21 research outputs found
Covariant calculation of strange decays of baryon resonances
We present results for kaon decay widths of baryon resonances from a
relativistic study with constituent quark models. The calculations are done in
the point-form of Poincare-invariant quantum mechanics with a spectator-model
decay operator. We obtain covariant predictions of the Goldstone-boson-exchange
and a variant of the one-gluon-exchange constituent quark models for all kaon
decay widths of established baryon resonances. They are generally characterized
by underestimating the available experimental data. In particular, the widths
of kaon decays with increasing strangeness in the baryon turn out to be
extremely small. We also consider the nonrelativistic limit, leading to the
familiar elementary emission model, and demonstrate the importance of
relativistic effects. It is found that the nonrelativistic approach evidently
misses sensible influences from Lorentz boosts and some essential spin-coupling
terms.Comment: 6 pages, 3 table
Quark-Model Identification of Baryon Ground and Resonant States
We present a new classification scheme of baryon ground states and resonances
into SU(3) flavor multiplets. The scheme is worked out along a covariant
formalism with relativistic constituent quark models and it relies on detailed
investigations of the baryon spectra, the spin-flavor structure of the baryon
eigenstates, the behaviour of their probability density distributions as well
as covariant predictions for mesonic decay widths. The results are found to be
quite independent of the specific types of relativistic constituent quark
models employed. It turns out that a consistent classification requires to
include also resonances that are presently reported from experiment with only
two-star status.Comment: 24 pages, 20 Fig
Electromagnetic nucleon form factors in instant and point form
We present a study of the electromagnetic structure of the nucleons with
constituent quark models in the framework of relativistic quantum mechanics. In
particular, we address the construction of spectator-model currents in the
instant and point forms. Corresponding results for the elastic nucleon
electromagnetic form factors as well as charge radii and magnetic moments are
presented. We also compare results obtained by different realistic nucleon wave
functions stemming from alternative constituent quark models. Finally, we
discuss the theoretical uncertainties that reside in the construction of
spectator-model transition operators.Comment: 13 pages, 9 figures, updated and extended version for publicatio
Extended Goldstone-Boson-Exchange Constituent Quark Model
We present an extension of the Goldstone-boson-exchange constituent quark
model including additional interactions beyond the ones used hitherto. For the
hyperfine interaction between the constituent quarks we assume pseudoscalar,
vector, and scalar meson exchanges and consider all relevant force components
produced by these types of exchanges. The resulting model, which corresponds to
a relativistic Poincare-invariant Hamiltonian (or equivalently mass operator),
provides a unified framework for a covariant description of all light and
strange baryons. The ground states and resonances up to an excitation energy of
about 2 GeV are reproduced in fair agreement with phenomenology, with the
exception of the first excitations above the Lambda and Xi ground states.Comment: 11 pages, 4 figures, 3 tables; substantial revisions, additional
author, bibliography extended and update
Wenn die Fakten der Anderen nur eine Alternative sind –»Fake News« in Verschwörungstheorien als überdauerndes Phänomen
Verschwörungstheorien beinhalten den Glauben an die geheime Zusammenarbeit von Verschwörer*innen mit destruktiven Zielen. Verschwörungstheoretiker*innen nutzen falsche oder stark verzerrte Informationen –»Fake News«–, um zu manipulieren, ihre Position zu stärken und ihren gesellschaftlichen Einfluss ausbauen zu können. Verschwörungstheorien sind jedoch bei weitem nicht neu. Sie gibt es, solange es Menschen gibt. Durch digitale Medien haben sich lediglich die Verbreitungskanäle und -geschwindigkeit verändert. Der nachfolgende Beitrag möchte einen detaillierten Blick auf die Gestalt und individuelle sowie gesellschaftliche Wirkung von Verschwörungstheorien werfen. Dabei bemühen wir aktuelle und historische Beispiele. Darüber hinaus werden verschiedene mögliche Ge-genmaßnahmen und ihre Limitationen diskutiert
Widespread detection of highly pathogenic H5 influenza viruses in wild birds from the Pacific Flyway of the United States
A novel highly pathogenic avian influenza virus belonging to the H5 clade 2.3.4.4 variant viruses was detected in North America in late 2014. Motivated by the identification of these viruses in domestic poultry in Canada, an intensive study was initiated to conduct highly pathogenic avian influenza surveillance in wild birds in the Pacific Flyway of the United States. A total of 4,729 hunter-harvested wild birds were sampled and highly pathogenic avian influenza virus was detected in 1.3% (n = 63). Three H5 clade 2.3.4.4 subtypes were isolated from wild birds, H5N2, H5N8, and H5N1, representing the wholly Eurasian lineage H5N8 and two novel reassortant viruses. Testing of 150 additional wild birds during avian morbidity and mortality investigations in Washington yielded 10 (6.7%) additional highly pathogenic avian influenza isolates (H5N8 = 3 and H5N2 = 7). The geographically widespread detection of these viruses in apparently healthy wild waterfowl suggest that the H5 clade 2.3.4.4 variant viruses may behave similarly in this taxonomic group whereby many waterfowl species are susceptible to infection but do not demonstrate obvious clinical disease. Despite these findings in wild waterfowl, mortality has been documented for some wild bird species and losses in US domestic poultry during the first half of 2015 were unprecedented
Patterns of long‐term vegetation change vary between different types of semi‐natural grasslands in Western and Central Europe
Questions: Has plant species richness in semi‐natural grasslands changed over recent decades? Do the temporal trends of habitat specialists differ from those of habitat generalists? Has there been a homogenization of the grassland vegetation?
Location: Different regions in Germany and the UK.
Methods: We conducted a formal meta‐analysis of re‐survey vegetation studies of semi‐natural grasslands. In total, 23 data sets were compiled, spanning up to 75 years between the surveys, including 13 data sets from wet grasslands, six from dry grasslands and four from other grassland types. Edaphic conditions were assessed using mean Ellenberg indicator values for soil moisture, nitrogen and pH. Changes in species richness and environmental variables were evaluated using response ratios.
Results: In most wet grasslands, total species richness declined over time, while habitat specialists almost completely vanished. The number of species losses increased with increasing time between the surveys and were associated with a strong decrease in soil moisture and higher soil nutrient contents. Wet grasslands in nature reserves showed no such changes or even opposite trends. In dry grasslands and other grassland types, total species richness did not consistently change, but the number or proportions of habitat specialists declined. There were also considerable changes in species composition, especially in wet grasslands that often have been converted into intensively managed, highly productive meadows or pastures. We did not find a general homogenization of the vegetation in any of the grassland types.
Conclusions: The results document the widespread deterioration of semi‐natural grasslands, especially of those types that can easily be transformed to high production grasslands. The main causes for the loss of grassland specialists are changed management in combination with increased fertilization and nitrogen deposition. Dry grasslands are most resistant to change, but also show a long‐term trend towards an increase in more mesotrophic species