1,363 research outputs found
SUSY in the sky
Spinning particles in curved space-time can have fermionic symmetries
generated by the square root of bosonic constants of motion other than the
Hamiltonian. We present a general analysis of the conditions under which such
new supersymmetries appear, and discuss the Poisson-Dirac algebra of the
resulting set of charges, including the conditions of closure of the new
algebra. An example of a new non-trivial supersymmetry is found in black-hole
solutions of the Kerr-Newman type and corresponds to the Killing-Yano tensor,
which plays an important role in solving the Dirac equation in these black-hole
metrics.Comment: 28, NIKHEF-H/93-04 and DAMTP R92/4
Killing tensors and a new geometric duality
We present a theorem describing a dual relation between the local geometry of
a space admitting a symmetric second-rank Killing tensor, and the local
geometry of a space with a metric specified by this Killing tensor. The
relation can be generalized to spinning spaces, but only at the expense of
introducing torsion. This introduces new supersymmetries in their geometry.
Interesting examples in four dimensions include the Kerr-Newman metric of
spinning black-holes and self-dual Taub-NUT.Comment: 20 pages (a4), standard LaTeX, no figure
Histopathological Characterization of the Lesions of Contagious Ovine Digital Dermatitis and Immunolabelling of Treponema-like Organisms
SummaryContagious ovine digital dermatitis (CODD) is a cause of severe lameness in sheep and the three Treponema phylogroups Treponema medium/Treponema vincentii-like, Treponema phagedenis-like and Treponema pedis have been associated with clinical disease. The aims of this study were: (1) to describe the histopathological changes associated with each previously established grade of clinical lesion, and (2) to investigate immunohistochemically the association of the Treponema-like organisms with the observed histopathological changes. Early lesions were characterized by lymphoplasmacytic infiltration of the distal digital skin, with suppurative coronitis and intracorneal pustules. In more advanced stages of the disease there was complete separation of the dorsal wall of the hoof with a necrotizing and fibrinosuppurative exudate and dermatitis. The later lesions were mostly resolved, but with milder suppurative changes remaining within the cornified layer and periosteal reaction of the dorsal aspect of the distal phalanx. Large numbers of Treponema-like organisms were identified within early grade lesions (as well as later, more advanced grade lesions) and were specifically associated with the observed histopathological changes. The results of this study provide some evidence in support of the hypothesis that the three CODD-associated Treponema phylogroups are involved in the aetiopathogenesis of this disease
Variational description of multi-fluid hydrodynamics: Uncharged fluids
We present a formalism for Newtonian multi-fluid hydrodynamics derived from
an unconstrained variational principle. This approach provides a natural way of
obtaining the general equations of motion for a wide range of hydrodynamic
systems containing an arbitrary number of interacting fluids and superfluids.
In addition to spatial variations we use ``time shifts'' in the variational
principle, which allows us to describe dissipative processes with entropy
creation, such as chemical reactions, friction or the effects of external
non-conservative forces. The resulting framework incorporates the
generalization of the entrainment effect originally discussed in the case of
the mixture of two superfluids by Andreev and Bashkin. In addition to the
conservation of energy and momentum, we derive the generalized conservation
laws of vorticity and helicity, and the special case of Ertel's theorem for the
single perfect fluid.
We explicitly discuss the application of this framework to thermally
conducting fluids, superfluids, and superfluid neutron star matter. The
equations governing thermally conducting fluids are found to be more general
than the standard description, as the effect of entrainment usually seems to be
overlooked in this context. In the case of superfluid He4 we recover the
Landau--Khalatnikov equations of the two-fluid model via a translation to the
``orthodox'' framework of superfluidity, which is based on a rather awkward
choice of variables. Our two-fluid model for superfluid neutron star matter
allows for dissipation via mutual friction and also ``transfusion'' via
beta-reactions between the neutron fluid and the proton-electron fluid.Comment: uses RevTeX 4; 20 pages. To appear in PRD. v2: removed discussion of
charged fluids and coupling to electromagnetic fields, which are submitted as
a separate paper for a clearer presentation v3: fixed typo in Eq.(9), updated
some reference
Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2C
Elastic and inelastic neutron scattering measurements have been carried out
to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The
inelastic measurements reveal that the lowest two crystal field transitions out
of the ground state occurat 11.28(3) and 16.00(2) meV, while the transition of
4.70(9) meV between these two levels is observed at elevated temperatures. The
temperature dependence of the intensities of these transitions is consistent
with both the ground state and these higher levels being magnetic doublets. The
system becomes magnetically long range ordered below 8K, and since this
ordering energy kTN ~ 0.69meV << 11.28meV the magnetic properties in the
ordered phase are dominated by the ground-state spin dynamics only. The low
temperature structure, which coexists with superconductivity, consists of
ferromagnetic sheets of Ho{3+ moments in the a-b plane, with the sheets coupled
antiferromagnetically along the c-axis. The magnetic state that initially forms
on cooling, however, is dominated by an incommensurate spiral antiferromagnetic
state along the c-axis, with wave vector qc ~0.054 A-1, in which these
ferromagnetic sheets are canted from their low temperature antiparallel
configuration by ~17 deg. The intensity for this spiral state reaches a maximum
near the reentrant superconducting transition at ~5K; the spiral state then
collapses at lower temperature in favor of the commensurate antiferromagnetic
state. We have investigated the field dependence of the magnetic order at and
above this reentrant superconducting transition. Initially the field rotates
the powder particles to align the a-b plane along the field direction,
demonstrating that the moments strongly prefer to lie within this plane due to
the crystal field anisotropy. Upon subsequently increasing the field atComment: RevTex, 7 pages, 11 figures (available upon request); Physica
Inhomogeneities in dusty universe - a possible alternative to dark energy?
