185 research outputs found
A spinor approach to Walker geometry
A four-dimensional Walker geometry is a four-dimensional manifold M with a
neutral metric g and a parallel distribution of totally null two-planes. This
distribution has a natural characterization as a projective spinor field
subject to a certain constraint. Spinors therefore provide a natural tool for
studying Walker geometry, which we exploit to draw together several themes in
recent explicit studies of Walker geometry and in other work of Dunajski (2002)
and Plebanski (1975) in which Walker geometry is implicit. In addition to
studying local Walker geometry, we address a global question raised by the use
of spinors.Comment: 41 pages. Typos which persisted into published version corrected,
notably at (2.15
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
Gastric cancer and Helicobacter pylori: a combined analysis of 12 case control studies nested within prospective cohorts
BACKGROUND: The magnitude of the association
between Helicobacter pylori and
incidence of gastric cancer is unclear. H
pylori infection and the circulating antibody
response can be lost with development
of cancer; thus retrospective studies
are subject to bias resulting from classifi-
cation of cases as H pylori negative when
they were infected in the past.
AIMS: To combine data from all case control
studies nested within prospective
cohorts to assess more reliably the relative
risk of gastric cancer associated with H
pylori infection.To investigate variation in
relative risk by age, sex, cancer type and
subsite, and interval between blood sampling
and cancer diagnosis.
METHODS: Studies were eligible if blood
samples for H pylori serology were collected
before diagnosis of gastric cancer in
cases. Identified published studies and two
unpublished studies were included. Individual
subject data were obtained for
each. Matched odds ratios (ORs) and 95%
confidence intervals (95% CI) were calculated
for the association between H pylori
and gastric cancer.
RESULTS: Twelve studies with 1228 gastric
cancer cases were considered. The association
with H pylori was restricted to noncardia
cancers (OR 3.0; 95% CI 2.3–3.8)
and was stronger when blood samples for
H pylori serology were collected 10+ years
before cancer diagnosis (5.9; 3.4–10.3). H
pylori infection was not associated with an
altered overall risk of cardia cancer (1.0;
0.7–1.4).
CONCLUSIONS: These results suggest that
5.9 is the best estimate of the relative risk
of non-cardia cancer associated with H
pylori infection and that H pylori does not
increase the risk of cardia cancer. They
also support the idea that when H pylori
status is assessed close to cancer diagnosis,
the magnitude of the non-cardia
association may be underestimated
How just and just how? A systematic review of social equity in conservation research
Background: Conservation decisions not only impact wildlife, habitat, and environmental health, but also human wellbeing and social justice. The inclusion of safeguards and equity considerations in the conservation field has increasingly garnered attention in international policy processes and amongst conservation practitioners. Yet, what constitutes an ‘equitable’ solution can take many forms, and how the concept is treated within conservation research is not standardized. This review explores how social equity is conceptualized and assessed in conservation research.Methods/Design: Using a structured search and screening process, we identified 138 peer-reviewed studies that addressed equity in relation to conservation actions. The authors developed a coding framework to guide the review process, focusing on the current state of, definitions used for, and means of assessing social equity in empirical conservation research.Review Results: Results show that empirical research on social equity in conservation is rapidly growing, with the majority of studies on the topic published only since 2009. Equity within conservation research is skewed toward distributional concerns and to a lesser extent procedural issues, with recognition and contextual equity receiving little attention. Studies are primarily situated in forested biomes of the Global South. Conservation interventions mostly resulted in mixed or negative impacts on equity.Synthesis and Discussion: Our results demonstrate the current limitations of research on equity in conservation, and raise challenging questions about the social impacts of conservation and how to ameliorate equity concerns. Framing of equity within conservation research would benefit from greater transparency of study motivation, more explicit definition of how equity is used within the study context, and consideration for how best to assess it.We recommend that the empirical conservation literature more deeply engage with different notions of equity when studying, planning, and implementing actions to address potential trade-offs among equity and conservation objectives and beneficiaries
Pseudo-hyperkahler Geometry and Generalized Kahler Geometry
We discuss the conditions for additional supersymmetry and twisted
supersymmetry in N = (2, 2) supersymmetric non-linear sigma models described by
one left and one right semi-chiral superfield and carrying a pair of
non-commuting complex structures. Focus is on linear non-manifest
transformations of these fields that have an algebra that closes off-shell. We
find that additional linear supersymmetry has no interesting solution, whereas
additional linear twisted supersymmetry has solutions with interesting
geometrical properties. We solve the conditions for invariance of the action
and show that these solutions correspond to a bi-hermitian metric of signature
(2, 2) and a pseudo-hyperkaehler geometry of the target space.Comment: Letters in Mathematical Physics : electronically published versio
Spinor condensates and light scattering from Bose-Einstein condensates
These notes discuss two aspects of the physics of atomic Bose-Einstein
condensates: optical properties and spinor condensates. The first topic
includes light scattering experiments which probe the excitations of a
condensate in both the free-particle and phonon regime. At higher light
intensity, a new form of superradiance and phase-coherent matter wave
amplification were observed. We also discuss properties of spinor condensates
and describe studies of ground--state spin domain structures and dynamical
studies which revealed metastable excited states and quantum tunneling.Comment: 58 pages, 33 figures, to appear in Proceedings of Les Houches 1999
Summer School, Session LXXI
Recommended from our members
Instrumentation and Measurement Strategy for the NOAA SENEX Aircraft Campaign as Part of the Southeast Atmosphere Study 2013
Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO2 measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions
Measurement of the View the tt production cross-section using eμ events with b-tagged jets in pp collisions at √s = 13 TeV with the ATLAS detector
This paper describes a measurement of the inclusive top quark pair production cross-section (σtt¯) with a data sample of 3.2 fb−1 of proton–proton collisions at a centre-of-mass energy of √s = 13 TeV, collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron–muon pair in the final state. Jets containing b-quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two b-tagged jets are counted and used to determine simultaneously σtt¯ and the efficiency to reconstruct and b-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be:
σtt¯ = 818 ± 8 (stat) ± 27 (syst) ± 19 (lumi) ± 12 (beam) pb,
where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented
Search for TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at sqrt [ s ] = 13TeV
A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb−1 of proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at √s = 13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions
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