to estimate SAA requirements. The digestible Met and Cys requirements from 2 to 5 weeks of age were 0.33% and 0.31%, respectively. Requirement estimates were lower than those predicted by PF or recommended by NRC, indicating that lower SAA levels may be utilized in a PF progra
Influence of correlated visual cues on auditory signal detection
Influence of correlated visual cues on auditory signal detectio
Hamilton-Jacobi method for Domain Walls and Cosmologies
We use Hamiltonian methods to study curved domain walls and cosmologies. This
leads naturally to first order equations for all domain walls and cosmologies
foliated by slices of maximal symmetry. For Minkowski and AdS-sliced domain
walls (flat and closed FLRW cosmologies) we recover a recent result concerning
their (pseudo)supersymmetry. We show how domain-wall stability is consistent
with the instability of adS vacua that violate the Breitenlohner-Freedman
bound. We also explore the relationship to Hamilton-Jacobi theory and compute
the wave-function of a 3-dimensional closed universe evolving towards de Sitter
spacetime.Comment: 18 pages; v2: typos corrected, one ref added, version to appear in
PR
Branon radiative corrections to collider physics and precision observables
In the context of brane-world scenarios, we study the effects produced by the
exchange of virtual massive branons. A one-loop calculation is performed which
generates higher-dimensional operators involving SM fields suppressed by powers
of the brane tension scale. We discuss constraints on this scenario from
colliders such as HERA, LEP and Tevatron and prospects for future detections at
LHC or ILC. The most interesting phenomenology comes from new four-particles
vertices induced by branon radiative corrections, mainly from four fermion
interactions. The presence of flexible branes modifies also the muon anomalous
magnetic moment and the electroweak precision observables.Comment: 23 pages, 4 figures, LaTe
A Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars
We introduce a new Rigid-Field Hydrodynamics approach to modeling the
magnetospheres of massive stars in the limit of very-strong magnetic fields.
Treating the field lines as effectively rigid, we develop hydrodynamical
equations describing the 1-dimensional flow along each, subject to pressure,
radiative, gravitational, and centrifugal forces. We solve these equations
numerically for a large ensemble of field lines, to build up a 3-dimensional
time-dependent simulation of a model star with parameters similar to the
archetypal Bp star sigma Ori E. Since the flow along each field line can be
solved for independently of other field lines, the computational cost of this
approach is a fraction of an equivalent magnetohydrodynamical treatment.
The simulations confirm many of the predictions of previous analytical and
numerical studies. Collisions between wind streams from opposing magnetic
hemispheres lead to strong shock heating. The post-shock plasma cools initially
via X-ray emission, and eventually accumulates into a warped, rigidly rotating
disk defined by the locus of minima of the effective (gravitational plus
centrifugal) potential. But a number of novel results also emerge. For field
lines extending far from the star, the rapid area divergence enhances the
radiative acceleration of the wind, resulting in high shock velocities (up to
~3,000 km/s) and hard X-rays. Moreover, the release of centrifugal potential
energy continues to heat the wind plasma after the shocks, up to temperatures
around twice those achieved at the shocks themselves. Finally, in some
circumstances the cool plasma in the accumulating disk can oscillate about its
equilibrium position, possibly due to radiative cooling instabilities in the
adjacent post-shock regions.Comment: 21 pages, 12 figures w/ color, accepted by MNRA
Guidance, flight mechanics and trajectory optimization. Volume 11 - Guidance equations for orbital operations
Mathematical formulation of guidance equations and solutions for orbital space mission
Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds
We present 2D MHD simulations of the radiatively driven outflow from a
rotating hot star with a dipole magnetic field aligned with the star's rotation
axis. We focus primarily on a model with moderately rapid rotation (half the
critical value), and also a large magnetic confinement parameter, . The magnetic field
channels and torques the wind outflow into an equatorial, rigidly rotating disk
extending from near the Kepler corotation radius outwards. Even with
fine-tuning at lower magnetic confinement, none of the MHD models produce a
stable Keplerian disk. Instead, material below the Kepler radius falls back on
to the stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outwards, leading to
episodic breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly K, high
enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection}
represents a novel mechanism for driving magnetic reconnection, and seems a
very promising basis for modeling X-ray flares recently observed in rotating
magnetic Bp stars like Ori E.Comment: 5 pages, 3 figures, accepted by ApJ
Conformal Theory of M2, D3, M5 and `D1+D5' Branes
The bosonic actions for M2, D3 and M5 branes in their own d-dimensional
near-horizon background are given in a manifestly SO(p+1,2) x SO(d-p-1)
invariant form (p=2,3,5). These symmetries result from a breakdown of ISO(d,2)
(with d=10 for D3 and d=11 for M2 and M5) symmetry by the Wess-Zumino term and
constraints. The new brane actions, reduce after gauge-fixing and solving
constraints to (p+1) dimensional interacting field theories with a non-linearly
realized SO(p+1,2) conformal invariance. We also present an interacting
two-dimensional conformal field theory on a D-string in the near-horizon
geometry of a D1+D5 configuration.Comment: 32 pages, two figures, Latex. A version to appear in JHEP. A comment
is added on infinite dimensional Kac-Moody type symmetry of D1+D5 system
observed by Brandt, Gomis, Sim'o
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