3,651 research outputs found
Neutron electric form factor at large momentum transfer
Based on the recent, high precision data for elastic electron scattering from
protons and deuterons, at relatively large momentum transfer , we
determine the neutron electric form factor up to GeV. The values
obtained from the data (in the framework of the nonrelativistic impulse
approximation) are larger than commonly assumed and are in good agreement with
the Gari-Kr\"umpelmann parametrization of the nucleon electromagnetic form
factors.Comment: 11 pages 2 figure
A pure S-wave covariant model for the nucleon
Using the manifestly covariant spectator theory, and modeling the nucleon as
a system of three constituent quarks with their own electromagnetic structure,
we show that all four nucleon electromagnetic form factors can be very well
described by a manifestly covariant nucleon wave function with zero orbital
angular momentum. Since the concept of wave function depends on the formalism,
the conclusions of light-cone theory requiring nonzero angular momentum
components are not inconsistent with our results. We also show that our model
gives a qualitatively correct description of deep inelastic scattering,
unifying the phenomenology at high and low momentum transfer. Finally we review
two different definitions of nuclear shape and show that the nucleon is
spherical in this model, regardless of how shape is defined.Comment: 20 pages and 10 figures; greatly expanded version with new fits and
discussion of DIS; similar to published versio
Joint density-functional theory for electronic structure of solvated systems
We introduce a new form of density functional theory for the {\em ab initio}
description of electronic systems in contact with a molecular liquid
environment. This theory rigorously joins an electron density-functional for
the electrons of a solute with a classical density-functional theory for the
liquid into a single variational principle for the free energy of the combined
system. A simple approximate functional predicts, without any fitting of
parameters to solvation data, solvation energies as well as state-of-the-art
quantum-chemical cavity approaches, which require such fitting.Comment: Fixed typos and minor updates to tex
Target normal spin asymmetry and charge asymmetry for elastic scattering and the crossed processes
Two kinds of asymmetry arise from the interference of the Born amplitude and
the box-type amplitude corresponding to two virtual photons exchange, namely
charge-odd and one spin asymmetries. In case of unpolarized particles the
charge-odd correlation is calculated. It can be measured in combination of
electron muon and positron muon scattering experiments. The forward-backward
asymmetry is the corresponding quantity which can be measured for the crossed
processes. In the case of polarized muon the one-spin asymmetry for
annihilation and scattering channels has been calculated. The additional
structure function arising from the interference is shown to suffer from
infrared divergencies. The background due to electroweak interaction is
discussed.Comment: 19 pages, 5 figure
High energy inelastic electron hadron scattering, in peripheral kinematics. Sum rules for hadron form factors
Relations between differential cross section for inelastic scattering of
electrons on hadrons and hadron form factors (sum rules) are derived on the
basis of analytical properties of heavy photon forward Compton scattering on
hadrons. Sum rules relating the slope of form-factors at zero momentum transfer
and anomalous magnetic moments of hadrons with some integrals on
photo-production on a hadrons is obtained as well. To provide the convergence
of these integrals we construct differences of individual sum rules for
different hadrons. Universal interaction of Pomeron with nucleons is assumed.
We derive the explicit formulae for processes of electro-production on proton
and light isobar nuclei. Sudakov's parametrization of momenta, for peripheral
kinematics relevant here, is used. The light-cone form of differential cross
sections is also discussed. The accuracy of sum rules estimated in frames of
point-like hadrons and it is shown to be at the level of precision achievable
by experiments. Suggestions and predictions for future experiments are also
given.Comment: 18 pages, 5 figure
Lichens as natural sources of biotechnologically relevant bacteria
International audienceThe search for microorganisms from novel sources and in particular microbial symbioses represents a promising approach in biotechnology. In this context, lichens have increasingly become a subject of research in microbial biotechnology, particularly after the recognition that a diverse community of bacteria other than cyanobacteria is an additional partner to the traditionally recognized algae-fungus mutualism. Here, we review recent studies using culture-dependent as well as culture-independent approaches showing that lichens can harbor diverse bacterial families known for the production of compounds of biotechnological interest and that several microorganisms isolated from lichens, in particular Actinobacteria and Cyanobacteria, can produce a number of bioactive compounds, many of them with biotechnological potential
Littoral lichens as a novel source of potentially bioactive Actinobacteria
International audienceCultivable Actinobacteria are the largest source of microbially derived bioactive molecules. The high demand for novel antibiotics highlights the need for exploring novel sources of these bacteria. Microbial symbioses with sessile macro-organisms, known to contain bioactive compounds likely of bacterial origin, represent an interesting and underexplored source of Actinobacteria. We studied the diversity and potential for bioactive-metabolite production of Actinobacteria associated with two marine lichens (Lichina confinis and L. pygmaea; from intertidal and subtidal zones) and one littoral lichen (Roccella fuciformis; from supratidal zone) from the Brittany coast (France), as well as the terrestrial lichen Collema auriforme (from a riparian zone, Austria). A total of 247 bacterial strains were isolated using two selective media. Isolates were identified and clustered into 101 OTUs (98% identity) including 51 actinobacterial OTUs. The actinobacterial families observed were: Brevibacteriaceae, Cellulomonadaceae, Gordoniaceae, Micrococcaceae, Mycobacteriaceae, Nocardioidaceae, Promicromonosporaceae, Pseudonocardiaceae, Sanguibacteraceae and Streptomycetaceae. Interestingly, the diversity was most influenced by the selective media rather than lichen species or the level of lichen thallus association. The potential for bioactive-metabolite biosynthesis of the isolates was confirmed by screening genes coding for polyketide synthases types I and II. These results show that littoral lichens are a source of diverse potentially bioactive Actinobacteria
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