There have been of late renewed debates on the role of inhomogeneities to
explain the observed late acceleration of the universe. We have looked into the
problem analytically with the help of the well known spherically symmetric but
inhomogeneous Lemaitre-Tolman-Bondi(LTB) model generalised to higher
dimensions. It is observed that in contrast to the claim made by Kolb et al the
presence of inhomogeneities as well as extra dimensions can not reverse the
signature of the deceleration parameter if the matter field obeys the energy
conditions. The well known Raychaudhuri equation also points to the same
result. Without solving the field equations explicitly it can, however, be
shown that although the total deceleration is positive everywhere nevertheless
it does not exclude the possibility of having radial acceleration, even in the
pure dust universe, if the angular scale factor is decelerating fast enough and
vice versa. Moreover it is found that introduction of extra dimensions can not
reverse the scenario. To the contrary it actually helps the decelerating
process.Comment: 14 pages, 4 figure
Targeting Conservation Investments in Heterogeneous Landscapes: A distance function approach and application to watershed management
To achieve a given level of an environmental amenity at least cost, decision-makers must integrate information about spatially variable biophysical and economic conditions. Although the biophysical attributes that contribute to supplying an environmental amenity are often known, the way in which these attributes interact to produce the amenity is often unknown. Given the difficulty in converting multiple attributes into a unidimensional physical measure of an environmental amenity (e.g., habitat quality), analyses in the academic literature tend to use a single biophysical attribute as a proxy for the environmental amenity (e.g., species richness). A narrow focus on a single attribute, however, fails to consider the full range of biophysical attributes that are critical to the supply of an environmental amenity. Drawing on the production efficiency literature, we introduce an alternative conservation targeting approach that relies on distance functions to cost-efficiently allocate conservation funds across a spatially heterogeneous landscape. An approach based on distance functions has the advantage of not requiring a parametric specification of the amenity function (or cost function), but rather only requiring that the decision-maker identify important biophysical and economic attributes. We apply the distance-function approach empirically to an increasingly common, but little studied, conservation initiative: conservation contracting for water quality objectives. The contract portfolios derived from the distance-function application have many desirable properties, including intuitive appeal, robust performance across plausible parametric amenity measures, and the generation of ranking measures that can be easily used by field practitioners in complex decision-making environments that cannot be completely modeled. Working Paper # 2002-01
A variational formulation of anisotropic geometric evolution equations in higher dimensions
Accepted versio
Detection and characterization of β-adrenergic receptors and adenylate cyclase in coated vesicles isolated from bovine brain
[No abstract available
Global modelling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution
We discuss 3D global simulations of the early Martian climate that we have
performed assuming a faint young Sun and denser CO2 atmosphere. We include a
self-consistent representation of the water cycle, with atmosphere-surface
interactions, atmospheric transport, and the radiative effects of CO2 and H2O
gas and clouds taken into account. We find that for atmospheric pressures
greater than a fraction of a bar, the adiabatic cooling effect causes
temperatures in the southern highland valley network regions to fall
significantly below the global average. Long-term climate evolution simulations
indicate that in these circumstances, water ice is transported to the highlands
from low-lying regions for a wide range of orbital obliquities, regardless of
the extent of the Tharsis bulge. In addition, an extended water ice cap forms
on the southern pole, approximately corresponding to the location of the
Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2
atmosphere, conditions are too cold to allow long-term surface liquid water.
Limited melting occurs on warm summer days in some locations, but only for
surface albedo and thermal inertia conditions that may be unrealistic for water
ice. Nonetheless, meteorite impacts and volcanism could potentially cause
intense episodic melting under such conditions. Because ice migration to higher
altitudes is a robust mechanism for recharging highland water sources after
such events, we suggest that this globally sub-zero, `icy highlands' scenario
for the late Noachian climate may be sufficient to explain most of the fluvial
geology without the need to invoke additional long-term warming mechanisms or
an early warm, wet Mars.Comment: Minor revisions to text, one new table, figs. 1,3 11 and 18 redon
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