3,147 research outputs found
Excited nucleon electromagnetic form factors from broken spin-flavor symmetry
A group theoretical derivation of a relation between the N --> Delta charge
quadrupole transition and neutron charge form factors is presented.Comment: 4 pages, Proc. of the 12 th Int'l. Workshop on the Physics of Excited
Nucleons, NSTAR 2009, Beijing, April 19-22, 200
Quark-hadron duality in neutrino scattering
We present a phenomenological model of the quark-hadron transition in
neutrino-nucleon scattering. Using recently extracted weak nucleon transition
form factors, we investigate the extent to which local and global quark-hadron
duality is applicable in the neutrino F_1, F_2 and F_3 structure functions, and
contrast this with duality in electron scattering. Our findings suggest that
duality works relatively well for neutrino-nucleon scattering for the F_2 and
F_3 structure functions, but not as well for F_1. We also calculate the
quasielastic, resonance and deep inelastic contributions to the Adler sum rule,
and find it to be satisfied to within 10% for 0.5 < Q^2 < 2 GeV^2.Comment: 28 pages, 6 figure
Explaining microbial population genomics through phage predation
The remarkable diversity of genes within the pool of prokaryotic genomes belonging to the same species or pan-genome is difficult to reconcile with the widely accepted paradigm which asserts that periodic selection within bacterial populations would regularly purge genomic diversity by clonal replacement. Recent evidence from metagenomics indicates that even within a single sample a large diversity of genomes can be present for a single species. We have found that much of the differential gene content affects regions that are potential phage recognition targets. We therefore propose the operation of Constant-Diversity dynamics in which the diversity of prokaryotic populations is preserved by phage predation. We provide supporting evidence for this model from metagenomics, mathematical analysis and computer simulations. Periodic selection and phage predation dynamics are not mutually exclusive; we compare their predictions to indicate under which ecological circumstances each dynamics could predominate
Global-in-time solutions for the isothermal Matovich-Pearson equations
In this paper we study the Matovich-Pearson equations describing the process
of glass fiber drawing. These equations may be viewed as a 1D-reduction of the
incompressible Navier-Stokes equations including free boundary, valid for the
drawing of a long and thin glass fiber. We concentrate on the isothermal case
without surface tension. Then the Matovich-Pearson equations represent a
nonlinearly coupled system of an elliptic equation for the axial velocity and a
hyperbolic transport equation for the fluid cross-sectional area. We first
prove existence of a local solution, and, after constructing appropriate
barrier functions, we deduce that the fluid radius is always strictly positive
and that the local solution remains in the same regularity class. To the best
of our knowledge, this is the first global existence and uniqueness result for
this important system of equations
Observation of a Turbulence-Induced Large Scale Magnetic Field
An axisymmetric magnetic field is applied to a spherical, turbulent flow of
liquid sodium. An induced magnetic dipole moment is measured which cannot be
generated by the interaction of the axisymmetric mean flow with the applied
field, indicating the presence of a turbulent electromotive force. It is shown
that the induced dipole moment should vanish for any axisymmetric laminar flow.
Also observed is the production of toroidal magnetic field from applied
poloidal magnetic field (the omega-effect). Its potential role in the
production of the induced dipole is discussed.Comment: 5 pages, 4 figures Revisions to accomodate peer-reviewer concerns;
changes to main text including simplification of a proof, Fig. 2 updated, and
minor typos and clarifications; Added refrences. Resubmitted to Phys. Rev.
Let
Impact of nuclear effects on the determination of the nucleon axial mass
We analize the influence of nuclear effects on the determination of the
nucleon axial mass from nuclear cross sections. Our work is based on a
formalism widely applied to describe electron-nucleus scattering data in the
impulse approximation regime. The results of numerical calculations show that
correlation effects, not taken into account by the relativistic Fermi gas
model, sizably affect the -dependence of the cross section. However, their
inclusion does not appear to explain the large values of the axial mass
recently reported by the K2K and MiniBooNE collaborations.Comment: 4 pages, three figure
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