2,684 research outputs found
Transitions to Nematic states in homogeneous suspensions of high aspect ratio magnetic rods
Isotropic-Nematic and Nematic-Nematic transitions from a homogeneous base
state of a suspension of high aspect ratio, rod-like magnetic particles are
studied for both Maier-Saupe and the Onsager excluded volume potentials. A
combination of classical linear stability and asymptotic analyses provides
insight into possible nematic states emanating from both the isotropic and
nematic non-polarized equilibrium states. Local analytical results close to
critical points in conjunction with global numerical results (Bhandar, 2002)
yields a unified picture of the bifurcation diagram and provides a convenient
base state to study effects of external orienting fields.Comment: 3 Figure
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
New measurement of charge asymmetry from HERA
After presenting the recent measurements of neutral current cross section in
DIS at HERA, we explain the effect of the interference at the
electro-weak scale, visible on these data. Then, the beam charge difference
is measured and the interference itself is extracted. Results are
discussed in the context of perturbative QCD.Comment: 4 pages, 3 figures, proceedings of the International Workshop on
Positrons at Jefferson Lab (March 25-27, 2009), Thomas Jefferson National
Accelerator Facility, Newport News, V
Variational Monte Carlo Calculations of H and He with a relativistic Hamiltonian - II
In relativistic Hamiltonians the two-nucleon interaction is expressed as a
sum of , the interaction in the rest frame,
and the ``boost interaction'' which depends upon the
total momentum and vanishes in the rest frame. The
can be regarded as a sum of four terms: , ,
and ; the first three originate from the
relativistic energy-momentum relation, Lorentz contraction and Thomas
precession, while the last is purely quantum. The contributions of and have been previously calculated with the
variational Monte Carlo method for H and He. In this brief note we
report the results of similar calculations for the contributions of and . These are found to be rather small.Comment: 7 pages, P-94-09-07
Relativistic calculation of the triton binding energy and its implications
First results for the triton binding energy obtained from the relativistic
spectator or Gross equation are reported. The Dirac structure of the nucleons
is taken into account. Numerical results are presented for a family of
realistic OBE models with off-shell scalar couplings. It is shown that these
off-shell couplings improve both the fits to the two-body data and the
predictions for the binding energy.Comment: 5 pages, RevTeX 3.0, 1 figure (uses epsfig.sty
Quantum Monte Carlo Studies of Relativistic Effects in Light Nuclei
Relativistic Hamiltonians are defined as the sum of relativistic one-body
kinetic energy, two- and three-body potentials and their boost corrections. In
this work we use the variational Monte Carlo method to study two kinds of
relativistic effects in the binding energy of 3H and 4He. The first is due to
the nonlocalities in the relativistic kinetic energy and relativistic one-pion
exchange potential (OPEP), and the second is from boost interaction. The OPEP
contribution is reduced by about 15% by the relativistic nonlocality, which may
also have significant effects on pion exchange currents. However, almost all of
this reduction is canceled by changes in the kinetic energy and other
interaction terms, and the total effect of the nonlocalities on the binding
energy is very small. The boost interactions, on the other hand, give repulsive
contributions of 0.4 (1.9) MeV in 3H (4He) and account for 37% of the
phenomenological part of the three-nucleon interaction needed in the
nonrelativistic Hamiltonians.Comment: 33 pages, RevTeX, 11 PostScript figures, submitted to Physical Review
The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbers
We study numerically the dependence of the critical magnetic Reynolds number
Rmc for the turbulent small-scale dynamo on the hydrodynamic Reynolds number
Re. The turbulence is statistically homogeneous, isotropic, and
mirror--symmetric. We are interested in the regime of low magnetic Prandtl
number Pm=Rm/Re<1, which is relevant for stellar convective zones, protostellar
disks, and laboratory liquid-metal experiments. The two asymptotic
possibilities are Rmc->const as Re->infinity (a small-scale dynamo exists at
low Pm) or Rmc/Re=Pmc->const as Re->infinity (no small-scale dynamo exists at
low Pm). Results obtained in two independent sets of simulations of MHD
turbulence using grid and spectral codes are brought together and found to be
in quantitative agreement. We find that at currently accessible resolutions,
Rmc grows with Re with no sign of approaching a constant limit. We reach the
maximum values of Rmc~500 for Re~3000. By comparing simulations with Laplacian
viscosity, fourth-, sixth-, and eighth-order hyperviscosity and Smagorinsky
large-eddy viscosity, we find that Rmc is not sensitive to the particular form
of the viscous cutoff. This work represents a significant extension of the
studies previously published by Schekochihin et al. 2004, PRL 92, 054502 and
Haugen et al. 2004, PRE, 70, 016308 and the first detailed scan of the
numerically accessible part of the stability curve Rmc(Re).Comment: 4 pages, emulateapj aastex, 2 figures; final version as published in
ApJL (but with colour figures
Structure of solar coronal loops: from miniature to large-scale
We will use new data from the High-resolution Coronal Imager (Hi-C) with
unprecedented spatial resolution of the solar corona to investigate the
structure of coronal loops down to 0.2 arcsec. During a rocket flight Hi-C
provided images of the solar corona in a wavelength band around 193 A that is
dominated by emission from Fe XII showing plasma at temperatures around 1.5 MK.
We analyze part of the Hi-C field-of-view to study the smallest coronal loops
observed so far and search for the a possible sub-structuring of larger loops.
We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal
loops. These have length of the coronal segment above the chromosphere of only
about 1 Mm and a thickness of less than 200 km. They could be interpreted as
the coronal signature of small flux tubes breaking through the photosphere with
a footpoint distance corresponding to the diameter of a cell of granulation. We
find loops that are longer than 50 Mm to have a diameter of about 2 arcsec or
1.5 Mm, consistent with previous observations. However, Hi-C really resolves
these loops with some 20 pixels across the loop. Even at this greatly improved
spatial resolution the large loops seem to have no visible sub-structure.
Instead they show a smooth variation in cross-section. The fact that the large
coronal loops do not show a sub-structure at the spatial scale of 0.1 arcsec
per pixel implies that either the densities and temperatures are smoothly
varying across these loops or poses an upper limit on the diameter of strands
the loops might be composed of. We estimate that strands that compose the 2
arcsec thick loop would have to be thinner than 15 km. The miniature loops we
find for the first time pose a challenge to be properly understood in terms of
modeling.Comment: Accepted for publication in A&A (Jun 19, 2013), 11 pages, 10 figure
Poincare' normal forms and simple compact Lie groups
We classify the possible behaviour of Poincar\'e-Dulac normal forms for
dynamical systems in with nonvanishing linear part and which are
equivariant under (the fundamental representation of) all the simple compact
Lie algebras and thus the corresponding simple compact Lie groups. The
``renormalized forms'' (in the sense of previous work by the author) of these
systems is also discussed; in this way we are able to simplify the
classification and moreover to analyze systems with zero linear part. We also
briefly discuss the convergence of the normalizing transformations.Comment: 17 pages; minor corrections in revised versio
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