388 research outputs found
No multi-graviton theories in the presence of a Dirac field
The cross-couplings among several massless spin-two fields (described in the
free limit by a sum of Pauli-Fierz actions) in the presence of a Dirac field
are investigated in the framework of the deformation theory based on local BRST
cohomology. Under the hypotheses of locality, smoothness of the interactions in
the coupling constant, Poincare invariance, (background) Lorentz invariance and
the preservation of the number of derivatives on each field, we prove that
there are no consistent cross-interactions among different gravitons in the
presence of a Dirac field. The basic features of the couplings between a single
Pauli-Fierz field and a Dirac field are also emphasized.Comment: 48 page
Geometric Second Order Field Equations for General Tensor Gauge Fields
Higher spin tensor gauge fields have natural gauge-invariant field equations
written in terms of generalised curvatures, but these are typically of higher
than second order in derivatives. We construct geometric second order field
equations and actions for general higher spin boson fields, and first order
ones for fermions, which are non-local but which become local on gauge-fixing,
or on introducing auxiliary fields. This generalises the results of Francia and
Sagnotti to all representations of the Lorentz group.Comment: 34 pages, LaTeX. Reference adde
Identification of phenolic constituents of cytisus multiflorus
The phenolic composition of the ethanolic extract obtained from the flowers of the medicinal plant Cytisus
multiflorus has been elucidated by high performance liquid chromatography, electrospray mass spectrometry
and nuclear magnetic resonance analysis. The extract was mainly composed of flavones,
including the common chrysin, orientin, luteolin-5-O-glucoside, luteolin-7-O-glucoside, apigenin and
apigenin-7-O-glucoside, which appeared as minor components. The major flavone in the extract was
chrysin-7-O-B-D-glucopyranoside, and it also contained moderate amounts of a dihydroxyflavone isomer
of chrysin, as well as of 2''-O-pentosyl-6-C-hexosyl-luteolin, 2''-O-pentosyl-8-C-hexosyl-luteolin and 6''-
O-(3-hydroxy-3-methylglutaroyl)-2''-O-pentosyl-C-hexosyl-apigenin, which are not commonly found in
the Fabaceae family. Other novel phenolic compounds found in the ethanolic extract of C. multiflorus
comprised the flavones 2''-O-pentosyl-6-C-hexosyl-apigenin, 2''-O-pentosyl-8-C-hexosyl-apigenin and
6''-O-(3-hydroxy-3-methylglutaroyl)-200-O-pentosyl-C-hexosyl-luteolin. The assessment of the biological
activities of the main compounds of this extract are now keen, in order to determine their relevance in
the beneficial properties of the plant
A low-memory algorithm for finding short product representations in finite groups
We describe a space-efficient algorithm for solving a generalization of the
subset sum problem in a finite group G, using a Pollard-rho approach. Given an
element z and a sequence of elements S, our algorithm attempts to find a
subsequence of S whose product in G is equal to z. For a random sequence S of
length d log_2 n, where n=#G and d >= 2 is a constant, we find that its
expected running time is O(sqrt(n) log n) group operations (we give a rigorous
proof for d > 4), and it only needs to store O(1) group elements. We consider
applications to class groups of imaginary quadratic fields, and to finding
isogenies between elliptic curves over a finite field.Comment: 12 page
Marine pelagic ecosystems: the West Antarctic Peninsula
The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 28C increase in the annual mean temperature and a 68C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.68C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in icedependent Ade´lie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along theWAPand the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response
Forward induction and entry deterrence: an experiment
The Dixit (Econ J 90:95–106, 1980) hypothesis that incumbents use
investment in capacity to deter potential entrants has found little empirical support.
Bagwell and Ramey (J Econ 27:660–680, 1996) propose a model where, in the unique game-theoretic prediction based on forward induction or iterated elimination
of weakly-dominated strategies, the incumbent does not have the strategic
advantage. We conduct an experiment with games inspired by these models. In the
Dixit-style game, the incumbent monopolizes the market most of the time even
without the investment in capacity. In our Bagwell-and-Ramey-style game, the
incumbent also tends to keep the market, in contrast to the predictions of an entrant
advantage. Nevertheless, we fin strong evidence that forward induction affects
the behavior of most participants. The results of our games suggest that players
perceive that the firs mover has an advantage without having to pre-commit
capacity. In our Bagwell–Ramey game, evolution and learning do not drive out this
perception. We back these claims with data analysis and a theoretical framework
for dynamics.Publicad
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Cost calculation and prediction in adult intensive care: A ground-up utilization study
Publisher's copy made available with the permission of the publisherThe ability of various proxy cost measures, including therapeutic activity scores (TISS and Omega) and cumulative daily severity of illness scores, to predict individual ICU patient costs was assessed in a prospective “ground-up” utilization costing study over a six month period in 1991. Daily activity (TISS and Omega scores) and utilization in consecutive admissions to three adult university associated ICUs was recorded by dedicated data collectors. Cost prediction used linear regression with determination (80%) and validation (20%) data sets. The cohort, 1333 patients, had a mean (SD) age 57.5 (19.4) years, (41% female) and admission APACHE III score of 58 (27). ICU length of stay and mortality were 3.9 (6.1) days and 17.6% respectively. Mean total TISS and Omega scores were 117 (157) and 72 (113) respectively. Mean patient costs per ICU episode (1991 6801 (2534, range 95,602. Dominant cost fractions were nursing 43.3% and overheads 16.9%. Inflation adjusted year 2002 (mean) costs were AUS). Total costs in survivors were predicted by Omega score, summed APACHE III score and ICU length of stay; determination R2, 0.91; validation 0.88. Omega was the preferred activity score. Without the Omega score, predictors were age, summed APACHE III score and ICU length of stay; determination R2, 0.73; validation 0.73. In non-survivors, predictors were age and ICU length of stay (plus interaction), and Omega score (determination R2, 0.97; validation 0.91). Patient costs may be predicted by a combination of ICU activity indices and severity scores.J. L. Moran, A. R. Peisach, P. J. Solomon, J. Martinhttp://www.aaic.net.au/Article.asp?D=200403
Aligning simulation models: A case study and results
This paper develops the concepts and methods of a process we will call “alignment of computational models” or “docking” for short. Alignment is needed to determine whether two models can produce the same results, which in turn is the basis for critical experiments and for tests of whether one model can subsume another. We illustrate our concepts and methods using as a target a model of cultural transmission built by Axelrod. For comparison we use the Sugarscape model developed by Epstein and Axtell.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44707/1/10588_2005_Article_BF01299065.pd
SUSY Higgs Boson Decays into Scalar Quarks: QCD Corrections
In supersymmetric theories, the decays of the neutral CP-even and CP-odd as
well as the charged Higgs bosons into scalar quarks, in particular into top and
bottom squarks, can be dominant if they are kinematically allowed. We calculate
the QCD corrections to these decay modes in the minimal supersymmetric
extension of the Standard Model, including all quark mass terms and squark
mixing. These corrections turn out to be rather large, altering the decay
widths by an amount which can be larger than 50%. The corrections can be either
positive or negative, and depend strongly on the mass of the gluino. We also
discuss the QCD corrections to the decays of heavy scalar quarks into light
scalar quarks and Higgs bosons.Comment: 17 pages, 6 figure
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